The ketogenic diet is a very low-carbohydrate diet with a high fat content. It was originally developed to treat epilepsy and in that context, it is recommended that 87%-90% of an individual’s daily calories come from fat with the remaining 10-13% being a mix of carbohydrates and protein.1 Recently the popular press has been citing the ketogenic diet as a way to lose body fat quickly, however these claims are premature. The scientific evidence shows that ketogenic diets are a promising weight loss strategy, but the results are not yet conclusive.
According to the laws of thermodynamics, simply the number of calories consumed and burned would determine whether weight is gained or lost over time. However, evidence from human and animal studies suggest that other factors such as the macromolecular composition of food may have implications for changes in body composition (i.e. percent body-fat)2, appetite3,4, and the body’s daily energy expenditure.5 For example, a randomized controlled crossover trial examining 19 obese men and women under tightly controlled conditions for six days of dieting found that low-fat diets result in more body fat loss than low-carbohydrate diets despite the latter leading to a reduction in insulin levels and enhanced fat oxidation.2 In this study, the low-carbohydrate diet contained too many carbohydrates to be considered a ketogenic diet, and the authors acknowledge that the ketogenic diet may cause similar amounts of body fat loss as compared to the low-fat diet they tested. They also acknowledged that the trial was done in a tightly controlled setting where the subjects were forced to adhere completely to the diet, which certainly does not reflect real world conditions.
As far as adherence is concerned, the ketogenic diet might offer a slight benefit over other isocaloric diets in that it may suppress appetite and be easier to monitor.3,6 According to a recent meta-analysis, ketogenic low-carbohydrate diets caused a significant decrease in hunger and the desire to eat.3 Unfortunately, this meta-analysis was limited by the fact that only three studies on the ketogenic diet were included. The mechanism underlying this potential reduction in appetite is not well-understood but, based on current human and animal studies, the authors proposed explanations ranging from the effects of neuroendocrine axes to gut microbiota.4 Regardless of the mechanism, a decrease in appetite, if true, would likely improve overall adherence to the ketogenic diet and promote weight loss better than other diets in free living adults. However, there are other downsides of a ketogenic diet. In an opinion paper published by Hall et al., it is argued that habitual eating patterns and the social pressures of eating might outweigh the appetite suppressing effects.6 Another factor that may promote weight loss on the ketogenic diet in a study setting is the ease of monitoring the ketogenic state. Subjects may be encouraged to continue dietary compliance by the positive feedback received from knowing their blood and urinary ketone levels,7 and the changes in the respiratory quotient,5 both of which can be easily measured. With other diets, adherence can only be assessed by self-report, which has been shown to be unreliable.8
It is commonly claimed that the ketogenic diet increases energy expenditure, however there is no support for this supposition in clinical trials. For example, in a controlled trial of 17 overweight and obese men over a 4-week intervention, a ketogenic diet increased energy expenditure by 57±13 kcal/d, a very small change that is inconsequential in the scheme of factors that impact weight loss.5 Others studies in children with epilepsy9 and obese adults10 confirm that the ketogenic diet does not significantly alter energy expenditure.
The previously discussed studies have been composed of small sample sizes, short durations, and tightly controlled conditions. Large studies, which allow subjects to moderate their own diets based on education offer less reliable data, but a more realistic depiction of dietary adherence and weight loss outcomes. These studies corroborate the findings that low-carbohydrate diets might be as effective as low-fat diets.11-13 In a 12-month study comparing 148 individuals randomized to either a low-carbohydrate diet (<40 g/d) or a low-fat diet (<30% of daily energy intake from total fat), the low-carbohydrate diet group experienced greater decreases in weight (mean difference in change, 3.5 kg, 95% confidence interval [CI] [5.6 – 1.4], p=0.002) and fat mass (mean difference in change, 1.5%, 95% CI [2.6 – 0.4], p=0.011).12 A meta-analysis of 13 studies comparing ketogenic diets to low-fat diets defined their threshold as carbohydrate intake <50 g/d.14 According to their analysis, those on the ketogenic diet lost a weighted mean difference of 0.91 kg (95% CI [-1.65, -0.17], p=0.02; I2 = 0%, p=0.47), so on average they lost 0.91 kg more than their counterparts assigned to a low-fat diet. The low-carbohydrate content of these diets induced some unreported level of ketosis, but perhaps not enough to be considered truly ketogenic diets. As compared to these diets, the original conception of the classical ketogenic diet used to manage epilepsy mandated a much lower carbohydrate consumption of <15 g/d.1 While some studies propose a threshold serum ketone concentration at which anti-seizure effects are maintained, no such correlate currently exists for weight loss.15 As such, it is difficult to define a ketogenic diet in a weight loss context. A classical ketogenic diet of the kind sometimes used to manage epilepsy might have different outcomes than those reported above for weight loss and, perhaps more importantly, different adherence.
Other versions of a very low carbohydrate diet have also been studied for weight loss. One example is the Atkins diet, which is similar to the classic ketogenic diet in that it initially limits carbohydrate intake to <20 g/d however differs by allowing substantially more protein. In a 24-week study of 120 overweight, hyperlipidemic individuals, the Atkins diet group lost more weight (12.0 kg, 95% CI [18.8 – 10.2]) than the low-fat diet group (6.5 kg, 95% CI [8.4 – 4.6]).16 Adherence was greater in Atkins group (76% as opposed to 57%), and the two dietary interventions were not isocaloric with the Atkins group consuming fewer calories. Another study published four years later compared a low-carbohydrate diet to a Mediterranean diet and a low-fat diet over the course of a much longer intervention period of two years.13 The low-carbohydrate group was again based on the Atkins diet rather than a classical ketogenic diet. Interestingly the Atkins diet was a non-restricted calorie diet as opposed to the other two, which did restrict calories. There was, however, no significant difference among the three groups with respect to the calorie deficit achieved from baseline. The Atkins group lost a mean of 4.7 kg of weight, while the low-fat diet led to only a 2.9 kg weight loss and the Mediterranean diet led to a 4.4 kg loss (p<0.001). The low-carbohydrate group had the lowest 24-month adherence rate (78%) in comparison to the Mediterranean (85.3%) and low-fat (90.4%) groups. Given that a classical ketogenic diet would impose even tighter carbohydrate restrictions, it is logical to wonder if, despite the appetite suppressing effects and ease of monitoring, ketogenic diets might have lower compliance than other plans. This would be in-line with the findings of a meta-analysis assessing the ketogenic diet as a seizure treatment that found a classical ketogenic diet has a very low compliance rate of only 38%.17
A further complicating factor when examining the efficacy of the ketogenic diet as a weight loss intervention is the idea of personalized nutrition plans. Certain genetic markers are believed to make individuals more or less prone to weight loss on different dietary plans.18 However, a recent randomized parallel group trial of 609 participants for 12 months found that the level of insulin secretion and previously identified diet-genotype patterns had no effect on the outcomes of healthy low-fat vs. healthy low-carbohydrate dietary interventions.11
It is remarkable that we still do not know what type of diet is best for weight loss. While there is ever increasing information on low-carbohydrate dietary patterns, there are still very few randomized controlled or parallel group trials being done with a strictly ketogenic diet. As such it is difficult to draw any definitive conclusions on their efficacy. Still, the studies done with low-carbohydrate diets suggest that if adherence is maintained and total caloric intake is appropriately monitored, the ketogenic diet is likely to be as effective as a low-fat diet.
References
- Peterman M. The Ketogenic Diet in the Treatment of Epilepsy: a preliminary report. Am J Dis Child. 1924;28(1):28-33. doi:doi:10.1001/archpedi.1924.04120190031004
- 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 Metab. Sep 1 2015;22(3):427-36. doi:10.1016/j.cmet.2015.07.021
- Gibson AA, Seimon RV, Lee CM, et al. Do ketogenic diets really suppress appetite? A systematic review and meta-analysis. Obes Rev. Jan 2015;16(1):64-76. doi:10.1111/obr.12230
- Paoli A, Bosco G, Camporesi EM, Mangar D. Ketosis, ketogenic diet and food intake control: a complex relationship. Front Psychol. 2015;6:27. doi:10.3389/fpsyg.2015.00027
- 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. Aug 2016;104(2):324-33. doi:10.3945/ajcn.116.133561
- Hall KD, Chung ST. Low-carbohydrate diets for the treatment of obesity and type 2 diabetes. Curr Opin Clin Nutr Metab Care. Jul 2018;21(4):308-312. doi:10.1097/MCO.0000000000000470
- Kossoff EH, Zupec-Kania BA, Amark PE, et al. Optimal clinical management of children receiving the ketogenic diet: recommendations of the International Ketogenic Diet Study Group. Epilepsia. Feb 2009;50(2):304-17. doi:10.1111/j.1528-1167.2008.01765.x
- Subar AF, Kipnis V, Troiano RP, et al. Using intake biomarkers to evaluate the extent of dietary misreporting in a large sample of adults: the OPEN study. Am J Epidemiol. Jul 1 2003;158(1):1-13.
- Groleau V, Schall Joan I, Stallings Virginia A, Bergqvist Christina A. Long-term impact of the ketogenic diet on growth and resting energy expenditure in children with intractable epilepsy. Dev Med & Child Neurol. 2014/09/01 2014;56(9):898-904. doi:10.1111/dmcn.12462
- Gomez-Arbelaez D, Crujeiras AB, Castro AI, et al. Resting metabolic rate of obese patients under very low calorie ketogenic diet. Nutr & Metab. 02/1709/21/received01/29/accepted 2018;15:18. doi:10.1186/s12986-018-0249-z
- 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. Feb 20 2018;319(7):667-679. doi:10.1001/jama.2018.0245
- Bazzano LA, Hu T, Reynolds K, et al. Effects of low-carbohydrate and low-fat diets: a randomized trial. Ann Intern Med. Sep 2 2014;161(5):309-18. doi:10.7326/M14-0180
- Shai I, Schwarzfuchs D, Henkin Y, et al. Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet. N Engl J Med. 2008/07/17 2008;359(3):229-241. doi:10.1056/NEJMoa0708681
- 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. Br J Nutr. Oct 2013;110(7):1178-87. doi:10.1017/s0007114513000548
- Gilbert DL, Pyzik PL, Freeman JM. The Ketogenic Diet: Seizure Control Correlates Better With Serum β-Hydroxybutyrate Than With Urine Ketones. J Child Neurol. 2000/12/01 2000;15(12):787-790. doi:10.1177/088307380001501203
- Yancy WS, Jr., Olsen MK, Guyton JR, Bakst RP, Westman EC. A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia: a randomized, controlled trial. Ann Intern Med. May 18 2004;140(10):769-77.
- Ye F, Li XJ, Jiang WL, Sun HB, Liu J. Efficacy of and patient compliance with a ketogenic diet in adults with intractable epilepsy: a meta-analysis. J Clin Neurol. Jan 2015;11(1):26-31. doi:10.3988/jcn.2015.11.1.26
- Gardner CD. Tailoring dietary approaches for weight loss. Int J Obes Suppl. Jul 2012;2(Suppl 1):S11-S15. doi:10.1038/ijosup.2012.4
