Low carb diets: What is the evidence?

This blog follows on from this week's #FactFriday all about low carb diets.

Low carb diets are EVERYWHERE at the moment, and proponents of low carb high fat (LCHF) diets are adamant that it is the BEST way to eat for health.

The problem with "low carbers" is that, although they believe wholeheartedly in the diet, there is very limited science to back there claims.

First up- what is LCHF?

These are very low carbohydrate, high fat diets, where dieters consume 5-10% energy from carbohydrates, 15-30% from protein and 60-75% from fat. The aim of the game is to force your body to use fat as it's main source of energy, rather than carbohydrates. Advocates of keto claim that when the body is in a state of ketosis it "burns fat", not only from food but from your body's stores. It can take several days, up to a few weeks to reach a state of ketosis.

What does the research say?

Bueno et al. 2013- This meta-analysis (compilation of research findings) of randomised controlled trials found that there were no significant differences between low carb and control groups for blood pressure, blood glucose, insulin and HbA1c. Of the studies with 24 month follow-up (4 studies), any weight loss or health improvements were non-significant by month 24 = this means that although participants on the low carb diets lost weight initially, by month 24 they had already started to regain that weight.

Dashti et al. 2004- Participants (83) were put on a 24 week ketogenic diet. Measurements were taken at week 8, 16 and 24. The authors conclude that "the present study shows the beneficial effects of a long-term ketogenic diet". However, I strongly disgree- here's why:

  • 24 weeks is not "long-term". Half a year. How does this prove efficacy over a lifetime?

  • 83 is a small sample size and therefore not representative of the population (also the authors give no indication of drop out rate- it is unlikely all participants completed ALL 24 weeks).

  • Participants were given a multi-vitamin throughout the study period, this may have played a role in improved health parameters.

  • There was no control group- control groups are used for comparison of results, without one the results mean very little.

  • No indication was given of the support received by participants. In some dietary intervention studies, participants receive multiple sessions with researchers, GPs, dietitians etc. This can have an impact on the "success" of the diet.

  • The research team behind this paper are heavily invested in the ketogenic diet- this is evident in the way they write. Statements and claims are made without referring to the literature, indicating that they are based on opinion, rather than fact.

Merra et al. 2016- This pilot study of 3 weeks put participants on either a very low calorie ketogenic diet or a very low restricted calorie diet. Women in both groups were to consume 450-500kcal/day, while men were allowed to consume 650-700kcal/day. Those on the low cal keto diet had 35-40% energy from fat, 55-60% from protein and 5% from carbs. The non-keto group had 35-40% energy from fat, 15-20% from carbs and 45-50% from protein. 25 people enrolled on the study, but only 18 completed it. Both groups lost weight, but the keto group lost less than the non-keto group.

The problems with this research:

  • 3 weeks indicates nothing in terms of life-time risk of disease, weight loss maintenance etc.

  • The sample size was very small and had a high drop out rate, particularly given that the intervention was only 3 weeks.

  • Consuming less than 1000kcal/day is not only dangerous, but is also completely unrealistic for people to maintain.

  • With such a low intake of energy, it is no surprise that all participants lost weight. If we could follow up with the participants now, it is VERY unlikely that they maintained any weight loss.

Foster et al. 2003- This study compared the Atkins diet (LCHF) to a conventional diet (the Learn Program for Weight Management). 63 participants were enrolled, however only 37 completed the year long intervention (59% completion rate). Data was gathered on weight and metabolic markers at baseline (the beginning), 3 months, 6 months and 12 months. All participants met with a registered dietitian at the beginning of the study and were given information on the diet to read. Although the low carb diet resulted in greater immediate weight loss, this group saw a steady increase in weight from 6 months until the final measurements were taken. A similar trend was observed with the conventional diet, however the rate of weight regain was slower. No significant changes were noted for blood pressure or glucose-tolerance. The authors note that results are in line with benefits from pharmacological and behavioural interventions, which have better adherence rates.

Bravata et al. 2003- This systematic review concluded that there is insufficient evidence to make recommendations for or against a low carb diet. The studies they reviewed which resulted in the greatest weight loss were noted to have restricted calorie intake, had a longer duration and involved participants who were significantly overweight to begin with. The authors conclude that these factors may have more to do with the level of weight loss achieved, than the composition of the diet. It is worth noting that some of the diet interventions reviewed here lasted less then 15 days, although the average duration was 50 days.

Of course these are only a handful of studies, but based on my reading of literature on LCHF similar issues consistently arise:

  • Short study duration.

  • Few participants.

  • Many studies on LCHF have been carried out in animals- mice, rats and dogs. These findings must be interpreted with care, animal models do not directly correspond with humans.

  • Lack of control groups- if you can't compare the intervention to anything, what is the point? There is no way to account for any confounding factors that could have impacted the results.

One of the key problems with #LCHF is that our body is built to run on carbs and glucose is the only fuel our brain can utilise. We can turn other macronutrients into glucose, but it is a longer process and costs energy.

Because of this, there are lots of potentially serious side effects associated with the keto diet. This includes fatigue from a lack of energy, brain fog from a lack of carbs, bad breath caused by ammonia from protein digestion, constipation due to a lack of fibre, increased cholesterol and potentially increased blood pressure from a high intake of saturated fat, dizziness caused by a lack of carbs and cravings because your diet lacks carbs, food you enjoy and energy.

If that isn't enough to deter those considering #keto: remember that the weight lost on this diet is water weight, not fat. Carbohydrates are stored with water molecules in the body, so if you run out of carbs, you will lose water too. Therefore you will regain any weight lost on the diet once you return to normal eating. (NB: Lower WEIGHT, does not necessarily improve health anyway, but in particular if that weight is just water).

I'm sure further research will continue on LCHF diets, and the quality of the research will improve. Future research may well indicate that low carb does have health benefits, but for the moment I would not recommend this diet for weight loss or general health. The research is lacking and very short term, so we have limited knowledge of the potential dangers.

If you need help navigating diet and health information online, contact me and find out how I can help you.

Lots of love,

Little O x

#littleonutrition #nutritionadvice #nondietnutrition #antidiet #keto #LCHF #lowcarb #research #ketogenic #riotsnotdiets #weightloss


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Protecting the public and promoting high standards in evidence-based science and professional practice of nutrition.

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