 Changes in your diet not only affect you physically, physiologically inside, but also mentally, how well you think, psychologically, how well you feel. But you'll never know just how good you can feel until you put it to the test and try eating healthier. Welcome to the Nutrition Facts Podcast. I'm your host, Dr. Michael Greger. Today we look at the relationship between exercise and weight loss, and we start out with the question, can exercising in cool water undermine weight loss by leading to an increase in post-workout calorie intake? Swimming and aquatic exercise in general are popular alternatives to land-based activities such as walking or biking. The buoyancy helps take some of the weight-bearing stress off the joints, but swimming appears to be less effective for weight loss. Obese women were randomized into an hour a day of walking, cycling, or swimming. Six months later, the walkers lost an average of 17 pounds. The cyclists lost an average of 19 pounds, and the swimmers didn't lose an ounce. In fact, they actually gained 5 pounds. Gaging skin-fulls to estimate body fat. The measurements slimmed more than 40% of the walking and cycling groups, but there was no change at all in the swimming group. What's going on? And check this out. The more the women walked, the more they lost weight. The more the women biked, the more they lost weight. But the more laps they did, didn't seem to matter. Even an hour a day, no weight loss. What is going on? Well, it turns out that some exercise boosts appetite more than others. See, while land-based exercise does not stimulate a compensatory increase in appetite and calorie intake, the same cannot be said for water-based exercise. In contrast to walking, in contrast to running, and in contrast to cycling, swimming can significantly heighten hunger within hours. This may explain why swimmers didn't have more body fat than runners of equal caliber, even though they may be actually expending more calories during training. If anything, you'd think swimming might lead to even greater weight loss since you're losing heat to the water, but swimming didn't seem to work at all. The cold, it turns out, may actually be the culprit. If you exercise in warm water about 90 degrees Fahrenheit, it does not boost your appetite more than exercising on land. After the same workout in cool water about 70 degrees Fahrenheit, people can end up eating more than twice as much at a meal an hour later. Maybe they're just burning off extra calories to stay warm? No. Even at the same number of calories expended, people eat hundreds of calories more after exercising in cold water. Offered a buffet after burning off about 500 calories in cool water, people eat nearly 900 calories, hundreds more than after exercising warm water or just staying dry. So they ended up taking in about twice as many calories as they exercised off. No wonder swimming doesn't appear useful for weight loss. Would the same thing happen under different temperatures on land? A team of British researchers sought to find out, randomizing people to briskly walk for 45 minutes on a treadmill in the cold, about 46 degrees Fahrenheit, or closer to room temperature, about 68 degrees Fahrenheit. Participants were then presented with a buffet meal in which their eating was covertly recorded, and calorie intake was significantly greater after exercising in the cold. The walking is often prescribed for overweight individuals, the researchers conclude. If walking was to take place in a cold environment such as in the winter, then this may stimulate food intake. In the warmer months, though, obesity researchers suggest exercising outdoors may be preferable to an air-conditioned gym. All studies to date on the effects of hot and cold environments have found that exercising in cool water or under cool conditions on land led to an increase in post-workout calorie intake. What about a quick dip in the pool after you exercise? Australian researchers found that immersion in water for 15 minutes, cool or warm after a running session, resulted in increased calorie intake. What is it about getting wet that wets your appetite? And maybe they got it chilled after getting out before they could change into dry clothes? This suggests that though a cool shower after workout may be invigorating, it might be better to stick to hot. Next up, we look at what role inactivity has played in the obesity epidemic. Right now, almost two-thirds of Americans are overweight, and by 2030, more than half our population may be clinically obese. Childhood obesity is tripled, and most of them will grow up to be overweight as well. The United States may be in the midst of raising the first generation since our nation's founding that will have a shorter predicted lifespan than that of the previous generation. The food industry blames inactivity. We just need to move more. But what is the role of exercise in the treatment of obesity? There is considerable debate in the medical literature today about whether physical activity has any role whatsoever in the epidemic of obesity that's swept the globe since the 80s. The increase in calories per person is more than sufficient to explain the U.S. epidemic of obesity. In fact, if anything, the level of physical activity over the last few decades has actually gone up in both Europe and North America. This has important policy implications. Yes, we still need to exercise more, but the priorities for reversing the obesity epidemic should focus on the overconsumption of calories. To work off the increased caloric intake, which for kids is like an extra can of soda and small fries, compared to what they're reading back in the 70s, and for adults like an extra Big Mac a day, to work that off, we'd have to walk like two hours a day, seven days a week. So exercise can prevent weight gain, but the amount required to prevent weight gain may be closer to twice the current recommendations. Public health advocates have been experimenting with including this kind of information. The fast food menu labeled with calories and the number of miles to walk to burn those calories appeared to be most effective in influencing the selection of lower calorie meals. Now, exercise alone may have a small effect, and that small effect can make a big difference on a population scale. A 1% decrease in BMI nationwide might prevent millions of cases of diabetes and heart disease, thousands of cases of cancer. But why don't we lose more weight from exercise? Maybe because we're just not doing it enough. I mean, the small magnitude of weight loss observed from the majority of exercise interventions, you know, where they make people exercise, may be primarily due to how low the doses are of the prescribed exercise. People tend to overestimate how many calories are burned by physical activity. For example, there's this myth that a bout of sexual activity burns a few hundred calories. So hey, you could get a side of fries with that. But if you actually hook people up and measure energy expenditure during the act, and your study subjects don't get too tangled up with all the wires and hoses, though it may be nearly the metabolic equivalent of calisthenics, given that the average bout of sexual activity only lasts about six minutes, a young man might expend approximately 21 calories during sexual intercourse. Of course, he would have spent roughly one third of that just lying around watching TV, just basal metabolism. So the incremental benefit is plausibly on the order of 14 calories. So maybe we could have like one fry with that. Finally today, what's more important, diet or exercise? When it comes to weight loss, you may be surprised at the answer. When trying to lose weight, which is most important, diet or exercise? This is what a survey found recently. The vast majority of those trying to lose or maintain weight believe that both monitoring food and beverage in Shumpton and physical activity are equally important in weight management and weight loss. Most people go with equally important, then exercise and then diet, and most people are wrong. Identified as one of the most common misconceptions about obesity in this recent review, the confusion about the leverage of exercise on body weight. Unfortunately, the energy balance equation, you know, calories in have to equal calories out, suggests that energy intake and energy expenditure occupy equivalent roles in determining energy balance, when in fact, the factors governing energy intakes influence the energy balance far more powerfully than the factors determining, you know, resting energy expenditures. What we put in our mouths is most important. For example, to walk off the calories found in a single pad of butter, you'd have to add an extra 700 yards to your stroll that evening. A quarter mile jog for each sardine we put in our mouths, and that's just the edible part. And those who choose to eat two chicken legs better get out on their own two legs and run an extra three miles that day to outrun weight gain. And that's for a steamed chicken skin removed. We would love it if you could share with us your stories about reinventing your health through evidence-based nutrition. Go to nutritionfacts.org slash testimonials, we may be able to share it on social media to help inspire others. If you'd like to see any of the graphs, charts, graphics, images, or studies mentioned here, go to the Nutrition Facts Podcast landing page, there you'll find all the detailed information you need plus links to all the sources we cite for each of these topics. My last two books were A Heart of Survival Pandemic and My How Not to Diet Cookbook. Get ready this year for the launch of How Not to Age, and of course all the proceeds for the sales of all my books goes directly to charity. NutritionFacts.org is a non-profit science-based public service where you can sign up for free daily updates on the latest new nutrition research with bite-sized videos and articles uploaded nearly every day. Everything on the website is free. There are no ads, no corporate sponsorships, no kickbacks, it's strictly non-commercial, not selling anything. I just put it up as a public service as a labor of love as a tribute to my grandmother, whose own life was saved with evidence-based nutrition.