The answer could be down to one type of bacteria in the gut, or the lack of it. The bacterium is called Akkermansia muciniphila, and it seems to regulate weight gain and even type 2 diabetes, which often occurs after people start becoming seriously overweight, or obese.
So, people who easily put on weight could be low in levels of the bacterium, while slimmer folk have ample stores of it in their gut.
You can increase your levels of the bacterium by taking food supplements that contain it, and that's just what researchers did when they gave Akkermansia capsules to laboratory mice.
Researchers from the University of Louvain first stumbled on the beneficial effects of the bacterium back in 2007, but that was just on mice, and now they've taken their research a stage further by testing a pasteurized form of Akkermansia on people.
The pasteurized variety seems to be more protective against diabetes-like symptoms such as insulin resistance than the live form of the bacterium, they reckon.
They've tested the pasteurized form on a group of overweight and obese people who were showing early signs of heart disease. The volunteers were put into three groups, and were either given a placebo, or dummy supplement, a 'live bacteria' supplement, or the pasteurized version.
After taking the supplements every day for three months, the ones that took the pasteurized form had lower inflammation markers in the liver, a slight drop in weight—the average loss was around 2.3 kg, or 5 lbs—and cholesterol levels were also down.
By comparison, those given the dummy supplements showed a continued deterioration of their pre-diabetic symptoms.