We all know someone like this. The person who orders dessert after every meal, goes for seconds without thinking twice, and hasn’t set foot in a gym in years. And they’re thin. They always have been.
For decades, the easy explanation was “genetics.” Fast metabolism. Biological luck. Something they were born with that the rest of us simply don’t have.
But in 2008, a team from the University of Helsinki analyzed over 5,000 pairs of identical twins — people with the exact same DNA — and found that genes only explained between 40 and 70% of differences in body weight. The rest depended on something else. Something that wasn’t in the DNA.
In 2013, researchers at the Weizmann Institute of Science in Israel found another piece of the puzzle. They showed that two people could eat the exact same meal and have completely opposite blood sugar responses. What was metabolically neutral for one person triggered an insulin spike in the other — activating fat storage.
The same food. The same portion size. Opposite results.
And in 2019, the PREDICT study — one of the largest ever conducted on individual metabolism, led by King’s College London — confirmed that even between identical twins, the way the body processes fats and sugars can vary by up to 50%.
So if it’s not genes, what is it?
Another study, this time from Washington University, transplanted gut bacteria from human twins — one lean, one obese — into mice raised in sterile conditions. The mice that received the lean twin’s microbiota stayed lean. The ones that got the obese twin’s bacteria gained weight. Eating the exact same food.
The lean twin’s body didn’t have better genetics. It had an internal ecosystem that processed food differently.
And that’s the key researchers keep finding again and again: it’s not what you eat. It’s what your body does with it afterward. Some people’s systems efficiently convert energy. Others accumulate it. Not from lack of willpower — from biology.
So that friend who eats whatever she wants and never gains a pound? She’s not lucky. Her body is doing something different with the exact same food — and it has nothing to do with genes, fast metabolism, or any of the explanations you’ve heard before.
A group of researchers recently figured out exactly what that difference is. They explain it here — it’s worth watching.