In which this was originally posted elsewhere, but I’m moving a copy over here because I wrote it and I can.

Overweight is a metabolic issue and not a behavioral one.

It’s currently the fashion to believe that overweight people suffer from sloth and gluttony; however, adipose tissue is not the result of character defects but rather of metabolic function. Most of what you know about diet and exercise and health, it turns out, is completely wrong.

For instance, the obese don’t consume more calories than the lean:

“Most studies comparing normal and overweight people suggest that those who are overweight eat fewer calories than those of normal weight.”
– [National Academy of Sciences, Diet and Health]

That is to say, people aren’t fat because they eat too much. They’re fat for another reason. Namely, that not all calories are created equal. The body metabolizes them differently. The current belief is that a calorie of protein provides the same amount of energy as a calorie of fat or carbohydrate. But:

Lost in this distillation is the fact that the effects of these different nutrients on metabolism and hormone secretion are so radically different, as is the manner in which the body employs the nutrients, that the energetic equivalence of the calories themselves is largely irrelevant to why we gain weight. As Rubner suggested more than a century ago, “the effect of specific nutritional substances upon the glands” may be the more relevant factor.
– [Taubes, Gary (2007). Good Calories, Bad Calories (p. 276). Anchor.]

Insulin regulates body fat. Modern diets are full of refined carbohydrates that cause excessively high blood glucose, and this in turn causes the pancreas to secrete insulin to force the tissues store it as fat.

The current wisdom is that to lose weight, one must eat less and exercise more (as if the human body were a car, and all food was petrol: GiGo), and yet anyone who has ever tried this approach knows it’s uncomfortable at best. Changing the types of calories we consume rather than the quantity is actually more appropriate:

• Limiting carbs, not calories, reduces liver fat faster
• A prolonged hypocaloric diet low in carbohydrates and high in fat has the same beneficial effects on intrahepatic lipid accumulation as the traditional low-fat hypocaloric diet.
• The low-carbohydrate diet produced a greater weight loss than did the conventional diet

We’re beginning to see meta studies as well:

An even more recent meta-study of randomized controlled studies that compared low-carbohydrate diets to low-fat/low-calorie diets found that measurements of weight, HDL cholesterol, triglyceride levels and systolic blood pressure were significantly better in groups that followed low-carbohydrate diets. The authors also found a higher rate of attrition in groups with low-fat diets. They conclude that “Evidence from this systematic review demonstrates that low-carbohydrate/high-protein diets are more effective at 6 months and are as effective, if not more, as low-fat diets in reducing weight and cardiovascular disease risk up to 1 year.”

What this means is that if you want to lose fat, you don’t have to endure a hunger-producing physical regime while suffering a hypo-caloric diet. You can, instead, change the types of calories you eat and lose weight. No need to live on twelve hundred calories a day for half a year.

It also explains why we see so many obese people with very active lifestyles. If physical activity and lower daily caloric intake actually worked, poor people in physical jobs would be thin like they were during the Great Depression, not clinically obese like they are now on diets made up almost entirely of refined carbohydrates.