Few nutrition debates have lasted as long as low-carb vs. low-fat. Each camp has passionate supporters, and each can point to a study that appears to prove its case. The more useful question is what happens when the two approaches are compared directly, in large trials, over a long enough period to matter. Several such studies now exist, and their conclusions are more consistent than the online arguments suggest.
This article compares the two approaches using the strongest available evidence: one of the largest randomized trials ever run on the question, a meta-analysis of 53 trials, and controlled feeding research that removes the guesswork from what people actually ate.
What "Low-Carb" and "Low-Fat" Actually Mean
Neither term has a single official definition, which is one reason results can seem contradictory. In general, a low-fat diet limits fat to a smaller share of daily calories and relies more on carbohydrates, while a low-carb diet limits carbohydrates and allows more fat. Very low-carb diets such as keto sit at the extreme end of the second category and are a separate topic from the moderate approaches compared here.
In the trial discussed below, participants in the low-fat group ended up eating about 48% of calories from carbohydrates and 29% from fat, while the low-carb group ended up at about 30% carbohydrates and 45% fat, with protein similar in both (about 21% vs. 23%). That is a real difference in eating pattern, but notably not an extreme one for the low-carb group.
The Largest Direct Comparison: The DIETFITS Trial
The DIETFITS randomized clinical trial (Gardner et al., published in JAMA in 2018) assigned 609 overweight adults to either a "healthy low-fat" or "healthy low-carbohydrate" diet for 12 months, with 481 participants (79%) completing the trial. The results at 12 months:
| Measure | Healthy Low-Fat Diet | Healthy Low-Carb Diet |
|---|---|---|
| Participants randomized | 305 | 304 |
| Average weight change at 12 months | -5.3 kg (about 11.7 lb) | -6.0 kg (about 13.2 lb) |
| Carbohydrate intake achieved | 48% of calories | 30% of calories |
| Fat intake achieved | 29% of calories | 45% of calories |
The difference between the groups was 0.7 kg, and it was not statistically significant (95% confidence interval -0.2 to 1.6 kg), meaning the data are consistent with no true difference at all. Put simply, both diets worked about equally well on average.
The trial also tested a popular idea: that certain people respond better to one diet based on their genetics or their insulin response to glucose. The researchers found no significant diet-genotype interaction (P = 0.20) and no significant diet-insulin secretion interaction (P = 0.47), concluding that neither genotype pattern nor baseline insulin secretion predicted which diet worked better for an individual.
What 53 Trials Together Show
A systematic review and meta-analysis by Tobias et al., published in The Lancet Diabetes and Endocrinology in 2015, pooled 53 randomized trials with 68,128 participants. It found no significant difference in average weight loss between low-fat diets and other diets. When low-fat diets were compared specifically with low-carb diets after a follow-up of a year or more, participants on low-carb diets were about 2.5 pounds (roughly 1.1 kg) lighter on average. That is a small difference relative to the total weight lost, and the analysis combined studies with differing designs, so it suggests a slight edge for low-carb in the pooled data rather than a decisive advantage.
What Controlled Feeding Studies Add
Real-world trials depend on what people report eating, which is notoriously imperfect. Controlled feeding studies avoid that problem by providing all of the food. A meta-analysis of such studies by Hall and Guo (Gastroenterology, 2017) compared diets with equal calories and protein but different carbohydrate and fat content. It found that diets predominating in carbohydrates or in fat had similar effects on energy expenditure and body fatness; if anything, lower-fat diets showed a very small advantage in fat loss per calorie (on the order of grams per day), which the researchers did not consider practically meaningful. The finding argues against the idea that carbohydrate restriction has a special "metabolic advantage" when calories are matched, though these studies are short and do not show how people behave outside a lab.
So Why Do Some People Succeed on One and Fail on the Other?
If averages are similar but individual experiences differ, the likeliest explanation is adherence and fit. In DIETFITS, both groups lost weight on average, but individual results varied widely within each group, which shows that the diet label tells you little about how any one person will do. A diet works when someone can follow it consistently for months and years, and that depends on factors such as food preferences, hunger, cultural eating patterns, budget, social situation, and how restrictive the plan feels.
The word "healthy" in the DIETFITS arms also matters. Both versions were designed to emphasize minimally processed, nutrient-dense foods rather than simply cutting a macronutrient. A low-fat diet built on refined snacks and a low-carb diet built on processed meats are very different experiences from the versions studied in the trial, and the quality of what fills the plate likely matters as much as the ratio.
Low-Carb vs. Low-Fat: Side-by-Side
| Factor | Low-Fat | Low-Carb |
|---|---|---|
| Average 12-month weight loss (DIETFITS) | -5.3 kg | -6.0 kg (difference not statistically significant) |
| Pooled trial data (Tobias 2015) | No better than other diets | About 2.5 lb lighter vs. low-fat at 1 year or more |
| Controlled feeding (Hall and Guo 2017) | Slight edge in fat loss per calorie, small | No special metabolic advantage at equal calories |
| Predicted by genotype or insulin response? | No (DIETFITS) | No (DIETFITS) |
| Best fit for | People who enjoy grains, fruit, and legumes and feel better with more carbohydrates | People who feel more satisfied with protein and fat and find cutting starches and sugar easier |
How to Choose: A Practical Approach
Since neither approach wins reliably, the evidence supports choosing based on what you can sustain. A few questions can help: Which foods would you miss least? Which pattern leaves you less hungry between meals? Which fits your household, culture, and budget? What have you been able to follow before, and what ended it? Many people also find that the best plan is a middle path, such as reducing refined carbohydrates and added sugars while keeping fruit, legumes, and whole grains, rather than adopting either extreme.
Whichever you choose, a few fundamentals show up across the research: prioritize minimally processed foods, keep protein adequate, include vegetables at most meals, and maintain a modest calorie deficit consistently over time. Our guides on protein targets, fiber intake, and maintaining weight loss cover those pieces in more detail.
Who Should Be Careful
Changing the carbohydrate or fat content of your diet can have medical implications for some people. Anyone with diabetes who takes insulin or other glucose-lowering medication can be at risk of low blood sugar if carbohydrates are cut without adjusting medication. People with kidney disease, liver disease, a history of pancreatitis or gallbladder problems, high cholesterol or a family history of it, or a history of disordered eating should discuss any significant change in diet with a clinician first. Low-carb diets can raise LDL cholesterol in some individuals while improving triglycerides, so lab monitoring is reasonable for people who already have cardiovascular risk. Pregnant and breastfeeding women have different nutritional needs, and this comparison does not address children or adolescents.
A Note on Using This Information
This article summarizes findings from published clinical research for general educational purposes and does not constitute medical or nutritional advice, a diagnosis, or a recommendation for any specific diet. The studies cited reported group averages in specific populations; individual results vary considerably and cannot be predicted from averages. Diet-related health outcomes depend on many factors, including age, sex, health status, medications, genetics, and the quality of the foods eaten, which a general article cannot assess.
Talk to a doctor or registered dietitian before starting a new diet, especially if you have diabetes, heart disease, kidney or liver disease, take prescription medications, are pregnant or breastfeeding, or have a history of an eating disorder. Seek medical attention promptly for symptoms such as persistent dizziness, fainting, palpitations, severe fatigue, or other symptoms that concern you after changing how you eat.
Primary sources referenced in this article:
1. Gardner CD, Trepanowski JF, Del Gobbo LC, et al. "Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial." JAMA, 2018;319(7):667-679: https://doi.org/10.1001/jama.2018.0245
2. Tobias DK, Chen M, Manson JE, Ludwig DS, Willett W, Hu FB. "Effect of low-fat diet interventions versus other diet interventions on long-term weight change in adults: a systematic review and meta-analysis." The Lancet Diabetes and Endocrinology, 2015: https://doi.org/10.1016/S2213-8587(15)00367-8
3. Hall KD, Guo J. "Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition." Gastroenterology, 2017: https://pubmed.ncbi.nlm.nih.gov/28193517/
4. U.S. Department of Agriculture and U.S. Department of Health and Human Services, Dietary Guidelines for Americans, 2020-2025: https://www.dietaryguidelines.gov/
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