GUIDES
The best diet for weight loss
What the trials comparing popular diets actually found, explained simply, and what it means for you
How scientists compare diets
The best evidence comes from randomized trials, in which volunteers are assigned a diet by chance, and from meta-analyses, which pool many trials together. A few words come up again and again in this research:
- Randomized trial
- People are assigned by chance to one diet or another, so the groups are alike at the start. The best way to test cause and effect.
- Meta-analysis
- A study that pools the results of many trials to get a more reliable answer than any one of them.
- Network meta-analysis
- A meta-analysis that compares many diets at once, even ones never tested against each other directly.
- Observational study
- Researchers follow what people eat and what happens to their health, without assigning a diet. It shows links, not proof of cause.
- Metabolic ward study
- Volunteers live in a research center where every bite is measured. Very precise, but short and small.
- Adherence
- How closely people actually follow the diet they were given.
What the biggest reviews found
Key study
14 popular diets, similar results
- Network meta-analysis
- 121 trials, 21,942 adults
- 6 to 12 months
Researchers compared popular diets such as Atkins, the Mediterranean diet, Weight Watchers and Ornish with eating as usual. At 6 months, low-carb diets led to about 4.6 kg more weight loss and low-fat diets about 4.4 kg more. By 12 months, the weight loss had shrunk on all of them, and the improvements in heart risk factors had mostly disappeared, except for the Mediterranean diet’s lower LDL cholesterol.
Key study
Low-carb and low-fat, about 8 kg each
- Network meta-analysis
- 48 trials, 7,286 adults
- 6 to 12 months
Compared with no diet, low-carb programs led to about 8.7 kg of weight loss at 6 months and low-fat programs about 8.0 kg. At 12 months both were about 7.3 kg. The differences between named diets were small.
In plain terms: almost any structured diet works better than doing nothing, and none of them is clearly better than the others for weight. This was published in two of the most respected medical journals, the BMJ in 2020 and JAMA in 2014.
What head-to-head trials found
Key study
Low-carb vs low-fat, tied after a year
- Randomized trial
- 609 adults
- 12 months
In the DIETFITS trial at Stanford, adults with overweight followed a healthy low-carb or a healthy low-fat diet, both based on whole foods. After a year, the low-carb group had lost 6.0 kg and the low-fat group 5.3 kg, a difference too small to matter. The researchers also tested whether genes or insulin levels predicted who would do better on which diet: they didn’t.
Key study
Mediterranean, low-carb and low-fat over two years
- Randomized trial
- 322 adults
- 2 years
In the DIRECT trial in Israel, the low-fat group lost about 2.9 kg, the Mediterranean group 4.4 kg and the low-carb group 4.7 kg. About 85% of participants were still following their diet after two years, helped by meals at their workplace.
Other long trials found the same thing. In the POUNDS LOST trial, published in the New England Journal of Medicine in 2009, four diets with very different amounts of fat, protein and carbohydrate led to similar weight loss after two years 5, and a two-year comparison of low-carb and low-fat diets found about 7 kg of weight loss in both groups 6.
Why sticking with it matters most
Key study
Adherence, not the diet, predicted weight loss
- Randomized trial
- 160 adults
- 1 year
Adults were assigned to the Atkins, Ornish, Weight Watchers or Zone diet. Average weight loss was modest and similar on all four, about 2 to 3 kg. What predicted weight loss was how closely people followed their diet, whichever it was. About half dropped out of the strictest diets, Atkins and Ornish, and about a third of the others.
In POUNDS LOST, people who attended more group sessions lost more weight, whatever diet they were on 5. So choosing a diet you enjoy and can fit into your life isn’t a weak choice: it’s the one the evidence supports.
The eight diets at a glance
| Diet | What changes | Strongest evidence | Hardest part |
|---|---|---|---|
| Mediterranean | More plants, olive oil and fish | Heart health | Portions of oil and nuts |
| DASH | More fruit, vegetables and dairy, less salt | Blood pressure | Cutting salt |
| Low-carb | Less sugar and starch | Weight loss, like low-fat; type 2 diabetes | Social meals |
| Keto | Very few carbs, mostly fat | Slightly more weight loss than low-fat | Keeping it up |
| High-protein | Protein at every meal | Keeping muscle, feeling full | Cost |
| Plant-based | Mostly or only plants | Weight loss; heart health in long studies | B12 and planning protein |
| Calorie counting | A daily calorie target | Weight loss | Logging every day |
| Intermittent fasting | When you eat | Weight loss, like counting | The first weeks |
Beyond the scale
If your goal isn’t only weight, the choice matters more. DASH has the strongest evidence for lowering blood pressure: in its original trial it lowered blood pressure by 11.4/5.5 mmHg in people with hypertension 8. The Mediterranean diet has the strongest evidence for preventing heart attacks and strokes, from the PREDIMED and CORDIOPREV trials 9,10.
Myths and facts
Myth Low-carb diets burn more fat.
Fact When calories are the same, they don’t. In a metabolic ward study, fat loss actually slowed when people switched to keto 11. People often lose weight on low-carb because they eat less.
Myth There’s a diet that matches your genes.
Fact In DIETFITS, genetic patterns thought to predict success on low-fat or low-carb diets didn’t predict anything 3.
Myth The faster you lose weight, the better.
Fact Fast loss is harder to keep up and leaves your appetite higher afterwards 12. About 0.5 kg (1 lb) a week is a pace most people can sustain.
Myth You have to cut out a whole food group.
Fact Diets with very different amounts of fat, protein and carbohydrate lead to similar weight loss 5. No food group has to go.
Questions
What is the fastest way to lose weight?
A larger calorie deficit gives faster loss at first, but it’s harder to keep up, costs more muscle and leaves you hungrier. About 0.5 kg (1 lb) a week is a pace most people can sustain.
Is low-carb or low-fat better?
For weight loss over a year, neither: they lead to similar results 3,2. Choose the one you prefer.
Which diet is healthiest?
The Mediterranean diet and DASH have the strongest evidence for heart health and blood pressure. Both are built on vegetables, fruit, whole grains, legumes and nuts.
Do I need a diet with a name?
No. Any way of eating that lowers your calories with enough protein, fiber and whole foods works. A named diet just gives you rules that make that easier.
Sources
- 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;369:m696. doi.org/10.1136/bmj.m696
- 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;312(9):923-933. doi.org/10.1001/jama.2014.10397
- 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. doi.org/10.1001/jama.2018.0245
- Shai I, Schwarzfuchs D, Henkin Y, et al. Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet. N Engl J Med. 2008;359(3):229-241. doi.org/10.1056/NEJMoa0708681
- Sacks FM, Bray GA, Carey VJ, et al. Comparison of Weight-Loss Diets with Different Compositions of Fat, Protein, and Carbohydrates. N Engl J Med. 2009;360(9):859-873. doi.org/10.1056/NEJMoa0804748
- Foster GD, Wyatt HR, Hill JO, et al. Weight and Metabolic Outcomes After 2 Years on a Low-Carbohydrate Versus Low-Fat Diet: A Randomized Trial. Ann Intern Med. 2010;153(3):147-157. doi.org/10.7326/0003-4819-153-3-201008030-00005
- Dansinger ML, Gleason JA, Griffith JL, et al. Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for Weight Loss and Heart Disease Risk Reduction: A Randomized Trial. JAMA. 2005;293(1):43-53. doi.org/10.1001/jama.293.1.43
- Appel LJ, Moore TJ, Obarzanek E, et al. A Clinical Trial of the Effects of Dietary Patterns on Blood Pressure. N Engl J Med. 1997;336(16):1117-1124. doi.org/10.1056/NEJM199704173361601
- Estruch R, Ros E, Salas-Salvadó J, et al. Primary Prevention of Cardiovascular Disease with a Mediterranean Diet Supplemented with Extra-Virgin Olive Oil or Nuts. N Engl J Med. 2018;378(25):e34. doi.org/10.1056/NEJMoa1800389
- Delgado-Lista J, Alcala-Diaz JF, Torres-Peña JD, et al. Long-term secondary prevention of cardiovascular disease with a Mediterranean diet and a low-fat diet (CORDIOPREV): a randomised controlled trial. Lancet. 2022;399(10338):1876-1885. doi.org/10.1016/S0140-6736(22)00122-2
- Hall KD, Chen KY, Guo J, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Am J Clin Nutr. 2016;104(2):324-333. doi.org/10.3945/ajcn.116.133561
- Sumithran P, Prendergast LA, Delbridge E, et al. Long-Term Persistence of Hormonal Adaptations to Weight Loss. N Engl J Med. 2011;365(17):1597-1604. doi.org/10.1056/NEJMoa1105816
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dieting.app is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes, kidney disease or a history of eating disorders, or take medication, talk to your doctor before changing how you eat.