It’s that time of the year when a middle-aged person’s fancy turns to treadmills and diets. Scientific literature on excess weight and health is expanding along with global waistlines, and yet, it’s hard to find a solid, coherent scientific explanation for why some people get fat and others don’t, and why some overweight people get Type 2 diabetes and heart disease, and others don’t.

In the United States, beliefs about fat follow a “sciencey” sounding quasi-religious narrative: our prehistoric ancestors had to scramble for food, and therefore evolved voracious appetites that we’ve inherited like original sin. Only self-control can save us, and the association between fat and disease goes without question; it is seen as punishment for the sins of gluttony and sloth.

This narrative acknowledges evolution, but it’s not real evolutionary biology. Last week, however, a real evolutionary biologist published a sweeping picture of human fat and health in the Proceedings of the National Academy of Sciences.

While traditional medical research tends to make very narrow hypotheses and test them with specific data, evolutionary biology often works as an observational science, seeking patterns that tie together and explain lots of diverse observations and measurements. Think Charles Darwin, or the big bang theory in cosmology.

The biologist, Mary Jane West-Eberhard, of the Smithsonian Tropical Research Institute in Costa Rica, has focused her work on understanding biological variation. Sometimes individuals with the same genes can show dramatic differences; a queen bee and her workers share the same genes but very different fates. A butterfly born at one time of year may live many times longer than those born in other seasons. Some fish can even change sex in response to changes in the environment.

She proposes that the same biological principle can explain why humans come in quite different shapes. Some people put on so-called visceral fat, surrounding vital organs, while others put on so-called subcutaneous fat on the limbs, hips and elsewhere. This makes a big difference in health because recent studies show it is the visceral fat that is associated with Type 2 diabetes and heart disease.

Because West-Eberhard is interested in the functions of things, she looked into visceral fat — also known as the omentum, a part of the immune system. It wraps around the vital organs and protects them from infection. But what’s protective early in life can have a downside later.

Our natural immune response often involves inflammation, and that has been associated with Type 2 diabetes and coronary heart disease. The omentum, she said, is the Rosetta stone of pathogenic obesity.

Why then do some people get an expanding omentum and others get “curves” or whatever the latest fashion calls attractively placed subcutaneous fat? She cites other biologists pointing to sexual selection as the driving force in the human tendency to put fat deposits in places where they serve as ornaments.

Her analysis of the data suggests that where your fat goes depends on how well fed you were as a foetus. It’s those who are most undernourished in utero — approximated by low birth weight — who are most likely to accumulate visceral fat in the abdomen.

Underweight, badly nourished babies are more vulnerable to infection and benefit from the short-term strategy of laying down protective visceral fat. The pattern is set by epigenetics — chemical changes surrounding the DNA that determine which genes become activated in which tissues.

“The tragic part is that if you look at people who move to urban areas to seek their fortune, they are on a tight budget and will buy these cheap foods that are bad for them,” she said. People whose mothers ate traditional, rural, sometimes sparse diets — people conditioned in utero to develop visceral fat — run into trouble when they move to urban areas and find that the only foods they can afford are full of fructose and cheap oils.

However the species came to be this way, it’s becoming clear that humans who tend to put on visceral fat may have to exercise more and maintain a lower weight than other people to remain healthy. It’s yet more evidence that life isn’t fair.

Distinguishing the kinds of human fat and the way they influence health may explain why scientists cannot agree over the value of body mass index, which is strictly a measure of your mass relative to your height.

One widely publicised analysis out of the Centres for Disease Control and Prevention showed an “overweight” BMI is not associated with increased mortality; people had to be considerably obese before it started to influence health. Other analyses have contradicted this, leading to more headlines about the dangers of being overweight, but still others have called their methods into question.

Perhaps researchers and doctors fixate on numbers on the scale because they are such an easy measurement. To predict health is much more difficult.

Analyses such as West-Eberhard’s paper may change the way we see our fellow humans. What makes a person with gorgeous, enviable curves different from someone with an unhealthy-looking gut? It’s not necessarily that one is more “out of shape” or less self-controlled.

It’s all part of the human story: evolution, epigenetics and the eternal human quest for a better life.

• Faye Flam is a Bloomberg Opinion columnist. She has written for The Economist, The New York Times, The Washington Post, Psychology Today, Science and other publications. She has a degree in geophysics from the California Institute of Technology