 Protein is the name given to a group of large, complex molecules that form the basis of tissues in living organisms. These groups, which consist of one or more long chains of amino acid residues, perform a huge number of essential roles in our bodies, including aiding the growth and repair of cells, maintaining immune function and much more. When we are deficient in protein, we begin to suffer from fatigue, slow healing, muscle atrophy and other unpleasant symptoms. Therefore, it's important that we obtain enough protein from our diets to remain in optimal health. But which whole foods contain the highest amounts of this essential macronutrient? Though opinions on this subject differ, the following foods tend to provide the highest protein-calorie ratio. The best sources of protein Chicken -- There's a good reason why chicken is a popular food with bodybuilders. One average-sized chicken breast supplies our bodies with a whopping 17 grams of protein, making it one of the most protein-dense foods in existence. The legs and thighs of the chicken are also packed with protein, as well as large amounts of selenium, niacin, vitamin B6 and phosphorous. Of course, the meat of chickens raised on organic, sustainable farms is the healthiest. If you are going to stick with non-organic chicken, consider removing the skin from the meat; this is where the toxins accumulate. Fish -- Most fish is an excellent source of protein, but salmon, tuna, snapper and halibut seem to be particularly rich sources of it. One fillet of each contains around 22 grams of protein , while perch, flounder, sole, cod and tilapia contain between 17 to 21 grams of it. Fish is also rich in omega-3 fatty acids, which keep our heart and brain healthy, as well as vitamin D, riboflavin, calcium and magnesium. Sadly, not all fish is safe to eat these days due to the ongoing contamination of our oceans. To minimize the risk of toxicity, favor sustainably caught fish sourced from the Atlantic Ocean when possible. Pork -- One pork chop provides 33 grams of protein, which is more than half of the recommended daily intake of protein for adult women (46 grams) and adult men (56 grams). Other pork products, such as pork roast, ham and bacon, are also good sources of protein. Pigs are widely considered to be dirty animals, and for good reasons. Aside from digesting their food in a mere 4 hours (compared to 24 hours in ruminant animals), pigs lack the sweat glands necessary for detoxification, making them walking vessels of harmful organisms. For this reason, pork should always be purchased from reputable organic farms. Beef -- Though red meat tends to contain a lower protein-calorie ratio than white meat, it's still famously rich in protein. One T-bone steak, for instance, supplies us with around 19 grams of protein, as well as large amounts of bio-available iron, zinc and vitamin B12. Even processed beef products, such as beef jerky, contain respectable amounts of protein. Eggs -- One large egg contains 6 grams of protein, making eggs one of the finest protein sources for vegetarians. Though most of this protein is contained in the white, don't forget to eat the yolk too; this is where the other essential nutrients, including the vision-boosting carotenoids, lutein and zeaxanthin are held. Greek yogurt -- According to Bodybuilding.com , an 8-ounce serving of Greek yogurt supplies us with 23 grams of protein, making it the most concentrated dairy source of protein. Greek yogurt, which has been strained to remove its whey content, is also a fantastic source of probiotics. Favor organic, unsweetened brands when possible. Other great whole food sources of protein include unprocessed cheeses, legumes, nuts, seeds, certain gluten-free grains (such as buckwheat and quinoa), spirulina and chlorella. Sources: http://www.healthaliciousness.com http://www.bodybuilding.com http://nutritiondata.self.com http://www.doh.wa.gov About the author: Michael Ravensthorpe is an independent writer whose research interests include nutrition, alternative medicine, and bushcraft. He is the creator of the website, Spiritfoods , through which he promotes the world's healthiest foods. 