 When trying to lose weight, which is more important, diet or exercise? A national survey found that a vast majority of Americans trying to control their weight believed that food and beverage consumption and physical activity were equally important. 7 out of 10 went with equally important. About 2 out of 10 thought exercise was more important, and only 1 out of 10 chose diet. The vast majority of Americans are wrong. It's easy to understand how people might think diet and exercise play equal roles, and after all, our weight is determined by the balance of calories in and calories out. What people may not understand about this energy balance equation is we have much more power over the calories in side. In fact, on a day-to-day basis, we have full control. We could choose to eat zero calories or 10,000 calories, but most of the calories out tend to be outside our control. Unlike wild animals who tend to burn most of their calories on activity, about 60% of our daily calories are used up just to keep us alive, what's called our resting or basal metabolic rate, thanks in part to our energy-intensive brains. Even if you stayed in bed all day, you'd still burn more than 1,000 calories just to fuel the basics, like thinking, breathing, keeping your heart pumping. In contrast, even most active people accrue less than two hours of exercise a week, which may average out to be less than 100 calories burned off each day. That's only about 5% of the total daily energy expenditure, the calories outside of the equation. So the 2,000 calories we may take in every day from our diet can exert 20 times more influence than exercise over our weight destiny. Most people believe that exercise is very effective. It's a way to lose weight, but this has been referred to as a myth in the scientific literature. In fact, it's been labeled one of the most common misconceptions in the field of obesity. Yet virtually all formal weight loss guidelines include some sort of physical activity recommendation. Can you outrun a bad diet? Let's see what the science says. Population studies have certainly found strong correlations between physical inactivity and obesity. But does a sedentary lifestyle lead to obesity, or does obesity lead to a sedentary lifestyle? It probably works a little bit in both directions. To prove cause and effect and also quantify the relationship, you really have to put it to the test. Dozens of randomized controlled trials involving thousands of participants have been published on the effects of exercise on weight loss. Physical activity was not found to be an effective strategy. For example, if you look at the studies that tried using exercise alone to induce weight loss, over an average of about 5 months, people only lost about 3 pounds. When you put all the studies together, it looks like it took around six weeks of exercise and they get people to lose a single pound. That was exercise alone, though. What about as an adjunct to diet? If you randomize people into a diet and exercise intervention versus just the diet alone, the added exercise group does do better, but the difference in weight loss only averaged about 2 pounds. The studies lasted between 3 and 12 months, and all that extra prescribed exercise only seemed to translate into a few pounds. The 2-pound difference was statistically significant, which means we're pretty sure it was a real effect, but losing 2 pounds over a year's time can hardly be considered clinically significant. As a general rule, researchers like to see at least a 5 or 6-pound drop. In a meta-analysis of 18 randomized controlled trials lasting a minimum of 6 months, the diet plus exercise group failed to significantly beat out the diet-only group at all. There appeared to be no long-term benefit to encouraging people to add exercise to their weight loss regimen. What is going on? Maybe exercise is just better at preventing people from regaining weight? No. The vast majority of randomized controlled trials examining weight loss maintenance also failed to show an exercise benefit. Part of the problem is compliance. It's one thing to tell people to adhere to an exercise regimen. It's another thing for them to actually do it. When the same randomized controlled trials were re-analyzed to exclude people who flouted the instructions, an analysis was limited to just those who actually put in the time and sweat a clear advantage to exercise emerges. Exercise only works if you actually do it. The one reason people may become rapidly disillusioned with their new gym membership is their gross over-estimation of the capacity of exercise to burn off extra calories. A slice of pizza has about 300 calories. That converts into an hour of brisk walking per slice. An hour? A slice? How many kids are jogging two hours a day to burn off their happy meals? Who's got time to climb 50 flights of stairs to take care of the calories in just one Oreo cookie? That's one of the reasons that what we put in our mouths is most important. Public health researchers have been experimenting with including labeling on junk food. Which label is more informative? This one? Or these? Still want that Toblerone? If it means you have to walk two hours? Or cookies you'd have to skip rope for 81 minutes for? I think I'd run an hour just to avoid having to eat shrimp-flavored potato chips. Labeling fast food menus with little pictograms of exercising stick figures was found to help nudge people towards lower calorie options. Seeing that the decision to supersize your fries would mean walking extra three miles that day, or that choosing the chicken salad over the garden salad could mean having to run nearly three miles. People are more likely to make the healthier choice.