Low Carb vs Low Fat for Weight Loss: What the Trials Show

For weight loss, low carb and low fat work about equally well. In the largest head-to-head trials, people lost almost the same amount of weight after one to two years whichever way they cut calories. The better choice is the one you can follow comfortably for a long time while keeping food quality high.

Here is what the research on low carb vs low fat actually found, where the two approaches do differ, and how to choose.

The short answer from the biggest trials

DIETFITS: 609 adults, 12 months

The DIETFITS trial at Stanford, published in JAMA in 2018, randomly assigned 609 adults with overweight or obesity to a “healthy low-fat” or a “healthy low-carb” diet for a year. Both groups had 22 small-group sessions that focused on reaching the lowest fat or carbohydrate intake they could keep up long term, with a strong emphasis on diet quality.

• Low fat: about 5.3 kg lost after 12 months
• Low carb: about 6.0 kg lost after 12 months
• Difference: 0.7 kg, not statistically significant

The researchers also tested whether a person’s genotype or how much insulin they released after a glucose drink predicted which diet worked better for them. Neither did. That is a useful finding if you have seen tests sold to tell you which diet suits your genes.

POUNDS Lost: 811 adults, 2 years

An earlier trial in the New England Journal of Medicine assigned 811 adults to four reduced-calorie diets with different mixes of fat (20% or 40% of calories), protein (15% or 25%) and carbohydrate (35% to 65%). At six months, people in every group had lost about 6 kg on average. By two years, weight loss was similar across the board: 3.3 kg in both the low-fat and high-fat groups, and 2.9 vs 3.4 kg in the highest- and lowest-carb groups.

One of the strongest predictors of success had nothing to do with macros: attending the group sessions. Each session attended was linked with about 0.2 kg more weight loss.

The wider evidence

A 2020 network meta-analysis in the BMJ pooled 121 trials with nearly 22,000 adults. At six months, low-carb and low-fat diets produced similar weight loss compared with usual eating (about 4.6 vs 4.4 kg). By 12 months, weight loss had shrunk for every diet pattern, and most improvements in blood pressure and cholesterol had largely faded.

The conclusion across these studies is consistent: reduced-calorie diets lead to meaningful weight loss regardless of which macronutrient they emphasize, and staying with the diet is what drives results.

What DIETFITS diets looked like in practice

It helps to see how different the two diets really were. These are the average macronutrient shares DIETFITS participants reached at 12 months, converted into grams for an example day of 1,800 kcal (4 kcal per gram of carbohydrate or protein, 9 kcal per gram of fat):

	Healthy low fat	Healthy low carb
Carbohydrate	48% = about 216 g	30% = about 135 g
Fat	29% = about 58 g	45% = about 90 g
Protein	21% = about 95 g	23% = about 104 g

Check the arithmetic for the low-carb column: 1,800 × 0.30 = 540 kcal ÷ 4 = 135 g carbohydrate; 1,800 × 0.45 = 810 kcal ÷ 9 = 90 g fat; 1,800 × 0.23 = 414 kcal ÷ 4 ≈ 104 g protein.

Notice that neither diet was extreme. The low-carb group still ate about 135 g of carbohydrate a day on this example, well above a ketogenic level. If you are curious about the very low-carb end, see our keto diet for beginners guide.

Where low carb and low fat do differ

The weight results are close, but the diets are not identical in every way.

Cholesterol

In the BMJ analysis, low-fat diets lowered LDL (“bad”) cholesterol more than low-carb diets, while low-carb diets raised HDL cholesterol slightly. If your LDL is already high, a low-carb plan built around butter, cheese and processed meat is not a great fit. A low-carb plan built around olive oil, nuts, fish and vegetables is a different story.

Fat loss under controlled conditions

In a tightly controlled metabolic-ward study, 19 adults with obesity ate diets with the same calories, cutting either carbohydrate or fat. The fat-restricted diet actually produced more body-fat loss (about 89 g a day vs 53 g a day over six days), a gap of roughly 36 g of fat a day. The researchers’ model predicted that over longer periods the body minimizes such differences. In other words, cutting carbs does not unlock special fat burning, and cutting fat does not either.

Appetite

People differ in which diet keeps them full. In one small inpatient study, 20 adults could eat as much as they liked of a plant-based, low-fat diet or an animal-based, ketogenic diet, each for two weeks. On the low-fat diet they ate about 689 kcal a day less. That challenges the idea that low-carb eating automatically reduces appetite, but it was short and small, and appetite responses vary from person to person. Your own experience is a good guide here.

Medication and health conditions

If you take insulin or other glucose-lowering medicines, cutting carbohydrates sharply can lower your blood sugar and change how much medication you need. Talk to your doctor before a big change. The same applies if you have kidney disease or are pregnant.

How to choose between low carb and low fat

Pick the approach that fits your tastes, culture and schedule. Some questions that help:

• What do you like eating? If you love bread, rice, beans and fruit, a lower-fat approach will feel natural. If you prefer eggs, fish, yogurt, nuts and big salads, lower-carb may suit you.
• What does your family eat? A diet you can share at the table is easier to keep.
• How do you feel after a week or two? Energy, hunger and mood tell you a lot.
• What do your blood tests say? High LDL leans toward unsaturated fats and fiber, whichever carb level you choose.

Whichever you pick, a few rules keep it healthy:

1. Focus on quality. Both DIETFITS diets were “healthy” versions that emphasized diet quality. Build meals around vegetables, fruit, legumes or whole grains, fish, eggs, dairy, nuts and olive oil rather than processed snacks.
2. Keep protein adequate. Our guide on how much protein per day gives targets.
3. Watch calories, even loosely. Neither diet works if total intake stays the same. Our guide to macros for weight loss shows how carbs, fat and protein add up to your calorie goal.
4. Track for a few weeks. Logging meals, for example by photographing them in PixNutri, shows your real carb and fat grams next to calories. The AI gives estimates, so adjust portions and add hidden oils or sauces yourself when you know better.

FAQ

Is low carb or low fat better for losing belly fat?

Neither targets belly fat specifically. Both reduce overall body fat when they create a calorie deficit, and waist size falls as total weight falls.

Why do people lose weight faster at the start of low carb?

Very low-carb eating has a diuretic effect, so the scale often drops quickly in the first week or two. Much of that early loss is water rather than fat, and the pace slows afterwards.

Does DIETFITS prove low carb doesn’t work?

No. It shows that a healthy low-carb diet and a healthy low-fat diet worked about equally well. Both led to average losses of 5 to 6 kg in a year.

Can I combine low carb and low fat?

Cutting both sharply usually leaves protein as the main energy source, which is hard to sustain. A moderate balance of all three macronutrients, with fewer total calories, is a reasonable middle ground.