In my reading, I've come to the conclusion that, in some people, overweight may be a type of carbohydrate intolerance.
Insulin is the master hormone that orchestrates the metabolic changes that cause fat accumulation. It instructs the body to transport glucose and fat from the blood into the cells. It tells the liver to synthesize fat from sugar. It inhibits the release of fat from fat cells. There are no other hormones that have a similar range of effects.
Insulin is the "storage hormone". Don't believe me? Ask a type I diabetic. Type I diabetes happens when the pancreas no longer secretes insulin. These people are rail-thin until they are given insulin injections, at which point they often gain excess weight. Many deliberately skip injections to lose weight. Unfortunately, this has serious consequences as it allows their blood glucose to rise to dangerous levels unchecked.
If insulin is kept low, fat synthesis and storage are inhibited, and fat release from fat cells is increased. Carbohydrate is particularly effective at elevating insulin, acutely and chronically. As carbohydrate digests, it's broken down into glucose, which enters the bloodstream. The pancreas releases insulin in an attempt to keep blood glucose within a healthy range, and the storage begins. Refined carbohydrate is the worst offender, because it causes a large and rapid rise in blood glucose.
Regular overconsumption of carbohydrate causes insulin to be chronically elevated in many people [update- I no longer believe this is true. I now believe that only certain types of carbohydrate- namely wheat and sugar- cause a pathological increase in fasting insulin over time]. This comes along with "insulin resistance", whereby most or all tissues become desensitized to insulin. This is the tissues' way of saying "Stop! My energy stores are already full! I can't handle any more glucose or fat!".
Some people are able to maintain normal insulin levels (and sensitivity) in the face of a high-carbohydrate diet. This is probably partly genetic and partly environmental. Certain people, for whatever reason, have fat tissue that is prone to fat accumulation. It could be because they oversecrete insulin, or because their fat tissue is sensitive to the action of insulin, but probably both. It likely has to do with a combination of insulin resistance in non-fat tissue, and insulin sensitivity in fat tissue. Inactivity and fructose consumption (from corn syrup or sucrose) are also high on the list of suspects.
Fat tissue is typically the last to become insulin resistant because it acts as a valuable buffer to remove excess (and potentially toxic) glucose from the bloodstream. Unfortunately, simply being thin is not a reliable indicator that your body tolerates carbohydrate well. It can indicate either that all tissues are insulin-sensitive and insulin levels are low, or all tissues (including fat) are insulin resistant and insulin levels are high. The latter scenario leads to type II diabetes, pronto.
Since fat accumulation revolves around carbohydrate intake and insulin production, it makes sense that reducing carbohydrate causes weight loss. No more carbohydrate = a lot less glucose, and a lot less insulin to deal with it. This completely sidesteps the problem of insulin resistance, although that seems to respond favorably to carbohydrate restriction as well. Every time true low-carbohydrate diets are matched head-to-head with reduced-calorie, carbohydrate-rich diets, subjects lose more weight and have fewer problems with hunger on the low-carbohydrate diet. I discussed a recent study here.
The idea that you can achieve and maintain a healthy weight without cutting calories sounds too good to be true. In fact, all it represents is a return to our natural pattern of eating as human beings. It may involve breaking an addiction to carbohydrate. True hunter-gatherers eat between 0 and 35% of their calories as carbohydrate, and no refined carbohydrate [correction: a number of hunter-gatherer groups ate more than 35% carbohydrate, typically from starchy tubers]. In industrial nations, we eat approximately 50% of our calories as carbohydrate. Hunter-gatherers also exercise regularly, and don't eat Frosted Sugar Bombs for breakfast. This helps maintain good insulin sensitivity. Since we are genetically very similar to our hunter-gatherer ancestors, we would be wise to learn from their example.