San Francisco -- Paul Williams has only run one marathon in his life, but by his own research, he could probably benefit from running a few more. A scientist at Lawrence Berkeley National Laboratory, Williams has put together the world's largest study on runners, and the evidence found over 20 years of research points to an important conclusion: When it comes to exercise, more is almost always better. 'When I started my study, everybody sort of knew exercise was beneficial. The government was saying you get benefits by walking three or four times a week. My data has shown the more you do, the greater the benefits,' Williams said. 'I've had people doing 100 miles a week of running, and you could see benefits up to that level.' To be sure, Williams is not suggesting that everyone try to run 100 miles a week, or even half of that. But for years, he's been a critic of national guidelines that recommend people get at least 150 minutes of exercise a week, or about 30 minutes a day, five days a week. That's a fine goal for the couch potatoes, Williams says, but it's shortchanging the millions of Americans who already get the minimum amount of exercise and might not realize that doing more -- maybe even doubling their workouts -- would improve their health. Williams' catalog of more than 100,000 runners has produced dozens of scientific and medical papers looking at the effect of running on everything from heart disease and stroke to vision problems and arthritis. The more miles people run, the less likely they are to develop heart disease or have strokes, Williams has found. The health improvements continue up to about 50 miles a week of running, roughly eight hours. Williams, for the record, runs about 35 miles a week. It's likely that health benefits keep growing above that level too -- with the 100-mile-a-week runners, for example -- but there aren't enough people in Williams' study running that much to provide hard data. 'Up until recently, the exercise research has pretty much focused on the couch potatoes,' Williams said. 'We've become fixated on how to get fat people to lose weight. But we shouldn't be pitching the weight loss and exercise thing only to the obese, sedentary people.' The health improvements don't just apply to runners -- any sort of regular aerobic activity helps, and the more hours people put in, the more benefits they'll see, Williams said. But Williams' findings haven't exactly caught on with the mainstream public health gurus. It's not that they disagree with Williams' findings. But doctors and public health officials worry that with half the country not meeting the current guidelines, even talking about running 50 miles a week will intimidate folks who aren't doing anything. 'The overwhelming majority of patients that I see really need the motivation to start, and seeing a 150-minute goal gives them something to work toward,' said Dr. MaryAlice Ambrose , chief of patient education at Kaiser Permanente in Santa Clara . 'Most often, once patients start exercising on a regular basis, they feel so much better that they try to exceed that level on their own.' Williams started his research project in 1991 , when he set out to study how much physical activity people needed to improve their health. He decided to focus on runners because they're an easy group to follow -- they usually know exactly how much exercise they get, in terms of miles run, and they can gauge their fitness based on race times. He started by buying the Runner's World subscription list, which yielded about 55,000 runners to study. Since then, he's doubled that group, often by recruiting people at races. He's also started studying walkers, about 45,000 of them, to compare their health to runners. The major benefit of his research cohort is its size -- it's unusual to have so many subjects to study. A downside is that most of the information he's gathered is self-reported -- it's up to the individual runners to be honest about how much they run, and their health data. Studying the same people for nearly 20 years has allowed Williams to look at the effects of exercise over time. Exercise, he said, seems to help prevent heart disease and stroke, and vision problems like glaucoma and cataracts. He hasn't found a relationship between running and cancer prevention, he said -- but that may just be a matter of time, since most of his runners were relatively young when the study began. The running community has changed since 1991 and since Williams ran his own marathon, in 1988 . More people have picked up the sport, and endurance events have become more popular. That's just fine by Williams, of course. Doctors may be reluctant to start encouraging all of their patients to go out and run marathons -- especially if they're starting an exercise regimen from scratch. But at the same time, doctors say they're increasingly viewing exercise as a critical health tool, equivalent to tracking a patient's weight or blood pressure, or prescribing a drug. 'I tell my patients, however much exercise you're getting, it would probably be even better to do a little more,' said Dr. Todd Weitzenberg , a sports medicine physician with Kaiser Permanente in Santa Rosa . 'I hope that patients, when they've done the minimum requirements, when they see that they've lost five pounds, that their blood pressure came down a couple of points, maybe they'll up the ante.' 