Physics can sometimes cut through the mess of complex problems with a simple conservation law. A year ago, in my column “The Physics Diet,” I applied conservation of energy to the problem of obesity. I argued that exercise burns so few calories that it cannot be a major way of losing weight.
But many people I have spoken to believe there is another benefit to exercise: it changes your metabolism. When that happens, you burn more calories naturally, and so your food doesnt turn into fat.
Let me address this issue by invoking another physics principle: conservation of mass. More specifically, let me talk about the conservation of carbon atoms. When you digest food, its carbon atoms enter your blood. Unless they are expelled from your body, they add to your weight. But here is the salient observation: the only effective way your body has to get rid of digested carbon is to combine it with oxygen to form carbon dioxide, and then expel it through your lungs. Unless you breathe out the carbon, you gain weight.
Here are some numbers, taken from books on exercise physiology. Fat, protein, and sugar all contain about 0.1 gram of carbon per food calorie consumed. So if you digest 2,000 calories of food (a typical daily diet for adults) then you take in about 200 grams of carbon. At rest, each breath exhales about 0.5 liter of air containing about 1 percent carbon, for about five milligrams per breath. After a day at 12 breaths per minute, you get rid of about 120 grams of carbon. Thats less than you ate, so youll gain weight.
But few of us spend the whole day resting. Walking increases your respiration by a factor of two to three. Running at eight kilometers per hour (five miles per hour) increases it by a factor of eight to 10. Put together a nice combination, and youll lose all the carbon you consumed, and your weight will be stable. Walking, running, and being active does increase your metabolism–the rate at which you burn calories–and it increases your breathing rate too.
The mistake people make is to think that an hour of moderate exercise will change their body chemistry enough so that theyll “burn” away the calories even when they are inactive. But unless you breathe more rapidly, the carbon will stay in your body. If you want to lose weight, eat less or breathe more. And the only effective way to breathe more is through increased activity. There is no such thing as stimulating your body into a higher resting metabolic rate.
But what about all those people who seem to eat more than we do, but stay thin? Dont they contradict my conclusion? I wondered about that too, so I started asking these people three questions: How often do you snack? Do you always finish your meals? How often do you have dessert after a meal?
My conclusion from this survey: thin people actually eat less. I’ve verified this by watching them. If they order a burger, they get it without the shake, and they leave the fries on their plate.
There are other mistaken beliefs about weight gain. The most widespread one is that eating junk food puts on pounds faster than “healthy” food. Some people have tried to sue McDonald’s as being responsible for their weight gain. In the recent award-winning documentary Super Size Me, director and star Morgan Spurlock ate nothing but junk food at McDonald’s for 30 days, to see how bad the results would be. Indeed, he gained 25 pounds, and his doctors said his health deteriorated significantly.
But was he actually testing junk food? Not really; he was testing the effects of overeating. He decided that every time he was asked if he preferred the “supersize” meal, he would say yes. He apparently knew that it was McDonald’s policy to recommend this choice to every customer, and as a result he had supersized meals for breakfast, lunch, and dinner. He was probably eating about 6,000 food calories a day–triple what his body needed. It’s not easy to do that, and Spurlock had trouble keeping it down.
In the end, Spurlock’s weight gain and poor health probably had little to do with the junkiness of the food. The same thing would have happened if he had similarly stuffed himself at a gourmet restaurant–or at a salad bar, if he heaped enough shredded cheddar over his blue cheese dressing for three meals a day.
Junk food may or may not be good for you. That depends on your cholesterol level and other factors, such as vitamin and mineral content. But for weight gain, the only thing that matters is how much you eat (specifically, how much you digest) and how much you breathe out. That’s just conservation of mass.
Should we blame McDonalds for recommending the super-sized meal? I don’t think so. In reaction to lawsuits and health concerns, the company is already phasing out super size fries and drinks. In any case, the explanation for the program’s success probably lies elsewhere. I owned a restaurant for six years, and I became very sensitive to customer complaints. Patrons really appreciate large servings. In fact, they often notice food quantity more than the quality.
What makes large servings especially dangerous is that many people find it hard to leave food on their plates; my own mom considered it a sin. (I seem to recall that some poor child in India was going to starve if I left food behind, although I never understood the logic of that argument.) I think she was reflecting her experience growing up during the Great Depression, when food was expensive and salaries were low. These days, the sin isn’t leaving food behind–it is cleaning your plate when you have been served too much.
But even if you have learned to moderate your mealtime intake, there are other hazards throughout the day. Coffee breaks, over the last two decades, have transformed into coffee and brownie and huge-chocolate-chip-cookie breaks. In recent years, nibbling has become pervasive. I have noticed that the students who take my classes at Berkeley now eat not only at meals but also between classes, during lectures, and even during exams. This is a real change, and I believe I see it in the size of many of my students. I don’t know how widespread this eating epidemic is, but my daughter lives in France, and she reports that over there it is considered rude to eat a candy bar in someone else’s presence. In the United States, eating is the chief form of instant gratification. Bored? Tired? Between tasks? Get a snack. It is the American way.
Gluttony was once considered one of the seven deadly sins. But we rarely hear that word these days. I think we need to bring it back into our everyday vocabulary. We also have to recognize that the problem is not junk food and bad metabolic rates. If we can end our epidemic of gluttony, then it will put an end to our epidemic of obesity.
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