Hall 2021: Keto vs Plant-Based — Who Actually Ate More?

The prediction was specific. Carbs spike insulin. Insulin drives hunger. Hunger makes you overeat. Cut the carbs, and appetite handles itself.

Kevin Hall's research group at the NIH designed an inpatient test of that prediction with both diets cut low enough to potentially reveal a difference, which no earlier inpatient study had done, the authors note. They admitted twenty adults — eleven men, nine women, ages eighteen to thirty-nine, BMI ranging from lean to obese — to a metabolic ward, where they lived for a continuous four weeks.

No trips home. No food from outside. Every meal prepared by NIH nutrition staff, every leftover weighed to the gram.

Each person ate two diets in random order, two weeks each. One was plant-based and low-fat — rice, lentils, vegetables, fruit, with 75% of calories from carbohydrates. The other was animal-based and ketogenic — meat, eggs, cheese, with 75% of calories from fat. Both diets offered as much food as anyone wanted. Eat until you're full. Eat again whenever you're hungry.

Ratings of how pleasant and how familiar the meals were showed no significant difference between the diets.

Then the intake data came back.

On keto, every single person ate more. Not some of them. Not most. All twenty. On average, the keto side consumed 689 extra calories per day — roughly an extra dinner's worth of calories. The difference was statistically significant (P < 0.0001).

The diet that was supposed to kill hunger led to about an extra meal's worth of calories a day, on average. And the hunger ratings showed no significant difference.

One in a Million

You might think that's just an average — some people ate a little more on keto, some ate a lot more, and it averaged out to 689. But this wasn't a trend. It was unanimous.

All twenty participants ate less on the high-carb side. Every single one. The probability of that happening by chance — of flipping a coin twenty times and getting heads every time — is less than one in a million.

And this held regardless of which diet people started with. The researchers checked for order effects — whether going keto-first or plant-based-first changed the outcome. It didn't. No significant effect of diet order. No significant effect of sex. The direction was the same for everyone, no matter how the study was arranged.

The hunger ratings make it stranger. On a hundred-point scale, participants rated their hunger, fullness, satisfaction, and eating capacity during the second week of each diet. Not a single measure showed a significant difference. Yet on one side they quietly consumed about an extra dinner's worth of calories a day, on average.

The prediction said carbs would drive hunger and overeating through insulin. The insulin part worked exactly as predicted — after a liquid test meal, insulin ran about three times higher on the high-carb diet than on keto. But the eating went the opposite direction. More insulin. Fewer calories. The mechanism fired. The prediction failed.

What the Scale Was Actually Measuring

Here's where it gets personal for anyone who's ever celebrated a keto weigh-in.

On the scale, keto looked ahead. In two weeks, participants on keto dropped 1.77 kg — nearly four pounds. On the low-fat plant-based side, they lost 1.09 kg. The authors call the two totals similar: the difference was not statistically significant.

But the researchers didn't stop at the scale. They used DXA scans to split what the body lost into fat and fat-free mass, which is everything else.

Of that 1.77 kg keto lost, 1.61 kg was fat-free mass. That's about 91%. The scan counts shifts in body water as fat-free mass, and the authors judged that keto's extra weight loss in the first week was likely mostly water, glycogen, protein and gut contents. The remaining 0.18 kg of fat loss wasn't even statistically significant.

Meanwhile, the low-fat side lost its 1.09 kg more slowly at first. But underneath, it was losing body fat at more than three times the rate — 51 grams of fat per day compared to keto's 16 — while eating 689 fewer calories a day. The low-fat fat loss was statistically significant. The keto fat loss was not.

If you turned keto's two-week scale result into a pie chart, you'd see a massive slice labeled fat-free mass, and a sliver labeled fat that the statistics couldn't distinguish from zero. The weigh-in everyone screenshots was tracking the wrong metric.

More Protein In, More Protein Out

This is the part that makes even keto-friendly researchers pause.

Participants ate more protein on keto — about 135 calories more per day than on the plant-based side, although the two diets as served were matched for protein. On the usual expectation, more protein eaten should mean less of the body's own protein lost. Protein is the building block. More blocks, more protection.

That's not what happened. On keto, people ate more protein and lost more body protein, along with 1.61 kg of fat-free mass. The low-fat side ate less protein and preserved its fat-free mass almost entirely — a loss of just 0.16 kg that wasn't statistically significant.

The body-protein measure was nitrogen balance: the nitrogen people ate minus the nitrogen in their urine, collected over five days in the second week of each diet. It came out more negative on keto, −7.8 versus −1.8 grams a day, which the authors read as a greater net loss of body protein despite more protein eaten.

The body was losing protein on net, even while more protein was coming in on the plate. The paper measured that balance, not the route the protein took. The protein-sparing promise that often accompanies keto didn't hold up in these two weeks on the ward.

The Three Strongest Objections

If this were a bad article, this is where it would declare keto dead and move on. The study's own authors didn't do that, and neither will this page.

Three objections deserve serious consideration — and the study's own data partially answers each one.

First: two weeks isn't long enough. Keto advocates argue that full adaptation takes longer. It's a fair point — and the study data engages with it directly.

In the fifteen people tested daily, blood ketones passed the 0.5 millimoles per liter threshold that defines nutritional ketosis within days, and averaged 1.8 in the second week. Their calorie intake on keto did drop by 312 calories per day from week one to week two, suggesting adaptation was actively occurring.

But even after that adaptation, the gap remained: 544 fewer calories per day on the plant-based side in the final week alone. Whether longer adaptation would eventually erase or reverse the gap is, in the authors' words, unknown.

Second: the crossover design may carry effects. Harvard's David Ludwig — one of the carbohydrate-insulin model's most prominent defenders — published a 2025 letter in Nature Medicine arguing that switching diets without a washout period could skew the results.

Hall replied in the same issue, noting no significant diet-order effect was found — the result held regardless of which diet came first. The strongest scientific critics of this study are engaging with it in the field's top journal.

Third: the diets differed in more than just macros. The plant-based side had half the energy density of the keto side — 1.1 versus 2.2 calories per gram. It had more fiber, and it was entirely plant-sourced, while the keto meals drew 82% of their calories from animal foods.

Any of these variables could explain part of the intake gap. This is the most technically valid criticism, and the study's own Discussion acknowledges it openly.

But here's the distinction that holds through all three objections, for these two weeks. The carbohydrate-insulin model made a directional prediction: high-carb food would drive MORE eating through insulin. That specific direction went the wrong way in every participant.

Duration might change the size of the gap, and the diets' other differences might explain part of it; the authors doubt any single factor explains it. What the two weeks do show is the direction, and on that the authors conclude it contradicts the predictions of the carbohydrate-insulin model.

Not Just One Ward

A separate 2025 study tested the same prediction from a different angle. Liu and colleagues at Shenzhen University gave 120 healthy adults meals that deliberately varied in glycemic index — the exact variable the carbohydrate-insulin model says should drive hunger and overeating [1].

Glucose and insulin followed the predicted pattern perfectly — higher-glycemic meals produced higher blood sugar and insulin responses. But hunger didn't follow. Participants reported identical hunger across all three glycemic groups.

And at the individual level, a weak correlation between insulin and subsequent eating went in the opposite direction the model predicts — higher insulin was associated with eating less, not more.

Two different designs. Two different populations. Two different countries. The CIM's hunger prediction failed in the same direction both times.

The Clip and the Evidence

The carbohydrate-insulin model hasn't survived as a scientific consensus — it's survived as a content format. The idea that carbs spike insulin and insulin drives hunger has been compressed into confident sixty-second clips that millions of people have absorbed as metabolic law.

This study didn't test whether keto is a bad diet. Both diets caused weight loss. Both had metabolic advantages the researchers documented — keto lowered fasting triglycerides and reduced glucose variability; the plant-based side lowered LDL cholesterol and improved fat loss.

The study tested one specific prediction made by one specific model — and that prediction went the opposite direction in every person who walked through the ward.

This hunger test was one of nine predictions about carbs that controlled trials have now examined. Which of the model's nine predictions survived and which didn't — from the metabolic edge to GI to fiber — is the question the rest of the evidence answers.

The question was never keto versus plant-based. The question was whether the mechanism behind the clip was real. Twenty people. No significant difference in hunger. Seven hundred extra calories per day on the side that was supposed to suppress appetite. All twenty the same direction.

The clip made a prediction. The ward returned a verdict.

That still leaves the model's other half unanswered. The CIM doesn't just predict that carbs drive hunger — it also predicts that cutting carbs unlocks a metabolic-rate advantage, burning more calories at rest. A different controlled trial tested that exact claim. The gap between what was promised and what was measured is worth knowing.