Weight loss — what the science says

In a nutshell

Low-carb and low-fat diets lead to similar weight loss after six months; what matters is which one you can stick to.^2,3,4
More protein during energy restriction helps preserve muscle.⁵
Exercise adds only about a kilo on top of a diet, but it improves health even without weight loss.⁶

1Energy comes first

The body loses weight when it spends more energy than it takes in over the long term. Different diets are just different ways of creating that gap.¹

Moderate evidence — a model based on human measurements ~1 year Body weight responds slowly to a change in eating: according to a model built on measurements in adults, half of the final change happens in about a year. People with more body fat lose more with the same restriction and take longer to settle.¹

That is why weight loss slows down over time: a lighter body uses less energy, so the same menu creates a smaller gap.¹

2The type of diet matters little

Moderate evidence — 48 studies 8.7 / 8.0 kg A summary of 48 randomised trials with 7,286 overweight people: compared with no diet, people lost an average of 8.7 kg on a low-carb diet and 8.0 kg on a low-fat diet after six months. Differences between named diets were small.²

Moderate evidence — 121 studies 4.6 / 4.4 kg The largest comparison: 121 trials with 21,942 people. After six months, people on a low-carb diet lost 4.6 kg and on a low-fat diet 4.4 kg more than on their usual diet. After a year, weight loss was smaller with every diet.³

Moderate evidence — a large randomised trial −5.3 / −6.0 kg The DIETFITS trial: 609 adults ate a healthy low-fat or a healthy low-carb diet for a year. They lost 5.3 and 6.0 kg — the difference was not significant. Neither genes nor insulin secretion predicted who would do better on which diet.⁴

Keto, carnivore, vegan and Mediterranean diets are compared in the article Popular diets.

3What makes it easier

Moderate evidence +0.4 kg muscle A summary of 24 trials with 1,063 people: a higher-protein diet with the same energy intake led to 0.8 kg more weight loss and 0.9 kg more fat loss, and people kept 0.4 kg more fat-free mass.⁵

Moderate evidence — Cochrane −1.1 kg A review of 43 trials with 3,476 people: exercise together with diet led to 1.1 kg more weight loss than diet alone. Exercise improved blood pressure, blood fats and blood sugar even in people who did not lose weight.⁶

Weak evidence — 22 studies 22 studies Recording food and activity and weighing yourself regularly were consistently linked with better weight loss in 22 studies. But the studies had methodological weaknesses, so they do not prove cause and effect.⁷

4Keeping it off is harder than losing it

Moderate evidence −1.6 kg A summary of 45 trials with 7,788 people who had lost at least 5% of their weight: programmes focusing on both food and activity reduced weight regain after a year by 1.6 kg on average compared with control groups.⁸

The results of every diet shrink after a year.^2,3 So it pays to choose a way of eating you can keep up even after the “diet” is over.

What to take from this

Pick a diet you can stick to — low-carb and low-fat work about equally well.^2,3,4
Eat enough protein and add exercise; it helps your muscles and your health.^5,6
In ForkRep you set a goal and a deadline in “Profile and goals”; the app works out the daily deficit and never pushes the target below your basal expenditure.

Sources

Written on 9 Oct 2026, based on studies available at that date. Newer research may change the conclusions.

Hall K. D. et al. (2011). Quantification of the effect of energy imbalance on bodyweight. Lancet 378:826–837. doi.org/10.1016/S0140-6736(11)60812-X
Johnston B. C. et al. (2014). Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. JAMA 312:923–933. doi.org/10.1001/jama.2014.10397
Ge L. et al. (2020). 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 369:m696. doi.org/10.1136/bmj.m696
Gardner C. D. et al. (2018). 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 319:667–679. doi.org/10.1001/jama.2018.0245
Wycherley T. P. et al. (2012). Effects of energy-restricted high-protein, low-fat compared with standard-protein, low-fat diets: a meta-analysis of randomized controlled trials. Am J Clin Nutr 96:1281–1298. doi.org/10.3945/ajcn.112.044321
Shaw K. et al. (2006). Exercise for overweight or obesity. Cochrane Database Syst Rev CD003817. doi.org/10.1002/14651858.CD003817.pub3
Burke L. E., Wang J., Sevick M. A. (2011). Self-monitoring in weight loss: a systematic review of the literature. J Am Diet Assoc 111:92–102. doi.org/10.1016/j.jada.2010.10.008
Dombrowski S. U. et al. (2014). Long term maintenance of weight loss with non-surgical interventions in obese adults: systematic review and meta-analyses of randomised controlled trials. BMJ 348:g2646. doi.org/10.1136/bmj.g2646

This is general information and does not replace a doctor or dietitian. If you have diabetes, kidney disease or an eating disorder, or are pregnant, talk to a doctor before changing your diet.