Low-carb vs low-fat diets: what the trials since Atkins found, from DIRECT and DIETFITS to four meta-analyses and NIH ward studies, plus blood fats and safety.
Fact-checked against FDA labels and official sources; not yet reviewed by a licensed clinician. This page is education, not medical advice. Talk to your doctor before starting, stopping or changing any treatment.
Key takeaways
- In DIETFITS, 609 adults lost 5.3 kg on a healthy low-fat diet and 6.0 kg on a healthy low-carb diet in a year, a difference that was not significant.[1]
- Across year-long weight-loss trials, low-carb led to about 1 kg more weight loss than low-fat (1.15 kg in one analysis, 0.91 kg for keto in another).[7]
- Compared with no diet, low-carb and low-fat diets led to almost identical weight loss at 12 months (7.25 vs 7.27 kg) in a 48-trial analysis.[6]
- Low-fat diets lowered LDL cholesterol more, while low-carb diets raised HDL.[8]
- In a metabolic ward, cutting fat led to more body fat loss per day than cutting the same calories from carbs (89 vs 53 g).[9]
In short: After five decades of trials, low-carb and low-fat diets lead to similar weight loss when people follow them well. Pooled analyses of year-long trials find low-carb ahead by about 1 kg (2 pounds), a small gap that shrinks over time, and the largest single trial of healthy versions found no significant difference. The two differ more in blood fats: low-carb tends to raise HDL and lower triglycerides more, while low-fat lowers LDL (“bad”) cholesterol more.
This page walks through the trials that have tested low-carb against low-fat eating since the 1970s, with the numbers each one reported. It does not name a winner. For the strictest form of low-carb, see our keto diet guide; for every eating pattern in one place, start at the diets hub; and for a related head-to-head, see keto vs Mediterranean. This is general education, not a personal plan. If you take medicines for diabetes, blood pressure or cholesterol, talk with your clinician before a big change in what you eat.
The two diets in one minute
| Low-carb | Low-fat | |
|---|---|---|
| The core idea | Cut carbohydrates (sugars, starches, grains), eat more fat and protein | Cut fat, especially added fats and fatty foods, eat more carbohydrate |
| How strict | From moderate (30% of calories from carbs in DIETFITS) to ketogenic (no more than 50 grams of carbs a day) | Usually under 30% of calories from fat; very low-fat versions go near 10% |
| In the DIETFITS trial (12 months) | 30% carbs, 45% fat, 23% protein | 48% carbs, 29% fat, 21% protein |
| Weight loss in long trials | Similar; about 1 kg more in pooled analyses | Similar; matched other diets of the same intensity |
| Blood fats | HDL up, triglycerides down; LDL can rise | LDL down more |
| Best-known caution | Very low-carb plans need care with some diabetes medicines and health conditions | Few specific cautions in the trials; the healthy version in DIETFITS emphasized diet quality |
Both labels cover a wide range. A “low-carb” diet in one trial may be a ketogenic plan with almost no bread or fruit, and in another a moderate plan that still includes beans and whole grains. That range is one reason trial results differ, and why reading the details matters more than the headline.
Fifty years in six steps
Low-carb vs low-fat: the research timeline
- 1972Dr. Atkins' Diet Revolution popularizes cutting carbs while eating fat and protein freely.
- 2003The first randomized trial of the Atkins diet: more weight lost at 6 months, no significant difference at a year.
- 2006The Women's Health Initiative: a low-fat pattern did not cause weight gain in 48,835 women.
- 2008DIRECT: a 2-year trial of low-fat, Mediterranean and low-carb diets.
- 2013 to 2020Meta-analyses pool dozens of trials; low-carb ahead by about 1 kg at a year, or no real difference.
- 2015 to 2021NIH metabolic-ward studies test the insulin theory directly; DIETFITS (2018) finds no significant difference.
The early trials: Atkins meets the low-fat era
Robert Atkins’ book Dr. Atkins’ Diet Revolution (copyright 1972) made low-carb eating famous. Its idea was simple: cut carbohydrates sharply and eat protein and fat freely. Low-fat eating was widely promoted in the decades that followed, and some low-carb supporters argued that low-fat, high-carb diets were partly to blame for rising obesity. Our history of diet fads tells the wider story.
For a long time there were no randomized trials to settle it. The first randomized trial of the Atkins diet appeared in the New England Journal of Medicine in 2003. In 63 adults with obesity, the low-carb group lost more at 3 months (6.8% vs 2.7% of body weight) and 6 months (7.0% vs 3.2%), but at 12 months the gap (4.4% vs 2.5%) was no longer significant. HDL rose and triglycerides fell more on low-carb. The authors noted that adherence was poor and dropout high in both groups.
The claim that low-fat eating causes weight gain was tested in the Women’s Health Initiative, which randomly assigned 48,835 postmenopausal women to a low-fat eating pattern or their usual diet, with no weight-loss or calorie goals. The low-fat group lost 2.2 kg in the first year and stayed slightly lighter over 7.5 years (0.4 kg lighter at the end). There was no tendency toward weight gain.
The long head-to-head trials
DIRECT, New England Journal of Medicine 2008 (2 years)
This trial assigned 322 adults with obesity (average age 52, average BMI 31, 86% men) to a calorie-restricted low-fat diet, a calorie-restricted Mediterranean diet or a low-carb diet without a calorie limit. Average weight loss over 2 years was 2.9 kg with low-fat, 4.4 kg with Mediterranean and 4.7 kg with low-carb; among the 272 who finished, 3.3, 4.6 and 5.5 kg. The ratio of total to HDL cholesterol improved 20% on low-carb and 12% on low-fat. The authors suggested that personal preferences and metabolic factors could guide the choice. Our keto vs Mediterranean comparison covers the Mediterranean side of this trial.
DIETFITS, JAMA 2018 (12 months)
DIETFITS is the largest trial to compare healthy versions of both diets. 609 adults aged 18 to 50 without diabetes (average BMI 33) were asked to cut either fat or carbohydrate as far as they could sustain, while focusing on diet quality, through 22 small-group sessions over a year. 79% completed the trial. Weight loss was 5.3 kg on healthy low-fat and 6.0 kg on healthy low-carb; the 0.7 kg difference was not significant. Here is how far apart the two diets actually were at 12 months:
| Item | Healthy low-fat | Healthy low-carb |
|---|---|---|
| Carbohydrate | 48% | 30% |
| Fat | 29% | 45% |
| Protein | 21% | 23% |
Average intakes reported at 12 months. Weight loss: 5.3 kg (low-fat) vs 6.0 kg (low-carb), not significantly different.
Source: Gardner CD et al., JAMA 2018 (DIETFITS, 609 adults) (checked on October 7, 2026)
DIETFITS also tested two popular ideas about matching a diet to the person: a genetic pattern thought to predict which diet suits you, and how much insulin your body releases after sugar. Neither one predicted who lost more on which diet. Adverse events were few (18) and evenly spread across both groups.
What the meta-analyses show
Meta-analyses pool many trials to get a steadier answer. The four below asked slightly different questions, so their numbers are not interchangeable:
- Bueno et al., British Journal of Nutrition 2013: 13 trials of 12 months or more compared very low-carb ketogenic diets (no more than 50 grams of carbs a day) with low-fat diets (under 30% of calories from fat). Keto led to 0.91 kg more weight loss.
- Johnston et al., JAMA 2014: 48 trials of named diets, 7,286 people. Compared with no diet, low-carb diets led to 8.73 kg lost at 6 months and 7.25 kg at 12; low-fat diets to 7.99 and 7.27 kg. The authors concluded that any diet a person sticks with is a reasonable recommendation.
- Tobias et al., Lancet Diabetes and Endocrinology 2015: 53 studies, 68,128 people, all lasting a year or more. In weight-loss trials, low-carb beat low-fat by 1.15 kg. Low-fat did no better than other higher-fat diets, and beat only a usual diet. The authors concluded that, against diets of similar intensity, the evidence does not support low-fat over other approaches.
- Ge et al., BMJ 2020: 121 trials, 21,942 people, 14 named diets. Compared with usual diet at 6 months, low-carb led to 4.63 kg lost and low-fat to 4.37 kg, a similar effect. Weight loss diminished by 12 months for every pattern.
Here are those results as a simplified forest-style summary: each bar is the extra weight lost on low-carb compared with low-fat in one study or analysis. They come from different designs and lengths, so they are not head-to-head with each other.
Put together, the picture is consistent: a small edge for low-carb in some analyses, about 1 kg (2 pounds) after a year, and no meaningful difference in others. For comparison, the gap between any of these diets and no diet at all was several kilograms. The choice between them matters much less than finding one you can keep, which is also the conclusion of our ranking of diets for weight loss. Over time, results for both drift back: by 12 months, the Ge analysis found smaller weight losses for every pattern.
| Item | Low-carb diets | Low-fat diets |
|---|---|---|
| 6 months | 8.73 kg | 7.99 kg |
| 12 months | 7.25 kg | 7.27 kg |
Network meta-analysis estimates relative to no diet, adjusted for behavioral support and exercise.
Source: Johnston BC et al., JAMA 2014 (48 trials, 7,286 people) (checked on October 7, 2026)
Inside a metabolic ward: the NIH studies
One popular explanation for low-carb weight loss is the “carbohydrate-insulin model”: carbs raise insulin, insulin drives fat storage and hunger, so cutting carbs should burn more fat and curb appetite. Kevin Hall’s team at the National Institutes of Health tested this in people who lived on a research ward, where every bite was measured.
2015, Cell Metabolism. 19 adults with obesity had the same number of calories cut from either carbohydrate or fat for 6 days, on two separate stays. Cutting carbs did raise fat burning, but cutting fat led to more body fat lost per day: 89 grams versus 53. The researchers’ models predicted that over longer periods the body would narrow these differences.
| Item | Value |
|---|---|
| Cutting fat | 89 g |
| Cutting carbohydrate | 53 g |
Calculated from fat intake and fat burned in a metabolic chamber. A short, tightly controlled study; it does not predict long-term weight loss.
Source: Hall KD et al., Cell Metabolism 2015 (19 adults with obesity) (checked on October 7, 2026)
2021, Nature Medicine. 20 adults ate as much as they wanted of a plant-based, low-fat diet (10% fat, 75% carbs) for 2 weeks and an animal-based ketogenic diet (76% fat, 10% carbs) for 2 weeks, in random order. On the low-fat diet they ate 689 fewer calories a day over the 2 weeks, and 544 fewer in the final week. One person withdrew because of low blood sugar on the low-carb diet. The authors concluded the results did not fit the carbohydrate-insulin model’s predictions.
These ward studies are short and small, and real life is different: in free-living trials like DIRECT and DIETFITS, low-carb did as well as or slightly better than low-fat. What they show is that low-carb has no special metabolic advantage, and that a calorie deficit is still what drives fat loss. Our guide to metabolism and weight explains why.
Heart and blood markers
The clearest differences between the two diets are in blood fats, and they point in different directions:
Blood fats: low-carb vs low-fat
- 7.08 vs 1.01 mg/dLLDL drop with low-fat vs low-carb diets compared with usual diet at 6 monthsGe L et al., BMJ 2020
- 2.31 mg/dLHDL rise with low-carb diets at 6 months; low-fat diets did not raise HDLGe L et al., BMJ 2020
- 0.12 mmol/Lhigher LDL (about 4.6 mg/dL) on very low-carb keto than low-fat after 12+ months, with lower triglycerides and higher HDLBueno NB et al., Br J Nutr 2013
- 20% vs 12%improvement in the total-to-HDL cholesterol ratio, low-carb vs low-fat, over 2 yearsShai I et al., NEJM 2008 (DIRECT)
Checked on October 7, 2026
Very low-carb plans also lowered diastolic blood pressure slightly (1.43 mmHg) in the Bueno analysis, and low-carb and low-fat reduced blood pressure similarly in the Ge analysis. How these changes add up for heart disease over decades is not known from these trials, which measured markers rather than heart attacks. If your LDL is high, or you have a family history of high cholesterol, ask your clinician whether to check your lipids after a few months on a new diet. For blood sugar, our guides to insulin resistance and type 2 diabetes and weight go further.
Safety: who should take care
The safety cautions in the research center on very low-carb (ketogenic) plans. A 2026 review in Annals of Medicine lists conditions that call for caution or avoidance, including acute pancreatitis, advanced liver or kidney disease, familial hypercholesterolemia (inherited high cholesterol), gallbladder disease, pregnancy or breastfeeding and being underweight. It flags interactions with SGLT2 inhibitors, insulin, sulfonylureas, GLP-1 medicines, blood pressure medicines and diuretics, and lists the common side effects as short-lived “keto flu” (tiredness, headache, nausea), digestive upset, passing more urine and low blood sugar.
The current prescribing information for Jardiance (empagliflozin), an SGLT2 inhibitor medicine, lists a ketogenic diet among the conditions that can bring on ketoacidosis, which can be life-threatening. On the low-fat side, the trials here reported few specific risks; the healthy low-fat diet in DIETFITS focused on diet quality, not just on cutting fat.
What people weigh up
Since weight loss is similar, most people decide on other grounds. These are the trade-offs worth discussing with your clinician or a registered dietitian:
- What you enjoy eating: the plan that fits your tastes and culture is the one you will keep. Johnston’s 48-trial analysis concluded that any diet a person will stick with is a reasonable recommendation.
- Your blood fats: low-carb tended to improve triglycerides and HDL, low-fat tended to lower LDL more. Your own numbers may tip the balance.
- Medicines: very low-carb eating can change how some diabetes medicines act; plan any change with your prescriber.
- Middle-ground options: the Mediterranean diet did as well as low-carb in DIRECT, and the Mediterranean diet guide and DASH diet guide describe two patterns that sit between the extremes. A high-protein diet is another route many people find filling.
- Quality either way: both versions in DIETFITS emphasized diet quality, aiming for the lowest fat or carb intake people could keep up for the long term rather than the lowest possible.
To see your current split of carbs, fat and protein, try the macro calculator, and for ideas on keeping hunger down on any plan, see our list of filling foods. If you are weighing timing-based approaches too, our intermittent fasting vs calorie counting comparison applies the same evidence test. Common beliefs about carbs and fat are checked in our weight-loss myths roundup, and for life after the diet, see maintaining weight loss. More side-by-side pages are in our comparisons hub.
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- Given my cholesterol and blood sugar results, does a low-carb or low-fat approach suit me better?
- Do any of my medicines need adjusting if I cut carbs sharply?
- Should my lipids be rechecked after a few months on a new diet?
- How do I keep fiber and nutrients up on the plan I pick?
- Could a registered dietitian help me build a version I can keep long term?
Frequently asked questions
Is low-carb or low-fat better for weight loss?
Neither clearly. The largest trial of healthy versions, DIETFITS, found no significant difference at a year. Pooled analyses show low-carb ahead by about 1 kg, a gap that shrinks over time. Sticking with the plan matters more than which one.
Does low-carb burn more fat because of insulin?
NIH metabolic-ward studies did not support that idea. With the same calorie cut, cutting fat led to more body fat loss per day, and people ate fewer calories on a low-fat diet than on a ketogenic one.
Which diet is better for cholesterol?
They help different numbers. Low-fat diets lowered LDL cholesterol more; low-carb diets raised HDL and lowered triglycerides more, and very low-carb diets raised LDL slightly. Ask your clinician which of your numbers matters most.
Can genes or insulin levels tell me which diet suits me?
Not according to DIETFITS. Neither a genetic pattern nor insulin release after sugar predicted who lost more weight on low-fat or low-carb.
Is a low-carb diet safe with diabetes medicines?
It needs care. Very low-carb eating can interact with insulin, sulfonylureas and SGLT2 inhibitors, and the Jardiance label lists a ketogenic diet as a trigger for ketoacidosis. Plan any change with your prescriber.
References
- 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 (DIETFITS randomized clinical trial). JAMA, 2018. doi:10.1001/jama.2018.0245 · PMID 29466592 · NCT01826591 (accessed October 7, 2026) Randomized trial
- Shai I, Schwarzfuchs D, Henkin Y, et al.. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet (DIRECT). New England Journal of Medicine, 2008. doi:10.1056/NEJMoa0708681 · PMID 18635428 · NCT00160108 (accessed October 7, 2026) Randomized trial
- Foster GD, Wyatt HR, Hill JO, et al.. A randomized trial of a low-carbohydrate diet for obesity. New England Journal of Medicine, 2003. doi:10.1056/NEJMoa022207 · PMID 12761365 (accessed October 7, 2026) Randomized trial
- Howard BV, Manson JE, Stefanick ML, et al.. Low-fat dietary pattern and weight change over 7 years: the Women’s Health Initiative Dietary Modification Trial. JAMA, 2006. doi:10.1001/jama.295.1.39 · PMID 16391215 · NCT00000611 (accessed October 7, 2026) Randomized trial
- Bueno NB, de Melo IS, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British Journal of Nutrition, 2013. doi:10.1017/S0007114513000548 · PMID 23651522 (accessed October 7, 2026) Meta-analysis
- Johnston BC, Kanters S, Bandayrel K, et al.. Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. JAMA, 2014. doi:10.1001/jama.2014.10397 · PMID 25182101 (accessed October 7, 2026) Meta-analysis
- 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. Lancet Diabetes and Endocrinology, 2015. doi:10.1016/S2213-8587(15)00367-8 · PMID 26527511 (accessed October 7, 2026) Meta-analysis
- Ge L, Sadeghirad B, Ball GDC, et al.. Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trials. BMJ, 2020. doi:10.1136/bmj.m696 · PMID 32238384 (accessed October 7, 2026) Meta-analysis
- Hall KD, Bemis T, Brychta R, et al.. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metabolism, 2015. doi:10.1016/j.cmet.2015.07.021 · PMID 26278052 (accessed October 7, 2026) Randomized trial
- Hall KD, Guo J, Courville AB, et al.. Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake. Nature Medicine, 2021. doi:10.1038/s41591-020-01209-1 · PMID 33479499 · NCT03878108 (accessed October 7, 2026) Randomized trial
- Dyńka D, Rodzeń Ł, Rodzeń M, et al.. The ketogenic diet is not for everyone: contraindications, side effects, and drug interactions. Annals of Medicine, 2026. doi:10.1080/07853890.2025.2603016 · PMID 41486865 (accessed October 7, 2026) Review
- JARDIANCE (empagliflozin) tablets: prescribing information and patient information. Boehringer Ingelheim Pharmaceuticals via DailyMed (U.S. National Library of Medicine). (accessed October 7, 2026) Drug label
- Atkins RC.. Dr. Atkins’ Diet Revolution (New York: McKay, [1973, c1972]), NLM Catalog record 355420. National Library of Medicine catalog, 1972. (accessed October 7, 2026) Other
Facts checked on October 7, 2026
Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.