Two groups ate identical calorie surpluses for seven weeks and gained the same 1.6 kilograms. But when researchers put them through an MRI, the image told a different story — one group had built roughly three times as much lean tissue as the other. The only difference in their diet was the cooking oil.
Whether eating fat makes you fat is a calorie question — across 37 trials and more than 57,000 people, the evidence settled that one. But the TYPE of fat changes what your body builds with those calories.
A team in Uppsala, led by Fredrik Rosqvist, fed two groups of lean adults identical surpluses for seven weeks. The extra calories came baked into muffins — identical except for the oil inside. Sunflower oil for one group. Palm oil for the other.
Both groups gained 1.6 kilograms. Your scale would have shown identical progress for both.
The MRI saw what the scale couldn’t.
One group built roughly equal parts lean tissue and body fat. The other stored roughly four times as much body fat for every unit of lean tissue.
Same surplus. Same number on the scale. Completely different bodies underneath.
The Oil Nobody Expected
The oil that built more lean tissue was sunflower oil — an unsaturated fat, one of the seed oils that 126 million TikTok viewers have been told is dangerous.
The oil that stored more fat per unit of lean tissue was palm oil — a saturated fat, the kind the anti-seed-oil movement calls safe and ancestral.
The body-comp evidence runs in the opposite direction from the most-watched fat story on the internet.
But before you change your grocery list, there’s a catch.
Who This Works For
The lean-to-fat ratio is the kind of finding that makes you want to tell everyone. But the evidence has a built-in boundary.
Rosqvist’s team ran the same experiment on overweight adults a decade later. Same surplus, same oils, same MRI protocol.
The lean tissue advantage vanished.
In overweight adults, the difference was 0.07 liters of lean tissue — practically nothing, where the lean group had shown a difference almost eight times larger. Where you start seems to decide whether fat type changes what you build or makes no difference at all.
If you’re carrying extra weight, the headline finding doesn’t apply to you.
But one finding survived the population boundary.
Even when the lean tissue advantage disappeared, saturated fat still drove a 53% increase in liver fat while unsaturated fat caused a 2% decrease. The lean tissue advantage is population-specific. The liver fat effect is not.
What the Evidence Points To
The evidence lands differently depending on where you start.
If you’re lean and in a surplus, these MRI studies point to unsaturated cooking fats building a better lean-to-fat ratio — not a guarantee from one lab, but the best body-composition signal this evidence has produced. If you’re carrying extra weight, that lean tissue edge doesn’t apply — but favoring unsaturated fats may still benefit your liver. The liver fat effect crossed every population line these studies tested.
One thing this evidence doesn’t address: all three studies tested a calorie surplus. Whether fat type matters during a deficit is a question these studies were not built to answer.
One Lab, No Backup
Everything above comes from Fredrik Rosqvist’s lab at Uppsala University. Three studies. Two cohorts totaling 98 people. No outside lab has replicated the lean tissue finding.
The experiments were well-designed — every variable controlled, every measurement done with MRI. But rigor and outside backup are different things. The researchers themselves wrote that this finding needs replication before anyone acts on it.
Confident direction from strong studies — not a settled answer.
If you’re now wondering whether swapping your cooking oil affects your testosterone — two meta-analyses pulled in opposite directions on that. The distance between the fear and the evidence is its own story.
The cooking oil in your pan during a surplus may determine whether your body partitions extra calories toward lean tissue or fat storage. In the MRI feeding studies, the practical difference came down to which oil was baked into the muffins — sunflower oil (polyunsaturated) versus palm oil (saturated). Same total fat, same total calories, same weight on the scale. The body composition underneath was completely different.