The largest investigation into fat and weight loss ever conducted found an answer small enough to hold in one hand.
57,079 adults across 37 trials cut fat from their diets. Total weight lost: 1.42 kg. A bag of flour.
Thirty-seven clinical trials. 57,079 adults. Decades of research across 11 countries. The Cochrane Collaboration, the most trusted name in evidence review, pulled it all together in 2020 and asked the question you've probably argued about at the gym: does cutting fat from your diet actually make you lose weight?
The answer: 1.42 kg.
That's a bag of flour. You could hold the entire result of the biggest fat-and-weight investigation ever conducted in one hand.
That number holds up under every test the researchers could throw at it. This is the most robust finding in 30 years of fat research, backed by the highest certainty rating the Cochrane system assigns. And what it found is that cutting fat barely moves the scale.
Three decades of evidence, 37 trials, and the highest certainty rating in nutrition science all say the same thing: the macronutrient you've been tracking hardest is the one that matters least for weight.
- A dose-response pattern emerged: for every 1% of calories replaced from fat, people lost 0.20 kg total — meaning a meaningful fat cut buys roughly the weight of a small bottle of water.
- The biggest counter-study — 53 trials concluding "stop recommending low-fat" — actually arrived at the same conclusion from the opposite direction: no macronutrient strategy produces impressive weight loss.
- The Cochrane certainty rating is HIGH — the top grade, meaning new research is very unlikely to change this conclusion.
- Across the trials that measured it, reducing fat intake showed no adverse effects on blood lipids, blood pressure, or quality of life.
Two Tablespoons of Olive Oil, Forever
Hooper and colleagues found a pattern linking how much fat you cut to how much weight you lose: for every 1% of your total calories you cut from fat, you lose 0.20 kg. Not per week. Not per month. Total.
If you eat around 2,500 calories a day and drop your fat intake from 35% to 25%, you're cutting roughly 28 grams of fat per day. That's about two tablespoons of olive oil. The bottle that sits on your counter every Sunday while you prep meals, wondering whether you're allowed to use it.
Two tablespoons, from every meal, indefinitely. Your return: about 2 kg total. Less than a newborn baby weighs.
You can calculate your own version. Take whatever fat you've been cutting, convert it to a percentage of your total calories, and multiply by 0.20. That number is what you're buying with every dry chicken breast and every undressed salad.
It's not just the scale. Body fat percentage dropped by 0.28 percentage points. Waist circumference shrank by less than half a centimeter. Every measure of body composition told the same tiny-effect story — cutting fat barely registers, no matter how you measure it.
The Counter-Argument That Proves the Point
If you've Googled "low fat diet" before, you've probably seen a 2015 systematic review by Tobias and colleagues in The Lancet Diabetes & Endocrinology. They pulled together 53 trials with 68,128 participants and arrived at what sounds like a death sentence for low-fat dieting.
Their conclusion: health and nutrition guidelines should stop recommending low-fat diets for weight loss. Low-carb diets beat low-fat by 1.15 kg. Low-fat showed no advantage over any other moderate-fat approach.
That looks like it contradicts everything above. It doesn't. Hooper tested whether cutting fat produces weight loss at all. Tobias tested whether one diet beats another. Different questions — and both got the same small answer.
They're both saying the same thing from opposite corners. Hooper found 1.42 kg. Tobias found that the average loss across every dietary approach was just 2.7 kg after a year. Both numbers fit in one hand.
Neither found that which macronutrient you cut makes a meaningful difference.
The two biggest systematic reviews on fat and weight appear to be fighting. They're actually agreeing: macronutrient composition is not a powerful lever for weight loss. Cut fat, cut carbs, or cut some combination. The numbers stay small either way.
No other analysis on the internet frames Hooper and Tobias as convergent. Everywhere else, they're opposing camps. Here, they're evidence for the same conclusion.
Different questions. Same small answer.
Systematic reviews compared · Hooper et al. 2020, Tobias et al. 2015Two tablespoons of olive oil, from every meal, indefinitely. Your return: about 2 kg total.
Why This Finding Won't Budge
The Cochrane review didn't produce 1.42 kg from a handful of questionable studies. 30 out of 33 direct comparisons pointed in the same direction — and the overall finding earned the highest certainty rating Cochrane assigns, meaning new research is very unlikely to change this conclusion.
The researchers stress-tested their own result. When they kept only the four most carefully designed trials, the effect shrank to -0.67 kg. Even smaller. Still real. Still pointing the same way.
One caveat deserves a direct mention. Most participants across these 37 trials started at a BMI above 30 (classified as obese). If you're a relatively lean lifter at 12-15% body fat, the effect of cutting dietary fat on your body might play out differently.
When researchers did test lean adults aged 20 to 38 in a controlled surplus, total fat intake was not the variable that mattered. The specific cooking oil determined whether the surplus built lean tissue or stored fat — a finding MRI scans confirmed at a three-to-one ratio.
The data can't confirm that either way, and we won't pretend otherwise. Whether cutting fat too low affects testosterone and recovery is a separate question — that research gets its own page.
This isn't a single study making this claim, either.
Kevin Hall's metabolic ward study at the NIH controlled everything. Nineteen adults lived in a sealed facility where researchers measured every calorie consumed and burned.
Under the most precise conditions science can create, cutting fat and cutting carbs produced virtually identical body fat loss. What Hooper found across 57,000 people, Hall confirmed at the individual metabolic level.
George Bray's team at Pennington Biomedical Research Center overfed 25 adults by roughly 954 extra calories per day for eight weeks. Body fat gain was identical regardless of whether the extra calories came from fat, carbs, or protein. In a surplus, the body stores excess energy as fat no matter which macronutrient delivered it.
And the POUNDS LOST trial, published in The New England Journal of Medicine, randomized 811 adults to diets with either 20% or 40% fat for a full two years. Weight loss was identical in both groups. The largest, longest controlled comparison of dietary fat levels ever run found that it doesn't matter.
The two biggest systematic reviews on fat and weight appear to be fighting. They're actually agreeing.
The Olive Oil Goes Back On
Across the studies that measured safety outcomes, reducing dietary fat produced no significant adverse effects on blood lipids, blood pressure, or quality of life. For anyone whose next question after "can I eat more fat?" is "but will it wreck my cholesterol?", the available trial data says it didn't.
The answer to "will eating fat make me gain fat?" is now as settled as nutrition science gets. Fat has 9 calories per gram, and that part is true. But cutting it from your diet buys you a bag of flour and costs you every tablespoon of olive oil along the way.
The dose-response confirms it. The counter-argument reinforces it. Three independent studies in three different settings all arrive at the same conclusion. The olive oil goes back on.
That still leaves a question the scale can't answer: whether the type of fat you eat changes where your body stores it. That's a different study with different data — and it's next in this series.
The research didn't find that fat is harmless. It found that the difference between eating it and not eating it is too small to build a diet around.
What the data actually suggests matters is total calories — not which macronutrient delivers them. The 37 research groups who tested this question all arrived at the same answer, and so did the biggest counter-study. The macronutrient on the label isn't the variable that moves the scale.
For anyone who has been dry-cooking chicken and skipping olive oil to "keep fat low" — the evidence says that sacrifice buys roughly a bag of flour. The meal prep decision just got simpler.
What other research found
What this means for you
The study tested a broad adult population — this takeaway applies the finding to your specific situation.
The fear that fat during a surplus turns straight into body fat has been directly tested. Bray's overfeeding study put it to the test: same surplus, different macros, identical fat gain. Your body doesn't care whether the extra calories came from olive oil or white rice — it stores the excess either way.
What drives fat storage on a bulk is the size of the surplus, not the macro split. Stop dry-cooking your chicken.
The POUNDS LOST trial randomised 811 adults to diets with either 20% or 40% fat for two full years. Weight loss was identical in both groups — 3.3 kg each.
The cognitive effort of tracking fat grams during a cut is real. The return on that effort, according to the largest and longest controlled comparison of dietary fat levels ever published, is zero.
Quitting a low-fat diet turns out to have been the rational response to the evidence — whether you knew it at the time or not.
The Cochrane data shows that the restriction you were enduring buys about 1.42 kg of weight loss. Meanwhile, the trials that measured safety found no harm from eating more fat — blood lipids, blood pressure, and quality of life were all unaffected.
Your instinct aligned with Cochrane-grade evidence before you had the data.
Before you change anything
Most participants were women over 50. The Women's Health Initiative trial — one single study enrolling postmenopausal women aged 50-79 — contributed 88% of the total participant weight to the body weight analysis. Other trials included both sexes across 11 countries, with ages ranging from 18 to 79.
If you're a lean lifter at 12-15% body fat, this data can't tell you what happens. Most participants started at a BMI above 30 (classified as obese). The effect of cutting dietary fat on someone who's already relatively lean might look different. The data doesn't confirm that either way.
The review included healthy adults, people at elevated cardiovascular risk, and people with diabetes risk factors. Athletes and highly active individuals were not represented in any of the 37 included trials.
Most of these trials weren't actually designed to measure weight loss. Body weight was a secondary outcome in cardiovascular or cancer prevention studies — the researchers were measuring heart disease and cancer risk, and recorded weight changes along the way.
The Women's Health Initiative dominated the numbers. One trial contributed 88% of the participants to the weight analysis. The sensitivity analysis excluding that trial showed a slightly larger effect (-1.51 kg), confirming the finding isn't driven by a single study, but the dominance is worth noting.
Dietary adherence relied on self-reporting. People told researchers what they ate, which is notoriously unreliable. The actual fat reduction achieved was smaller than what was prescribed in most trials.
This is about as certain as nutrition science gets. GRADE HIGH is the top rating, reserved for evidence where new research is very unlikely to change the conclusion. This finding earned it — not downgraded for any of the five standard criteria.
Thirty-seven research groups across 11 countries, spanning decades of investigation, all pointing in the same direction. 30 out of 33 individual comparisons agreed. Even the strictest analysis — using only the four trials with the lowest risk of bias — still found the effect.
The remaining uncertainty isn't about whether cutting fat produces weight loss. It does. The uncertainty is about exactly how small that effect is in populations the review didn't heavily represent — like lean, active adults.
If the amount of fat in your diet barely moves the scale, the question that follows is whether the type of fat changes how your body actually looks.
Separate research using MRI scans found that the kind of fat you eat may dramatically affect where your body stores it — and the answer has nothing to do with total calories or weight on the scale. That's a completely different question with a completely different evidence base, and it's the next study in this cluster.
What This Study Found
All findings from this paper, in plain language.
- People who cut fat from their diet lost an average of 1.42 kg more than people who didn't — roughly the weight of a bag of flour.
- The more fat people cut, the more weight they lost — but the rate was tiny: 0.20 kg for every 1% of calories removed from fat.
- BMI dropped by less than half a point (0.47 kg/m²) in the groups that cut fat compared to those that didn't.
- Waist circumference shrank by less than half a centimeter (0.47 cm) — barely enough to measure with a tape.
- Body fat percentage dropped by 0.28 percentage points — a change so small most body composition tools wouldn't detect it.
- Even when the researchers used only the highest-quality trials, the effect survived — it just got even smaller (0.67 kg).
- Cutting fat from the diet didn't cause any measurable harm — blood lipids, blood pressure, and quality of life were all unaffected.
- 30 out of 33 individual study comparisons pointed in the same direction — an almost unheard-of level of agreement in nutrition research.
- The certainty rating earned the highest grade possible — meaning new research is very unlikely to change this conclusion.
- The weight effect persisted whether studies lasted 6 months or over 8 years — it didn't fade with time.
- Most trials couldn't fully blind participants to their diet (people know what they're eating), but the effect held up even in the most carefully controlled studies.