Diets · 6 min read ·

Keto, carnivore, vegan, Mediterranean: what studies say

Every diet has its fans and success stories. How they did in randomised trials — and which one has no trials yet.

In a nutshell

1Keto and low-carb

A ketogenic diet limits carbohydrates to no more than 50 g a day, so the body burns mainly fat instead of glucose. Milder low-carb diets get up to a fifth of their energy from carbohydrates.1,2

Moderate evidence −0.9 kg A summary of randomised trials lasting at least a year (1,415 people): people on a ketogenic diet lost 0.9 kg more than on a low-fat diet. Triglycerides and blood pressure fell and “good” HDL rose — but so did “bad” LDL cholesterol.1
Moderate evidence −2.2 kg A summary of 11 trials with 1,369 people lasting at least six months: people on a low-carb diet lost 2.2 kg more than on a low-fat diet, but their LDL cholesterol rose. The authors say the benefit has to be weighed against the higher LDL.2

2Carnivore

The carnivore diet leaves out all plant foods: people eat meat, fish, eggs and sometimes dairy. As of the date of writing, we found no randomised trial in PubMed comparing it with another diet.

Weak evidence — a social media survey 2,029 people A survey of 2,029 people who had eaten carnivore for at least six months: 85% ate red meat daily, fewer than 10% ate vegetables, fruit or grains more often than once a month and 37% took no vitamins. Most reported better health and lower weight, but their LDL cholesterol was markedly raised (172 mg/dL, about 4.5 mmol/L).3

A survey like this only captures enthusiasts who stayed on the diet; it says nothing about people it did not suit or about long-term risk. A diet without plants also contains no fibre at all.

3Vegetarian and vegan

Moderate evidence −2.0 kg A summary of 12 randomised trials with 1,151 people: people on a vegetarian diet lost 2.0 kg more than on a non-vegetarian diet, and on a vegan diet 2.5 kg more.4
Weak evidence — observational studies −25% A summary of observational studies: vegetarians had a 25% lower risk of ischaemic heart disease and 8% fewer cancers than people who eat meat. There was no difference in overall mortality. But vegetarians often live more healthily in other ways too.5

A vegan diet has no reliable source of vitamin B12, so it needs to be supplemented — more in the article Vitamins.

4Mediterranean

Moderate evidence — a large randomised trial −30% The PREDIMED trial: 7,447 Spaniards at high risk of heart disease. On a Mediterranean diet with extra-virgin olive oil or nuts, they had about 30% fewer heart attacks, strokes and deaths from them over almost 5 years than with advice to eat less fat. Because of randomisation errors at some sites, the trial was reanalysed; the result stayed similar.6
Moderate evidence — 121 studies 12 months In a comparison of 121 trials, the Mediterranean diet was the only one whose improvement in heart disease risk factors lasted after a year.7

What to take from this

Sources

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

  1. Bueno N. B. et al. (2013). Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr 110:1178–1187. doi.org/10.1017/S0007114513000548
  2. Mansoor N. et al. (2016). Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials. Br J Nutr 115:466–479. doi.org/10.1017/S0007114515004699
  3. Lennerz B. S. et al. (2021). Behavioral characteristics and self-reported health status among 2029 adults consuming a “carnivore diet”. Curr Dev Nutr 5:nzab133. doi.org/10.1093/cdn/nzab133
  4. Huang R. Y. et al. (2016). Vegetarian diets and weight reduction: a meta-analysis of randomized controlled trials. J Gen Intern Med 31:109–116. doi.org/10.1007/s11606-015-3390-7
  5. Dinu M. et al. (2017). Vegetarian, vegan diets and multiple health outcomes: a systematic review with meta-analysis of observational studies. Crit Rev Food Sci Nutr 57:3640–3649. doi.org/10.1080/10408398.2016.1138447
  6. Estruch R. et al. (2018). Primary prevention of cardiovascular disease with a Mediterranean diet supplemented with extra-virgin olive oil or nuts. N Engl J Med 378:e34. doi.org/10.1056/NEJMoa1800389
  7. 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

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.