Stanford spent $8.2 million testing whether cutting carbs beats cutting fat. The answer fits in a jar of peanut butter.
“Two people followed the exact same diet for a year. One lost thirty kilograms. The other gained ten. The diet wasn’t the variable.”
You’ve been staring at two grocery lists for a week. One has rice crossed off. The other has butter crossed off.
Your training partner swears by keto. Your coworker lost weight eating pasta. Somewhere between their advice and a hundred Instagram posts, you landed here — looking for the actual answer.
Stanford gave it to you. In 2018, a research team led by Christopher Gardner published the largest trial designed to settle this exact question — the DIETFITS study, published in JAMA.
They recruited 609 overweight adults between 18 and 50, without diabetes, roughly 57% women. Half were randomly assigned to a healthy low-fat diet. Half to healthy low-carb. Each group got a registered dietitian, 22 coaching sessions, and a full year. No calorie targets. Just follow your plan and live your life.
After twelve months, the low-fat group lost an average of 5.3 kg. The low-carb group lost 6.0 kg. The entire difference between the two diets: 0.7 kg — and the confidence interval crossed zero, meaning even that tiny gap might be nothing more than statistical noise.
Pick up a jar of peanut butter from your kitchen. A standard jar weighs about 700 grams. That jar is the entire year-long difference between low-carb and low-fat. The diet war you’ve been agonizing over amounts to something you can hold in one hand. And it might not even be real.
The diet type you've been agonizing over? It accounts for 700 grams over a year. The person following the diet accounts for 40 kilograms.
- Both groups cut about the same calories naturally — without being told to — because both focused on real food and vegetables.
- Individual results ranged from losing 30 kg to gaining 10 kg on the same diet — the person mattered far more than the plan.
- Stanford spent $8.2 million testing whether your genes or insulin response could match you to the right diet — and found nothing.
- The study was partially funded by an organization created to prove low-carb is superior. The results said otherwise.
Both Groups Did the Same Thing Without Knowing It
Here’s the part that makes the result make sense. Neither group was told to count calories. The low-fat group was told to cut fat. The low-carb group was told to cut carbs. Both were told to eat real food — maximize vegetables, minimize added sugar, cook at home.
When researchers tracked what everyone was actually eating, they found something striking. Both groups had quietly cut approximately 500 to 600 calories per day below their starting intake — at every check-in, for the entire year. Nobody told them to do this. It just happened.
The mechanism hiding behind both diet labels was the same: focus on whole food, eat less processed junk, and your appetite naturally settles into a lower range. The macro they restricted was different. The calorie reduction was identical. And identical calorie reduction produced identical weight loss.
Which raises a question you probably haven’t considered. If both diets landed at the same place — why did some people in those groups lose dramatically more than others?
Same Coach. Same Year. Forty Kilos Apart.
Picture two women sitting in the same classroom. Same dietitian. Same low-carb protocol. Same twelve months of coaching.
One of them walked out 30 kg lighter. The other walked out 10 kg heavier.
This isn’t hypothetical. It’s what the data showed. Within each diet group, weight change ranged from losing about 30 kg to gaining about 10 kg — a spread of roughly 40 kg among people following the exact same rules.
Now compare those two numbers. The gap between the two diets was 0.7 kg. The gap between individuals on the same diet was 40 kg. That ratio is 57 to 1.
The variable everyone argues about — which diet to choose — accounts for 700 grams over a year. The variable nobody discusses — what makes one person sustain a diet where another can’t — accounts for 40 kilograms. You were solving a 700-gram problem while ignoring a 40-kilogram one.
But maybe your genes could predict which person would thrive on which plan. Maybe your insulin response holds the key. That’s exactly what the Stanford team spent $8.2 million trying to find out.
The weight numbers were identical. The blood markers weren't.
Low-carb slightly improved HDL cholesterol and triglycerides. Low-fat slightly improved LDL cholesterol. Neither diet won the full lipid panel — each one traded a small advantage for a small disadvantage on a different marker.
Eight Million Dollars Searching for an Answer That Doesn’t Exist
DIETFITS wasn’t just a diet comparison. It was specifically built to test whether your genetic profile or your insulin response could match you to the right diet. Three gene markers. Baseline insulin secretion measured after a glucose challenge. $3.2 million from the National Institutes of Health, plus $5 million from a private nutrition research organization.
The sample was large enough to detect a meaningful effect if one existed — 609 people, roughly 90% statistical power, which means the study had an excellent chance of catching a real signal. This wasn’t a side analysis. The gene-matching question was the study’s entire reason for existing.
After a year: genotype didn’t predict who succeeded on which diet (P = .20 — nowhere near the threshold for a real effect). Insulin secretion was even further off (P = .47). Not borderline. Not trending. Nothing.
Every DNA-based diet quiz on your social media feed is selling confidence in a question that Stanford spent $8.2 million failing to answer. Which raises an obvious concern — who exactly funded a study that so thoroughly dismantles the low-carb-is-superior narrative?
Funded by the People Who Wanted It to Go the Other Way
Part of that funding — $5 million — came from the Nutrition Science Initiative, or NuSI. NuSI was founded in 2012 with a specific mission: prove that carbohydrates, not calories, are the primary driver of obesity. [1]
Its founders raised over $40 million from a major philanthropic foundation. They funded DIETFITS expecting the trial to validate their core hypothesis. Instead, the study came back null — no advantage for cutting carbs — directly undermining the idea NuSI was created to champion. [1]
After the results came in, NuSI attempted to interfere with the researchers’ work. The scientists had to formally push back to protect their academic independence.
NuSI’s co-founder resigned in December 2015. The primary funder stopped writing checks. NuSI officially dissolved on December 31, 2021. [1]
A study funded by advocates against its own conclusion is nearly impossible to dismiss as biased. The people who most wanted low-carb to win funded the trial, got a null result, and the organization didn’t survive it.
“The organization that partially funded this study was created to prove low-carb is superior. It didn’t survive the result.”
The Strongest Objection — and What It Concedes
Some researchers argue DIETFITS didn’t test a truly ketogenic diet. By month twelve, the low-carb group had drifted to about 30% of their calories from carbohydrates — well above the strict ketogenic threshold. Researchers David Ludwig and Cara Ebbeling published a reanalysis suggesting the most extreme low-carb adherents may have done slightly better.
It’s a fair point — and worth hearing. But notice what the strongest opposition actually concedes. The population-level result stands unchallenged.
The rebuttal narrows the claim to a tiny subset of extreme adherents in an analysis the trial wasn’t designed to test. The smartest critics of this finding don’t dispute it. They retreat to a smaller, untestable question.
It Was Never About Which Diet
Gardner’s 609 participants aren’t alone. A meta-analysis pooling 19 calorie-matched trials across over 3,200 participants found the same pattern — when calories are held equal, the macro ratio doesn’t change the outcome [2].
A separate two-year Harvard trial with 811 adults tested four different macro combinations and found no meaningful difference between any of them [3]. The question has been answered from multiple directions.
And the answer changes the question entirely. The choice you were agonizing over — keto or balanced, low-carb or low-fat — was never the decision that determined your result. What separated the person who lost 30 kg from the person who gained 10 wasn’t the plan. It was the fit between the plan and the person.
The research doesn’t say all diets are identical. It says the differences between diets are so small relative to the differences between people that obsessing over diet type is like choosing paint colors while the foundation needs work. The diet that matches your life — the one you can sustain through a year of real dinners, gym sessions, and holidays — is the one the data supports.
That still leaves questions DIETFITS can’t answer alone. The type of diet didn’t matter for weight loss — but does the quality of the carbs you eat matter? And if you’re training seriously, will keeping carbs in your diet help you perform under the bar?
Both questions have large trials behind them. Both answers are worth knowing.
The study didn't just find that both diets work. It found why both diets work — and the reason had nothing to do with macros.
Both groups were told to eat real food, cook at home, and fill their plates with vegetables. Both quietly dropped the same number of calories without being asked to count. The shared foundation mattered more than the macro split.
That changes the decision. Instead of asking which macronutrient to restrict, the research points toward a different question: which way of eating real food fits your actual life — your dinners, your schedule, your kitchen?
The approach that survives contact with a bad week is the one the data supports.
What other research found
What this means for you
Nothing in this trial says you need to switch. The data showed no weight disadvantage for low-carb — the same result as low-fat, not a worse one.
The low-carb group also showed slightly better HDL cholesterol and triglyceride numbers, though LDL went the other direction.
If keto fits your life and you're sustaining it, the study says that's a perfectly supported choice. The finding isn't that keto fails — it's that it isn't uniquely superior.
This is the part that might actually help: the choice between low-carb and low-fat didn't determine who succeeded. Both groups lost the same weight over a year.
What did matter was whether someone could stick with their plan through twelve months of real life. The individual gap within each group was enormous.
The research points toward the approach that matches your actual kitchen and your actual social life — not the one that sounds best on paper.
Stanford designed part of this trial specifically to test whether your genes could match you to the right diet. Three genetic markers. Baseline insulin response. Neither predicted who would succeed on which approach.
The study had the sample size and statistical power to catch a real signal if one existed. It found nothing.
That doesn't rule out future genetic discoveries — but the specific markers these tests currently sell didn't predict outcomes in the most rigorous trial testing them.
One important caveat: both groups in this trial had 22 sessions with a registered dietitian over the year. That's far more support than most people get when they start a diet on their own.
The equal weight loss across diets happened in a setting with substantial professional guidance. Whether the same pattern holds for self-directed dieting is an open question the trial can't answer.
The quality-first approach both groups followed — real food, minimal processed junk, more vegetables — doesn't require a coach to try.
Before you change anything
Overweight or obese adults aged 18-50 without diabetes — that's who was in the trial. BMI between 28 and 40, roughly 57% women, based in the San Francisco Bay area with relatively high education levels.
Not tested: people with diabetes, anyone over 50, lean individuals, or people with severe obesity above BMI 40.
Both diet groups emphasized real, whole food — vegetables, home cooking, minimal sugar and refined grains. These findings apply to healthy versions of each approach, not to a low-fat diet built on white bread or a low-carb diet built on bacon and processed meat.
Twenty-two dietitian-led sessions over a year is far more professional support than most real-world dieters get. The results happened in a privileged coaching environment.
Single location. Stanford's Bay Area population had high education levels (67-70% with college degrees) and good access to quality food. Whether results generalize to different communities and food environments is unknown.
Both groups drifted toward moderate macros by month twelve — the low-carb group ended at about 30% carbs. Whether a strictly ketogenic version (under 10% carbs) would have produced different results remains untested at this scale.
This is one of the most robust diet-comparison trials ever conducted. 609 people, twelve months, published in JAMA, with 90% statistical power to detect meaningful effects.
The primary finding — no significant weight difference between diets — is confirmed by two independent research teams using different designs.
The adversarial funding adds another layer of trust. The organization that partially funded the study was created to prove the opposite of what the study found.
Confidence is slightly lower for the genetic-matching null: the trial tested three specific gene markers, and other unmeasured genetic factors might still matter. The door isn't closed on genetic diet-matching — but these specific markers didn't open it.
The type of diet didn't matter for weight loss. But DIETFITS didn't test what happens when the quality of your carbs changes — when ultra-processed food replaces whole food with the same macros. That question has its own controlled feeding study where twenty adults ate 508 extra daily calories on ultra-processed food, and the answer surprised the researchers who ran it.
And if you're training, there's a separate question entirely: does keeping carbs help you perform under the bar? A meta-analysis pooling six keto-vs-normal trials in 131 trained lifters has an answer worth knowing.
What This Study Found
All findings from this paper, in plain language.
- After a year, the low-carb and low-fat groups lost almost exactly the same amount of weight — the tiny difference wasn't statistically meaningful.
- Three genetic markers tested couldn't predict who would do better on which diet — DNA didn't help match people to plans.
- Baseline insulin response also failed to predict which diet would work better for a given person.
- Within each diet group, results ranged from losing 30 kg to gaining 10 kg — individual variation was enormous compared to the between-diet difference.
- Both groups naturally cut the same number of calories without being told to count — the calorie reduction happened on its own.
- Low-carb produced slightly better HDL cholesterol and triglyceride numbers than low-fat.
- Low-fat produced slightly better LDL cholesterol numbers than low-carb — neither diet won the full lipid picture.
- BMI barely favored low-carb, but body fat percentage and waist size showed no meaningful difference between the groups.
- Both groups actually followed their assigned diets for a year — the macro split was real and confirmed by independent body measurements.
- Resting metabolism dropped in both groups as they lost weight, but neither diet preserved metabolic rate better than the other.
- Side effects were rare and evenly split between the two diet groups — neither approach was riskier.