Same surplus. Same scale weight. Same seven weeks. But underneath, one cooking oil built a completely different body.
Both groups gained 1.6 kg. The scale saw nothing different. Then the MRI told a story the scale could not.
You have tracked everything. The macros. The surplus. The protein per kilogram. You have logged meals, weighed rice, and split your fats and carbs to the gram. The oil in the pan? An afterthought. Olive oil, butter, whatever was on sale last Sunday. Because a calorie is a calorie. As long as the total adds up, the bottle doesn't matter.
Researchers at Uppsala University tested that exact assumption. They used MRI scanners, muffins, and 37 young, healthy adults who were doing exactly what you're doing: eating at a surplus to gain weight.
The oil in the pan was the variable nobody was tracking — and MRI scans showed it changed everything about what the surplus actually built.
- The sunflower oil group built lean tissue at a roughly 1-to-1 ratio with fat — the palm oil group stored at 1-to-4.
- The palm oil group accumulated twice as much visceral fat around their organs, invisible to mirrors and calipers.
- Both groups had identical blood glucose and insulin levels — standard blood work could not detect the body composition difference.
- When the same lab tested overweight adults, the lean tissue advantage disappeared entirely — the effect is population-dependent.
- The study tested palm oil and sunflower oil specifically — butter, coconut oil, and olive oil were not tested.
Three Muffins a Day for Seven Weeks
The setup was clean. A double-blind, randomized controlled trial: neither the participants nor the researchers handling the muffins knew who got what.
For seven weeks, 37 lean, healthy adults between 20 and 38 years old added roughly 750 extra calories per day to their normal diet. The calories came from muffins, about three per day, baked to the same recipe with one difference.
Half the muffins were baked with palm oil, high in saturated fat. The other half with sunflower oil, high in polyunsaturated fat.
The participants were lean, non-athletes, eating at a monitored surplus. They were, by any practical definition, doing a bulk. A verified, measured, controlled caloric surplus for seven straight weeks.
At the end, both groups stepped on the scale.
The Scale Saw Nothing
Both groups gained exactly 1.6 kg. No difference between them. From the scale's perspective, the case was closed. Both oils produced the same weight gain from the same surplus. A calorie is a calorie. Debate over.
Then the researchers put everyone inside an MRI scanner.
Your blood work is as blind as your scale. Both groups had identical glucose, insulin, and insulin resistance levels throughout the entire trial. A standard blood panel would show no difference between the person building lean tissue and the person storing fat — the body composition change is invisible to every routine test your doctor runs.
Same Weight. Different Bodies.
The MRI told a story the scale could not.
The sunflower oil group gained lean tissue and body fat at roughly a 1-to-1 ratio. For every unit of fat they added, they gained about the same amount of lean tissue. By almost any standard, that is a clean bulk.
The palm oil group gained at a 1-to-4 ratio. For every unit of lean tissue, four units of fat. Same calories. Same scale reading. But one group was building muscle while the other was mostly storing fat.
In total, the sunflower oil group gained nearly three times more lean tissue than the palm oil group. Over seven weeks at the same surplus, the type of fat in the muffin determined whether the extra calories built the body or just padded it.
The Oil the Internet Told You to Avoid
Sunflower oil sits on a list. The "Hateful Eight," as the seed oils movement calls it. One of eight oils that TikTok creators, podcast hosts, and an online movement with over 100 million views have spent years warning people away from. Swap to butter. Cook with tallow. Use coconut oil. Anything but the seed oils.
In this trial, the oil the internet calls poison built three times more lean tissue than the fat it calls safe.
This does not settle the entire seed oils debate. That argument spans inflammation, industrial processing, heart disease, and a dozen questions this study was never designed to answer. What it does settle is one specific question nobody in the debate is asking: which fat builds more muscle during a caloric surplus? The MRI data answered that. The seed oil won.
The oil the internet calls poison built three times more lean tissue than the fat it calls safe.
The Fat You Cannot See
The lean tissue difference was the headline. The visceral fat difference was the finding most people will never scroll down far enough to read.
Visceral fat wraps around your organs. You cannot see it in the mirror. You cannot pinch it. You cannot measure it with any tool in your gym bag. The only reliable way to detect it is an MRI or CT scan, which is exactly what this study used.
The palm oil group accumulated twice as much visceral fat as the sunflower oil group. Same calories, same surplus, same number on the scale. But the saturated fat was routing surplus calories into organ-wrapping storage that no mirror or caliper could detect.
Where the Evidence Stops
The same Uppsala group kept testing. In 2019, they ran a similar protocol in overweight adults and confirmed that saturated fat still drove significantly more fat into the liver. That finding held across body types.
But in 2024, they asked the lean tissue question in overweight people (BMI 25 to 32, 61 participants, 8 weeks). The lean tissue advantage disappeared. The between-group difference was essentially zero. The authors themselves noted that the earlier results "may have been chance findings or explained by population differences."
The effect works in lean, young people eating at a surplus. If you are already carrying significant extra body fat, the fat-type advantage on lean tissue does not show up in the current data. The research group that proved the effect is the same group that proved its boundary.
Other limitations worth knowing: 37 people is a small sample. Seven weeks is a short window. Only two fat sources were tested, palm oil and sunflower oil, not butter, coconut oil, or olive oil specifically. The participants were young, lean, and Swedish. The authors used careful language: the type of fat "seems to be a novel and important determinant" of body composition.
None of that erases what the MRI showed. It draws edges around it. This was a double-blind, randomized, controlled trial with MRI-measured body composition and a clear result in a specific population.
The research group that proved the effect is the same group that proved its boundary.
One Bottle
Across seven weeks, the entire body composition difference between these two groups came down to which oil was baked into the muffins. No training difference. No protein difference. No calorie difference. Just the fat source.
The study did not test every oil on the shelf. It did not run for a year. It did not include older adults or people with higher body fat. But for lean adults in a surplus, it showed something no other page on the internet has shown with MRI data: the bottle of cooking oil appears to be a body composition variable most lifters have never considered.
That still leaves a question the MRI cannot answer on its own: whether the amount of fat you eat determines your weight at all. That is a different question, backed by a different study, and 57,000 people who already answered it.
The standard surplus equation has three variables: calories, protein, and training. This study adds a fourth that most lifters have never considered — which fat delivers the surplus calories.
The study tested two specific oils, palm oil and sunflower oil, and only in lean young adults eating at a surplus. It did not test every oil on the shelf and it did not test people already carrying significant body fat. What it did show is that body composition during a bulk is not entirely determined by calories and training.
Fat type appears to be a variable. Whether that changes your next grocery trip depends on which oil is currently in your pan — and whether you match the population where the effect showed up.
What other research found
What this means for you
You are the study population. The 37 participants were young, lean, non-athletes eating at a monitored surplus — close to the definition of a controlled bulk.
In this group, the type of fat in the surplus determined the lean-to-fat ratio of the weight gained. The difference was not subtle: roughly one-to-one for sunflower oil versus one-to-four for palm oil, from the same caloric surplus.
The effect showed up over seven weeks. If your bulk runs longer, the cumulative difference in body composition may be larger — though the study did not test beyond that window.
The same research group tested this question in your population. In 2024, they ran a similar protocol in 61 overweight adults (BMI 25 to 32) for eight weeks.
The lean tissue advantage disappeared entirely. Both fat types produced the same amount of lean tissue gain. The between-group difference was essentially zero.
The liver fat finding from the same lab's 2019 study did hold for overweight adults — saturated fat still drove significantly more fat into the liver. But the body composition split that makes this page's headline does not show up in the current data for higher body fat levels.
This study tested two specific oils: palm oil and sunflower oil. It did not test butter, coconut oil, beef tallow, or olive oil.
All of those contain saturated fat, but in different proportions and with different fatty acid profiles. Coconut oil is roughly 90% saturated fat. Butter is about 65%. Palm oil — the one tested here — is around 54%. Whether the body treats all saturated fat sources the same way during a surplus is a question this study cannot answer.
The data shows what happened with two oils. Extending that to every fat source on the shelf would be speculation the study does not support.
Before you change anything
Lean, young, healthy adults only. The 37 participants were between 20 and 38 years old, had a BMI between 18 and 27 (median around 20), and had no diabetes or liver disease. About 70% were male. All were recruited in Uppsala, Sweden.
Not tested: anyone with significant body fat. When the same lab later tested overweight adults (BMI 25 to 32), the lean tissue advantage disappeared entirely. The body composition finding on this page applies to people who are already lean and eating at a surplus — not to the general population.
Non-athletes by design. Anyone exercising more than three hours per week was excluded. The results come from people doing little or no structured training during the surplus.
Small sample, short window. Thirty-seven people for seven weeks. The study was powered for liver fat (the primary outcome), not lean tissue (a secondary outcome). The body composition findings may be underpowered.
Two oils, not the full shelf. Palm oil versus sunflower oil. Not butter, not coconut oil, not olive oil, not lard. Different saturated fat sources have different fatty acid profiles and may behave differently during a surplus.
Free-living beyond the muffins. The 750-calorie surplus from muffins was controlled, but the rest of each participant's diet was self-reported. Total dietary intake outside the muffins was not locked down.
One ethnic population. All participants were from Sweden. The authors explicitly state the results need confirmation in other ethnic groups.
Strong method, narrow scope. A double-blind, randomized, controlled trial with MRI body composition — the measurement standard for fat distribution research. The design is excellent. The findings are real within this population.
Single lab, single trial for lean tissue. All three LIPOGAIN studies come from the same research group at Uppsala University. The lean tissue finding has not yet been independently replicated by a different team. The liver fat finding is more robust (confirmed across two LIPOGAIN trials with different populations).
The authors are honest about the edges. They call fat type a variable that "seems to be" important — not one that definitively is. That hedging reflects the evidence stage: promising, proven in one context, awaiting broader confirmation.
Fat type changed what the surplus built. But there is a louder question most lifters hear first: does eating fat make you fat at all?
A Cochrane review — the gold standard of evidence synthesis — pooled 37 trials and 57,079 adults to answer that question. The result surprised everyone who had been avoiding butter to stay lean. That evidence is the first study in this cluster, and the answer is already live.
What This Study Found
All findings from this paper, in plain language.
- Saturated fat drove significantly more fat into the liver than polyunsaturated fat, despite identical weight gain.
- Polyunsaturated fat built nearly three times more lean tissue than saturated fat from the same caloric surplus.
- Saturated fat deposited twice as much fat around internal organs as polyunsaturated fat.
- Blood markers tracked with body changes — more saturated fat in the blood correlated with more liver and organ fat.
- The two fats switched on different genes in fat tissue, including one involved in energy storage.
- Blood sugar and insulin levels were identical in both groups — routine blood work could not detect the body composition difference.
- The saturated fat group gained more total body fat than the polyunsaturated fat group, confirmed by two independent measurement methods.
- Pancreatic fat unexpectedly decreased in both groups during weight gain, regardless of which fat they ate.
- A liver enzyme marker linked to fat production was lower in the polyunsaturated fat group, suggesting the liver may process these fats differently.