Why is it so easy to get fat and yet being fat predisposes one to heart disease, cancer, and adult onset diabetes? It seems like a maladaption to the presence of plenty of food.
First of all, it’s not nearly so bad if you exercise. For each condition, heart disease, cancer, and diabetes, being sedentary is as much or nearly as much of a risk factor as being overweight. It’s also the primary reason people put on weight. So weight may be as much a symptom as a cause of increased morbidity. People who appear thin because they diet may not get the health benefits [source]. It’s just that weight is an easy thing to measure, and it’s an easy thing to discriminate against, so it’s been the factor a lot of studies have examined.
Imagine our hunter gatherer ancestors. Any healthy animal is active, either finding food, socialising, or just being curious. You’re sedentary when you’re sick or when you’re starving. At the end of winter or when food can’t be found the tribe expends as little energy as possible while waiting for things to improve. When there’s an abundance of food it’s stored as fat, within reason. There’s a huge gene-survival advantage in being one of the few in your tribe to survive a bad famine or a bad winter. When food returns and the other homo-sapiens aren’t competing for it, the remaining individuals’ children and grandchildren are going to thrive.
To get that advantage, humans are adapted to
- put on as much fat as won’t interfere with their normal activity and
- quickly slow down their metabolism (including losing energy-expensive muscle mass) to burn energy slowly when they don’t need to be active
The 21st century human evolved primarily before agriculture and definitely before computers. So being inactive pushes the body into a high-risk strategy. Slow down the metabolism and conserve energy. It’s high risk because a slow metabolism easily gets sick. It’s high benefit if you’re the famine survivor. That’s not a big problem for the average hunter gatherer, who isn’t sedentary without good reason and doesn’t live long enough to suffer most of the downsides. Things are different now. We’re very well adapted for things which just don’t apply in the modern west.