it is known that swimming is a non - weight - bearing activity that can increase aerobic capacity and lean body mass , however , it has no positive effects on bone mineral density ( bmd ) [ 1 - 5 ] . the loss of bone volume and quality due to a lack of mechanical stimuli can be explained by wolff s law . bone loss may induce osteoporosis later in life , a skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue , leading to increased susceptibility to fractures . osteoporosis has become a poignant health problem , particularly for women , and it has long been known that female athletes , especially swimmers , have menstrual irregularity [ 9 - 11 ] . females are more vulnerable to bone loss as a result of having smaller skeletons compared with males , and the estrogen patterns that accompany menarche and menopause . hence , low bmd associated with menstrual irregularity in female athletes may cause severe health problems later in life . previous cross - sectional studies have compared the bmd of competitive swimmers to other competitive athletes , showing that the bmd of collegiate swimmers is significantly lower than that of their counterparts who participate in impact exercise and equal to or lower than that of non - athletic controls . the results of these studies indicate that swimming is not beneficial for the promotion , maintenance , or increase of bmd . conversely , several studies have addressed that there is no significant difference between swimmers and controls with respect to bmd [ 14 - 16 ] . as shown in a previous study , bone - exercise - nutrition interaction exists , and therefore , studies that evaluate the effect of dietary supplementation in swimmers are needed in order to find out the possible interaction between training and bone health . nutritional aspects such as calcium , magnesium , and vitamin d may also affect bone health in swimmers . to date , swim training still seems to have conflicting effects on bone health maintenance in female athletes . therefore , the present review focuses on swim training , dietary supplementation , and bmd in athletes from prior representative research from the 1990s to date , with a view to elucidating the effect of swim training on bmd in the athletic population . the international society of clinical densitometry ( iscd )   recommends the use of z - scores rather than t - scores , because they compare bmd with age- and sex - matched controls . iscd defines a z - score of -2 or below as low bmd under the expected age , and recommends that a diagnosis of osteoporosis be made if additional risk factors for poor bone health are identified . low bmd as a history of nutritional deficiencies , hypoestrogenism , stress fractures , and/or other secondary clinical risk factors for fracture , together with a z - score between -1.0 and -2.0 due to the fact that athletes tend to have 10 - 15% higher bmd than the non - athletic population . although low bmd has been defined as a z - score of -2 or below , there is no report of the characteristics of normal distribution , and therefore , the exact range of low bmd can not be obtained . moreover , a z - score between -1.0 and -2.0 can be applied to members of the entire population including well - trained athletes and untrained individuals . hence , bmd measurement in the athletic population needs to consider characteristics of the sport type and bone - loaded regions . according to wolff s law , in the healthy population bone mass can be increased by physical activities requiring high force and/or generating high impact on bone . in previous studies , several exercise training programs have been used to improve bmd [ 8 , 20 ] . in general , swimming is considered to be an unloaded exercise that has no impact on bmd , whereas exercises that include walking , jumping , running , and stopping such as basketball , running , and gymnastics are considered to be high impact sports that have a positive effect on bmd . however , bmd can be also be directly affected by weight bearing and indirectly affected by repeated muscle contraction . therefore , recent studies have focused on the relationship between bmd and the characteristics of muscle fibers based on genetic factors in animals and humans . although swim training may not actually cause positive phenotypic effects on bmd , regular and long - term exercise is repeatedly performed and thus may appear to exert positive changes in bmd . to support this suggestion , the joint effect of phenotypic activation , considering age and physiological characteristics and major and minor trace minerals affecting bone metabolism , should be conducted in order to elucidate an exact mechanism for bmd change . in the 1990 s , studies examined and compared bmd in swimmers and athletes who participated in impact sports , concluding that swimming does not promote high bmd , based on their attempts to mechanistically explain their results . prior cross - sectional studies have demonstrated higher bmd among athletes who engage in weight - bearing and impact sports when compared with their non - athlete counterparts [ 23 - 26 ] , as well as with their non - weight - bearing athlete counterparts . the goal of the study was to clarify the relationship between athletic training and bone density in young eumenorrheic athletes . swimmers had a significantly lower mean density in the lumbar spine compared with all other groups . the major conclusion of the paper is that the data support the concept that athletes in sports involving impact have a higher bmd than non - athletes and swimmers . taaffe et al . ,   examined the role of skeletal loading patterns on bmd in eumenorrheic athletes who chronically trained using various forms of skeletal loading ; the intensive impact of gymnastics , the non - weight - bearing of swimming , and the usual activity among controls . they examined 39 athletes who competed in their respective sports at the ncaa division 1 level and 19 non - athletes who exercised three hours or less a week . the study found no difference in bmd among the groups in either the lumbar spine or the whole body . the gymnasts and controls had greater bmd at the greater trochanter and femoral neck than the swimmers . the study concluded that high - impact weight - bearing activity is beneficial for bone accumulation , and the authors proposed that high ground reaction forces and extreme muscle contraction on bone may contribute to the higher bmd in gymnasts . ,   compared the bmd of collegiate female athletes who competed in impact - loading sports ( volleyball and gymnastics ) and swimming . impact - loading players had greater bmd than swimmers and controls , however , there was no significant difference in bmd between swimmers and controls , at any site . this study also concluded that the prevalence of menstrual dysfunction in some participants does not appear to negatively influence bmd . in the 2000 s , creighton et al . ,   examined bmd and markers of bone turnover in female athletes with different levels of impact in their sports . they determined the effect of regular training in three different types of competitive sport on bmd , bone formation , and bone resorption in young women . 50 women participated in the study and were separated into three groups based on the degree of impact , which was determined by ground reaction forces associated with their sport . the high impact group was found to have significantly higher bmd at the femoral neck , ward 's triangle , trochanter , and a higher total bmd than the medium impact , non - impact ( swimmers ) , and control groups . markers of bone formation were significantly lower in the swimming group compared with the high and medium impact groups . swimmers had high bone resorption and low bone formation markers compared with the high , medium , and control groups . the study concluded that female athletes involved in high impact sports have the greatest bmd at weight - bearing sites as well as the highest markers for bone formation . duncan and colleagues   investigated the influence of different exercise types on bmd in elite female athletes including cyclists , runners , swimmers , triathletes , and controls ( 15 per group ) . total and areal bmd using dxa scans were conducted and it was found that running , a weight - bearing exercise , was associated with higher bmd than swimming or cycling , which is in accordance with previous studies . in 2007 , mudd and colleagues   found that bone mass and sport type were important determinants of bone health in female athletes when comparing impact sports and swimming . this cross - sectional study recruited 99 collegiate female athletes in gymnastics , softball , cross country , field hockey , soccer , crew , and swimming / diving . runners had higher overall bmd values than other sports athletes , and swimmers and divers had a significantly lower leg bmd ( 1.1170.086 g / cm ) than every other sports athlete . this study concluded that greater differences among sports were seen when comparing lumbar , pelvis , and leg bmds , and recommended that a longitudinal study would need to be conducted in order to investigate bone changes in response to training over a longer period of time in female athletes . maimoun and colleagues   investigated the bmd of gymnasts ( impact activity ) , swimmers ( non - impact activity ) , and controls using dxa for total and regional measurement . at baseline and the 12-month follow - up , gymnasts showed a significantly higher bmd than swimmers and controls . moreover , the total bmd of swimmers ( 0.9600.013 g / cm ) was lower than that of the controls ( 0.9950.013 g / cm ) at baseline and the 12-month follow - up ( 0.9910.014 g / cm and 1.0090.013 g / cm , respectively ) . in accordance with previous findings , this study demonstrated that the osteogenic effect of impact activity was greater than non - impact activity . summarizing previous studies , it has been shown that the loading - induced bmd of impact sports athletes seems to be higher compared with swimmers and/or the non - athletic population . thus , it appears that different bone - loading history and type may highly affect skeletal adaptation in athletes . future studies should consider this point in order to elucidate a better and clearer mechanism of the effect of swim training on bmd . several previous studies on bmd and other markers of bone strength have shown an advantage for swimmers compared with inactive controls and/or other sports athletes . some researchers insist that swim training increases muscle contraction and strain on the skeleton , leading to increased mechanical loading , which would have a positive effect on bmd despite the fact that swimming generally represents a non - weight - bearing sport . matsumoto et al . ,   examined biomarkers of bone metabolism and bmd in 103 male and female athletes . the subjects were japanese collegiate athletes who specialized in long distance running , judo , and swimming at the national level , however , no controls were used in the study . whole body bmd was measured using dxa , and urine samples were collected for pyriodinoline ( pyr ) and deoxypyridinoline ( dpd , bone resorption markers ) using high pressure liquid chromatography ( hplc ) . blood was collected for bone alkaline phosphatase ( balp ) and carboxyterminal propeptide of type one collagen ( picp , bone formation markers ) measurement . researchers found that the total body bmd was significantly higher in judo athletes compared with runners and swimmers . male runners had a significantly lower balp level than male judoists , and found in balp among the female athletes . the dpd level of male and female runners and swimmers was significantly lower than that of male and female judoists . however , the pyd level of female swimmers was not significantly different from that of runners or judoists . the conclusion by the researchers in this study was that differences in total body bmd are in part due to the demands of the specific sport , and that they are reflected in the levels of bone metabolic markers . this was the first study of its kind to examine biomarkers of bone metabolism among various types of athletes , although its results are limited and there were no control subjects . biomarkers are best if assessed over time to determine change in bone metabolism with a given stimulus , and bmd is best when examining clinically relevant sites such as the hip and spine . similar to this study , summarizing previous studies shows that swimmers seem to have higher levels of bone turnover biomarkers [ 33 , 34 ] , although they do not have a higher bmd compared with other sports athletes and/or controls . greenway et al . ,   examined the negative effect of long - term swim training participation in 43 swimmers compared with 44 controls . swimmers had a swimming career of over 5 years , and dxa was used to determine the total and regional bmd . there were no differences in bone mass or bmd between the two groups , at any site . the total body z - score of swimmers was 1.24 and of the controls was 1.18 . this study concluded that long - term swim training participation did not compromise regional bmd , and swim training coupled with weight - bearing activities may induce positive effects on bmd . it appears that longitudinal swim study investigating other physical activity history is needed to verify the exact effect of swim training on bmd . in 2014 , stanforth and colleagues compared the bmd in female athletes of various sports ( basketball , soccer , swimming , volleyball ) aged 18 - 23 . they reported that female swimmers in this study had an increased bmd from baseline to year 3 post - season , although the increased amount was still smaller than that of impact sports athletes . they concluded that differences in bmd between impact and non - impact sports are large , however , this study provides evidence that swim training may increase bmd in female swimmers . akgl and colleagues   examined the effect of swim training on bmd in 79 swimmers . all swimmers had engaged in at least 2 years swim training , and dxa was used to measure the total and regional bmd . the bmd of 68 swimmers ( 86% ) was normal and a total of 9 swimmers ( 11.4% ) had low bmd . this study implies that swim training may positively affect bmd , although it did not induce increased bmd in female swimmers . summarizing previous studies , it has been shown that swimming may be preferable for maintaining bone health , although swimming has not been shown to result in an improvement in bmd like impact sports . therefore , swim training combined with other types of training would be beneficial on the bone health of swimmers , in order to ensure a better health condition . it has been stressed that reliable reports concerning the dietary habits of athletes , especially elite swimmers , are needed . several studies have shown significantly higher calcium intake in swimmers than in controls [ 36 - 38 ] . other nutritional aspects such as magnesium , iron , and vitamin d may also affect the maintenance of bone health in swimmers . a healthy and well - balanced diet , including the proper amount of calcium , vitamin d , magnesium , and iron , may provide sufficient nutrients . in 2009 , hoogenboom et al . , determined the nutritional knowledge and eating behaviors of female collegiate swimmers . they proposed that swimming , due to the importance of a lean body weight , is associated with nutritional deficiency , and that this would lead to the development of the female athletic triad ; osteoporosis , menstrual dysfunction , and eating disorders . to determine nutritional intake , they used a 24-hour recall food survey , with 85 collegiate female swimmers participating in the study . they reported a mean daily calcium intake of 1578.88 mg in all participants , which was a higher level compared with the recommended dietary allowance ( rda ) of calcium ( 1200 mg / day ) . dugocka and colleagues   investigated bmd using dxa in young female swimmers ( n=41 , aged 11 - 14 years ) , considering nutritional aspects using a 3-day food intake recall survey . they reported that swimmers had a higher intake of calcium ( 602229 mg vs. 454236 mg ) and phosphorus ( 1390420 mg vs. 1027292 mg ) compared with non - athletic controls , although both groups had a deficiency in average calcium intake . however , they found that the mean value of bmd in both groups did not differ , indicating that in this study , a higher intake of calcium and phosphorus in swimmers may positively affect bone health when compared to controls who were not active in sports . czeczuk et al . ,   investigated the dietary habits ( calcium intake ) of 18 former swimmers and 18 current swimmers , reporting that calcium intake in both groups was sufficient and did not exceed 3/4 of the daily normal . these previous studies reported that calcium consumption in swimmers was higher than the rda and/or controls . in contrast , several previous studies have shown that female swimmers have calcium intakes below the rda and/or other sports athletes and controls . berning et al . ,   investigated the dietary food records of adolescent male and female swimmers , and reported that over 50% of elite adolescent female swimmers consumed lower calcium and iron intake compared with the rda and other sports athletes . similar to this study , hawley and williams   investigated 20 elite swimmers ( 11 females , 9 males ) using a 4-day food intake survey , and found that 55% of swimmers consumed an intake of calcium below the rda and 65% of swimmers had a lower intake of iron . moreover , the mean iron intake of female swimmers was significantly lower than the rda , and 82% of female swimmers consumed a low iron intake in this study . based on previous studies [ 42 - 44 ] , a low iron intake is common in female athletes . greenway et al . ,   examined the calcium intake for the previous 12 months through a questionnaire , and they also found that calcium intake was low in swimmers and controls . swimmers achieved 83% of the recommended daily intake ( rdi ) , however , when compared with the controls ( achieved 66% of the rdi ) , swimmers consumed a higher amount of calcium . czeczelewski and colleagues   conducted a 3-year monitoring of bmd and nutrient intakes in adolescent female swimmers ( n=20 ) and non - athletic controls ( n=20 ) . the calcium intake of swimmers was below the recommended values , and throughout the 3 years , the bmd of swimmers decreased , suggesting that this could have been due to insufficient calcium intake . akgl and colleagues   examined the dietary information of 79 swimmers using a 3-day food diary , and also reported low calcium intake in the swimmers . moreover , they found that swimmers with a normal bmd consumed a higher amount of calcium ( median 700 mg ) than the swimmers with a low bmd ( median 600 mg ) . furthermore , 31 ( 39.2% ) swimmers in this study had a vitamin d deficiency . vitamin d supplementation is also becoming more popular , especially in the athletic population , because it may reduce the incidence of stress fractures when combined with calcium , and may also have an ergogenic factor for athletes . summarizing previous studies , many authors have reported common dietary irregularities concerning female adolescent and adult swimmers in relation to swim training . therefore , to elucidate the exact mechanism and association between dietary supplementation and swim training , the international society of clinical densitometry ( iscd )   recommends the use of z - scores rather than t - scores , because they compare bmd with age- and sex - matched controls . iscd defines a z - score of -2 or below as low bmd under the expected age , and recommends that a diagnosis of osteoporosis be made if additional risk factors for poor bone health are identified . the american college of sports medicine defines the term  low bmd as a history of nutritional deficiencies , hypoestrogenism , stress fractures , and/or other secondary clinical risk factors for fracture , together with a z - score between -1.0 and -2.0 due to the fact that athletes tend to have 10 - 15% higher bmd than the non - athletic population . although low bmd has been defined as a z - score of -2 or below , there is no report of the characteristics of normal distribution , and therefore , the exact range of low bmd can not be obtained . moreover , a z - score between -1.0 and -2.0 can be applied to members of the entire population including well - trained athletes and untrained individuals . hence , bmd measurement in the athletic population needs to consider characteristics of the sport type and bone - loaded regions . according to wolff s law , in the healthy population , bone usually responds to stress by increasing its mass and bmd . bone mass can be increased by physical activities requiring high force and/or generating high impact on bone . in previous studies , several exercise training programs have been used to improve bmd [ 8 , 20 ] . in general , swimming is considered to be an unloaded exercise that has no impact on bmd , whereas exercises that include walking , jumping , running , and stopping such as basketball , running , and gymnastics are considered to be high impact sports that have a positive effect on bmd . however , bmd can be also be directly affected by weight bearing and indirectly affected by repeated muscle contraction . therefore , recent studies have focused on the relationship between bmd and the characteristics of muscle fibers based on genetic factors in animals and humans . although swim training may not actually cause positive phenotypic effects on bmd , regular and long - term exercise is repeatedly performed and thus may appear to exert positive changes in bmd . to support this suggestion , the joint effect of phenotypic activation , considering age and physiological characteristics and major and minor trace minerals affecting bone metabolism , should be conducted in order to elucidate an exact mechanism for bmd change . in the 1990 s , studies examined and compared bmd in swimmers and athletes who participated in impact sports , concluding that swimming does not promote high bmd , based on their attempts to mechanistically explain their results . prior cross - sectional studies have demonstrated higher bmd among athletes who engage in weight - bearing and impact sports when compared with their non - athlete counterparts [ 23 - 26 ] , as well as with their non - weight - bearing athlete counterparts . the goal of the study was to clarify the relationship between athletic training and bone density in young eumenorrheic athletes . swimmers had a significantly lower mean density in the lumbar spine compared with all other groups . the major conclusion of the paper is that the data support the concept that athletes in sports involving impact have a higher bmd than non - athletes and swimmers . taaffe et al . ,   examined the role of skeletal loading patterns on bmd in eumenorrheic athletes who chronically trained using various forms of skeletal loading ; the intensive impact of gymnastics , the non - weight - bearing of swimming , and the usual activity among controls . they examined 39 athletes who competed in their respective sports at the ncaa division 1 level and 19 non - athletes who exercised three hours or less a week . the study found no difference in bmd among the groups in either the lumbar spine or the whole body . the gymnasts and controls had greater bmd at the greater trochanter and femoral neck than the swimmers . the study concluded that high - impact weight - bearing activity is beneficial for bone accumulation , and the authors proposed that high ground reaction forces and extreme muscle contraction on bone may contribute to the higher bmd in gymnasts . ,   compared the bmd of collegiate female athletes who competed in impact - loading sports ( volleyball and gymnastics ) and swimming . impact - loading players had greater bmd than swimmers and controls , however , there was no significant difference in bmd between swimmers and controls , at any site . this study also concluded that the prevalence of menstrual dysfunction in some participants does not appear to negatively influence bmd . in the 2000 s , creighton et al . ,   examined bmd and markers of bone turnover in female athletes with different levels of impact in their sports . they determined the effect of regular training in three different types of competitive sport on bmd , bone formation , and bone resorption in young women . 50 women participated in the study and were separated into three groups based on the degree of impact , which was determined by ground reaction forces associated with their sport . the high impact group was found to have significantly higher bmd at the femoral neck , ward 's triangle , trochanter , and a higher total bmd than the medium impact , non - impact ( swimmers ) , and control groups . markers of bone formation were significantly lower in the swimming group compared with the high and medium impact groups . swimmers had high bone resorption and low bone formation markers compared with the high , medium , and control groups . the study concluded that female athletes involved in high impact sports have the greatest bmd at weight - bearing sites as well as the highest markers for bone formation . duncan and colleagues   investigated the influence of different exercise types on bmd in elite female athletes including cyclists , runners , swimmers , triathletes , and controls ( 15 per group ) . total and areal bmd using dxa scans were conducted and it was found that running , a weight - bearing exercise , was associated with higher bmd than swimming or cycling , which is in accordance with previous studies . in 2007 , mudd and colleagues   found that bone mass and sport type were important determinants of bone health in female athletes when comparing impact sports and swimming . this cross - sectional study recruited 99 collegiate female athletes in gymnastics , softball , cross country , field hockey , soccer , crew , and swimming / diving . runners had higher overall bmd values than other sports athletes , and swimmers and divers had a significantly lower leg bmd ( 1.1170.086 g / cm ) than every other sports athlete . this study concluded that greater differences among sports were seen when comparing lumbar , pelvis , and leg bmds , and recommended that a longitudinal study would need to be conducted in order to investigate bone changes in response to training over a longer period of time in female athletes . maimoun and colleagues   investigated the bmd of gymnasts ( impact activity ) , swimmers ( non - impact activity ) , and controls using dxa for total and regional measurement . at baseline and the 12-month follow - up , gymnasts showed a significantly higher bmd than swimmers and controls . moreover , the total bmd of swimmers ( 0.9600.013 g / cm ) was lower than that of the controls ( 0.9950.013 g / cm ) at baseline and the 12-month follow - up ( 0.9910.014 g / cm and 1.0090.013 g / cm , respectively ) . in accordance with previous findings , this study demonstrated that the osteogenic effect of impact activity was greater than non - impact activity . summarizing previous studies , it has been shown that the loading - induced bmd of impact sports athletes seems to be higher compared with swimmers and/or the non - athletic population . thus , it appears that different bone - loading history and type may highly affect skeletal adaptation in athletes . future studies should consider this point in order to elucidate a better and clearer mechanism of the effect of swim training on bmd . several previous studies on bmd and other markers of bone strength have shown an advantage for swimmers compared with inactive controls and/or other sports athletes . some researchers insist that swim training increases muscle contraction and strain on the skeleton , leading to increased mechanical loading , which would have a positive effect on bmd despite the fact that swimming generally represents a non - weight - bearing sport . matsumoto et al . ,   examined biomarkers of bone metabolism and bmd in 103 male and female athletes . the subjects were japanese collegiate athletes who specialized in long distance running , judo , and swimming at the national level , however , no controls were used in the study . whole body bmd was measured using dxa , and urine samples were collected for pyriodinoline ( pyr ) and deoxypyridinoline ( dpd , bone resorption markers ) using high pressure liquid chromatography ( hplc ) . blood was collected for bone alkaline phosphatase ( balp ) and carboxyterminal propeptide of type one collagen ( picp , bone formation markers ) measurement . researchers found that the total body bmd was significantly higher in judo athletes compared with runners and swimmers . male runners had a significantly lower balp level than male judoists , and found in balp among the female athletes . the dpd level of male and female runners and swimmers was significantly lower than that of male and female judoists . however , the pyd level of female swimmers was not significantly different from that of runners or judoists . the conclusion by the researchers in this study was that differences in total body bmd are in part due to the demands of the specific sport , and that they are reflected in the levels of bone metabolic markers . this was the first study of its kind to examine biomarkers of bone metabolism among various types of athletes , although its results are limited and there were no control subjects . biomarkers are best if assessed over time to determine change in bone metabolism with a given stimulus , and bmd is best when examining clinically relevant sites such as the hip and spine . similar to this study , summarizing previous studies shows that swimmers seem to have higher levels of bone turnover biomarkers [ 33 , 34 ] , although they do not have a higher bmd compared with other sports athletes and/or controls . greenway et al . ,   examined the negative effect of long - term swim training participation in 43 swimmers compared with 44 controls . swimmers had a swimming career of over 5 years , and dxa was used to determine the total and regional bmd . there were no differences in bone mass or bmd between the two groups , at any site . the total body z - score of swimmers was 1.24 and of the controls was 1.18 . this study concluded that long - term swim training participation did not compromise regional bmd , and swim training coupled with weight - bearing activities may induce positive effects on bmd . it appears that longitudinal swim study investigating other physical activity history is needed to verify the exact effect of swim training on bmd . in 2014 , stanforth and colleagues compared the bmd in female athletes of various sports ( basketball , soccer , swimming , volleyball ) aged 18 - 23 . they reported that female swimmers in this study had an increased bmd from baseline to year 3 post - season , although the increased amount was still smaller than that of impact sports athletes . they concluded that differences in bmd between impact and non - impact sports are large , however , this study provides evidence that swim training may increase bmd in female swimmers . akgl and colleagues   examined the effect of swim training on bmd in 79 swimmers . all swimmers had engaged in at least 2 years swim training , and dxa was used to measure the total and regional bmd . the bmd of 68 swimmers ( 86% ) was normal and a total of 9 swimmers ( 11.4% ) had low bmd . this study implies that swim training may positively affect bmd , although it did not induce increased bmd in female swimmers . summarizing previous studies , it has been shown that swimming may be preferable for maintaining bone health , although swimming has not been shown to result in an improvement in bmd like impact sports . in addition to swim training , extra strength training may also positively affect bmd . therefore , swim training combined with other types of training would be beneficial on the bone health of swimmers , in order to ensure a better health condition . it has been stressed that reliable reports concerning the dietary habits of athletes , especially elite swimmers , are needed . several studies have shown significantly higher calcium intake in swimmers than in controls [ 36 - 38 ] . other nutritional aspects such as magnesium , iron , and vitamin d may also affect the maintenance of bone health in swimmers . a healthy and well - balanced diet , including the proper amount of calcium , vitamin d , magnesium , and iron , may provide sufficient nutrients . in 2009 , hoogenboom et al . , determined the nutritional knowledge and eating behaviors of female collegiate swimmers . they proposed that swimming , due to the importance of a lean body weight , is associated with nutritional deficiency , and that this would lead to the development of the female athletic triad ; osteoporosis , menstrual dysfunction , and eating disorders . to determine nutritional intake , they used a 24-hour recall food survey , with 85 collegiate female swimmers participating in the study . they reported a mean daily calcium intake of 1578.88 mg in all participants , which was a higher level compared with the recommended dietary allowance ( rda ) of calcium ( 1200 mg / day ) . dugocka and colleagues   investigated bmd using dxa in young female swimmers ( n=41 , aged 11 - 14 years ) , considering nutritional aspects using a 3-day food intake recall survey . they reported that swimmers had a higher intake of calcium ( 602229 mg vs. 454236 mg ) and phosphorus ( 1390420 mg vs. 1027292 mg ) compared with non - athletic controls , although both groups had a deficiency in average calcium intake . however , they found that the mean value of bmd in both groups did not differ , indicating that in this study , a higher intake of calcium and phosphorus in swimmers may positively affect bone health when compared to controls who were not active in sports . czeczuk et al . ,   investigated the dietary habits ( calcium intake ) of 18 former swimmers and 18 current swimmers , reporting that calcium intake in both groups was sufficient and did not exceed 3/4 of the daily normal . these previous studies reported that calcium consumption in swimmers was higher than the rda and/or controls . in contrast , several previous studies have shown that female swimmers have calcium intakes below the rda and/or other sports athletes and controls . berning et al . ,   investigated the dietary food records of adolescent male and female swimmers , and reported that over 50% of elite adolescent female swimmers consumed lower calcium and iron intake compared with the rda and other sports athletes . similar to this study , hawley and williams   investigated 20 elite swimmers ( 11 females , 9 males ) using a 4-day food intake survey , and found that 55% of swimmers consumed an intake of calcium below the rda and 65% of swimmers had a lower intake of iron . moreover , the mean iron intake of female swimmers was significantly lower than the rda , and 82% of female swimmers consumed a low iron intake in this study . based on previous studies [ 42 - 44 ] , a low iron intake is common in female athletes . ,   examined the calcium intake for the previous 12 months through a questionnaire , and they also found that calcium intake was low in swimmers and controls . swimmers achieved 83% of the recommended daily intake ( rdi ) , however , when compared with the controls ( achieved 66% of the rdi ) , swimmers consumed a higher amount of calcium . czeczelewski and colleagues   conducted a 3-year monitoring of bmd and nutrient intakes in adolescent female swimmers ( n=20 ) and non - athletic controls ( n=20 ) . the calcium intake of swimmers was below the recommended values , and throughout the 3 years , the bmd of swimmers decreased , suggesting that this could have been due to insufficient calcium intake . akgl and colleagues   examined the dietary information of 79 swimmers using a 3-day food diary , and also reported low calcium intake in the swimmers . moreover , they found that swimmers with a normal bmd consumed a higher amount of calcium ( median 700 mg ) than the swimmers with a low bmd ( median 600 mg ) . furthermore , 31 ( 39.2% ) swimmers in this study had a vitamin d deficiency . vitamin d supplementation is also becoming more popular , especially in the athletic population , because it may reduce the incidence of stress fractures when combined with calcium , and may also have an ergogenic factor for athletes . summarizing previous studies , many authors have reported common dietary irregularities concerning female adolescent and adult swimmers in relation to swim training . therefore , to elucidate the exact mechanism and association between dietary supplementation and swim training , the close and precise monitoring of nutritional habits of swimmers should be conducted . upon the review of previous studies , it is obvious that the majority of studies did not collect physical activity data on swimmers activities outside of their swimming activities . these extra activities may have some influence on the bmd of swimmers , and therefore , future studies need to examine additional physical activity history data in addition to swim training . this additional information may help explain why swimmers ' bmd tends to be lower than the bmd of the controls in many studies . if a swimmer participated at a young age in impact - based sports , such as running or gymnastics , this may be reflected in their current bmd . a thorough knowledge of swimmers ' past physical activity can help to better understand the results seen with dxa and biomarker analysis . since bone adaptation to exercise is limited to loaded regions , exercise types should be carefully chosen . nutritional intake of calcium , magnesium , and vitamin d for swimmers also needs to be considered when conducting training and bmd interaction studies . the compilation of results in this review suggests that further exercise intervention studies are needed in the attempt to introduce various exercise programs to female swimmers , in order to determine the optimal exercise prescription for bone health . moreover , longitudinal studies and randomized control trials are necessary to better understand the association of swim training with bmd in female athletes .