Dr. Maya Rosman: Can someone born with a "thin genetics" eat without limits?

"I have a friend who eats whatever she wants – chocolates, pasta, cakes – and she’s thin." This is what a listener told me on my radio show, raising a question that turned into an important and fascinating discussion: Are there really people who can eat anything without gaining a single gram?

Let’s clarify: Yes, some people have a faster metabolism, a genetic tendency to be thin, and even some who don’t like sweets. But there’s no such thing as eating 20,000 calories a day without gaining weight. It’s simply physiologically impossible. Everyone has a limit.

Sometimes it seems from the outside that someone "eats whatever they want," but in reality, it’s not very high-calorie eating.

For example, someone who skips dinner, doesn’t snack between meals, and doesn’t drink calories – can eat a plate of pasta in the evening and feel like they’ve “treated themselves” all day. This feeling is so common that researchers noticed it too – calling it the “illusion of the thin eaters.”

What about genetics?

The listener rightly asked if it could simply be genetic. The answer? Definitely yes – but not entirely. Studies show genetics influences the tendency to gain weight, but not to the extent that it cancels out the effects of environment, diet, and physical activity. Here are some key studies that help explain this:

The study examined identical twins raised separately in completely different conditions. The results were clear and striking: The BMI of the twins was significantly similar despite completely different life environments.

Researchers concluded that heredity plays a very significant role in determining weight and maintaining it over the years. However, they also emphasized that the environment – despite lower heritability – still matters and must be considered when discussing obesity and prevention.

Later research added to this: A large review published in the American Journal of Clinical Nutrition examined 45 twin studies and also found that heredity accounted for 75% of the variance in BMI at age 19, but individual environment still played a significant role.

What does this mean? We have a strong genetic basis for weight, as shown by the high similarity in BMI among identical twins even when environments differ. But environment still matters – diet, physical activity, and overall lifestyle influence outcomes. Genetics sets the baseline, but the environment determines whether that baseline is exceeded.

The result was clear: Although the children grew up in different dietary environments, their BMI was more similar to their biological parents. This suggests that the tendency to gain weight is likely inherited rather than acquired from the adoptive family’s lifestyle.

But – importantly – this doesn’t mean every child with a genetic tendency to gain weight will become overweight. The study also showed exceptions: Children who chose a healthier lifestyle maintained a normal weight.

In other words, heredity dictates the tendency, the “starting point” – but personal choices and environment ultimately determine whether this potential is realized.

Conclusion: Genetics is complex and involves hundreds of small genes that influence weight – but they do not cancel out the impact of diet and habits.

In summary: There are real genetic differences – some of us burn more calories at rest, some feel hungrier or less full – but ultimately, body weight is determined by calories in versus calories out.

There’s no magic, and no genetics that completely overrides physiology. So if you see someone who "eats all the time" and stays thin – maybe they are truly lucky, but maybe, just maybe, they simply don’t eat as much as it seems.