Low-Carb vs Low-Fat Diet: Which Actually Loses More Fat?

Low carb or low fat, which actually loses more fat? Essentially neither, at least not in the largest and longest trial built to answer exactly this question. DIETFITS, a 2018 randomized trial of 609 adults published in JAMA, put people on a "healthy" low-fat or a "healthy" low-carb diet for 12 months and found no significant difference in weight loss between the two groups. Median losses were close, about 5.3 kg on low-fat versus 6.0 kg on low-carb, well within the range you'd expect from individual variation rather than a real group difference.

That's the headline. The more useful story is why shorter, tightly controlled studies sometimes disagree with it, and what that disagreement actually tells you.

What did the biggest trial actually find?

DIETFITS, led by Christopher Gardner's group at Stanford, randomly assigned 609 adults without diabetes to a low-fat or low-carb diet for a full year. Both diets emphasized whole foods and minimizing added sugar and refined grains rather than "eat bacon" versus "eat pasta," which matters, since a junk-food low-carb diet and a junk-food low-fat diet aren't the comparison most people actually care about. At 12 months, weight change didn't differ significantly between the two groups, and the range of outcomes within each group, some people lost 30 kg, some gained a little, was similar on both sides.

The researchers also tested two popular theories for predicting which diet suits which person: insulin secretion and a genetic profile tied to fat or carb metabolism. Neither one meaningfully predicted who'd do better on which diet. The authors' takeaway was that diet quality and consistency mattered more than the macronutrient split itself.

But didn't a meta-analysis find low-carb wins?

Yes, and it's worth taking seriously rather than dismissing. A 2015 meta-analysis in PLOS ONE pooled 17 randomized trials covering 1,797 patients, comparing low-carbohydrate diets against low-fat diets over periods of 8 weeks to 24 months. It found a modest but statistically significant edge for low-carb, around 2 kg more weight loss on average, along with a lower predicted cardiovascular risk score.

Two kilograms over months of dieting is a real but small effect, and it's consistent with a simpler explanation than "carbs are metabolically worse than fat": protein and fat tend to be more filling than carbs gram for gram, so a lower-carb diet often ends up higher in protein and more satiating almost by default, which can mean people eat less without consciously trying. That's a behavioral effect working through appetite, not necessarily a fixed metabolic advantage for cutting carbs specifically.

What about the study where fat restriction beat carb restriction?

This is the finding that tends to surprise people, since it points the opposite direction from the "low-carb wins" meta-analysis. In 2015, Kevin Hall's team at the NIH put 19 adults with obesity through a tightly controlled metabolic ward study, each person getting six days of calorie-matched carb restriction and six days of calorie-matched fat restriction, in random order. Cutting carbs raised the body's fat-burning rate and lowered insulin, exactly what the "carbs drive fat storage" theory would predict. But measured body fat loss was still higher during the fat-restricted phase, about 89 grams per day, versus about 53 grams per day during the carb-restricted phase.

That's a real, carefully measured result, and also a narrow one. It was six days per phase, in a metabolic ward, in 19 people, and Hall was explicit that it shouldn't be read as long-term dietary advice. What it does show is that "lower insulin means more fat burned" isn't a complete explanation for how body fat loss actually works over a short window, since the diet with the higher insulin response lost more fat in this specific setup. Longer, free-living trials like DIETFITS, where that effect doesn't show up as a lasting advantage either way, are a better guide for an actual multi-month plan.

So which one should you actually pick?

Whichever one keeps your calorie total in a deficit and is something you'll still be doing in month three. The honest summary of this research is that the carb-versus-fat split matters a lot less than the diet marketing around both camps suggests, and total calories plus adherence explain most of the real-world difference in outcomes. If cutting carbs makes you less hungry and easier to stay consistent, that's a legitimate reason to go lower-carb. If you'd rather keep rice and bread and cut fat instead, that's equally legitimate, and DIETFITS is the best evidence we have that neither path has a built-in ceiling the other doesn't.

This is also where a moderate, sustainable deficit beats either extreme, something how to lose fat without losing muscle covers if protein and training are part of your goal alongside fat loss, which they usually should be.

Picking a lane only matters if you can actually see whether it's working. Ori doesn't care whether you're counting carbs or fat grams, it logs whatever you scan and tracks your weight and training trend over weeks, so you can tell early if your chosen approach is actually moving the needle instead of guessing until the next weigh-in.

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FAQ

Which loses more fat, low carb or low fat? In the largest and longest real-world trial, neither. DIETFITS, a 2018 randomized trial of 609 adults published in JAMA, found no significant difference in 12-month weight loss between a healthy low-fat diet and a healthy low-carb diet. Shorter, tightly controlled studies show small differences in one direction or the other, but they don't overturn that main result.

Didn't a study find low-carb diets lose more weight? A 2015 meta-analysis of 17 randomized trials in PLOS ONE did find a modest edge for low-carb diets, about 2 kg more weight loss on average than low-fat diets over 8 weeks to 24 months. The individual trials it pooled varied a lot in design and adherence, which likely explains part of that gap.

What about the study where cutting fat burned more body fat than cutting carbs? That's a 2015 metabolic ward study by Kevin Hall's team in Cell Metabolism. In 19 adults with obesity, cutting fat produced more measured body fat loss per day than cutting an equal number of calories from carbs, even though the low-carb phase increased fat burning. It was a six-day, inpatient, tightly controlled comparison, and Hall himself cautioned against drawing long-term diet advice from it.

So does it actually matter which one I pick? Less than most diet marketing suggests. The research points to total calories and how long you can actually stick with a diet as the bigger drivers of fat loss, with the carb-versus-fat split mattering mostly through its effect on hunger and how easy a diet is to follow. Pick the one you can eat consistently.

Is one better for health markers like cholesterol? DIETFITS found some secondary differences, LDL cholesterol improved more on the low-fat diet, for instance. These are real but modest findings on specific markers, not evidence that one diet is overall healthier. If you have a specific condition like high LDL or diabetes, that's a conversation for your doctor, not a blog post.