Interestingly carbohydrates are the only macronutrient with no specified minimum requirement. Although fat and protein are essential, carbohydrate intake is not needed for survival. Although there is no exact definition of how much carbohydrate accounts to a low-carbohydrate diet, it can generally be defined as a diet where carbohydrates are restricted in favour of protein or fat intake, whereas traditional low-fat diets are diets where fat contributes to < 30% of total energy intake. Now the idea of reducing carbohydrates for weight loss has dominated the ‘diet’ industry ever since the introduction of diet books such as The Atkins Diet, promoting this idea that carbohydrates are the evil things that are the reason why we can no longer fit in those favourite pair of jeans we always loved. This lead to a shift from the idea that eating fat made us fat, and would be dieters moved away from low-fat diets in favour of low-carbohydrate diets.
Now to begin with there is definitely evidence for the use of low-carbohydrate diets for weight loss. In 2016 a study published by Mansoor and colleagues in the British Journal of Nutrition conducted a meta-analysis by pooling randomised controlled trials (RCTs) comparing low-carbohydrate diets to low-fat diets on body weight. Eleven RCTs consisting of 1,369 participants were included in the analysis. The RCTs included in the analysis ranged from 6 months to 2 years in duration, with results showing that participants in the low-carbohydrate groups lost more weight than those who were assigned to a low-fat diet (Mansoor et al. 2016). Now on first glance, this would seem to suggest that the low-carbohydrate diet was superior for weight loss than that low-fat diet. But as with all studies, there are always limitations. The problem with the RCTs includes in this analysis was that there was poor reporting of energy intakes for the different groups. Put simply we do not know the caloric intakes of the groups, or more importantly the difference in caloric intakes between the groups.
So if the participants in the low-carbohydrate group were consuming less total calories than those in the low-fat, we would not be able to make the conclusion that it was the low-carbohydrate nature of the diet which led to better improvements. Interestingly the authors themselves stated that ‘unfortunately there is a general lack of information detailing actual energy intake among subjects, and therefore some uncertainty is associated with the effect of the macronutrient composition vs the energy intake on weight loss’. We know that caloric intake is the key to weight loss. So if for some reason people are more able to stick to a low-carbohydrate diet, then as a result of consuming fewer calories it would be logical that they would lose more weight.
Now, let’s move forward to 2018 and see if we can shine some more light on this battle between low-carbohydrate vs. low-fat diets. In the United States, Gardner and colleagues from Stanford University conducted The Diet Intervention Examining The Factors Interacting with Treatment Success, or more interestingly referred to as the DIETFIT trial (Gardner et al. 2018). This randomised controlled trial included 609 overweight and obese adults looking to see the effects of either a low-fat or a low-carbohydrate diet on weight loss over a 12 month period. At the end of the 12 months, there were no statistically significant differences in weight loss between dietary groups (5.3 kg for low-fat vs. 6.0 kg for low-carbohydrate). Furthermore, reductions in both body fat and waist circumference were also similar between groups.
Although adherence to diets is often a difficult task in intervention trials, from the figure below we can see that at 3 months, 6 months and 12 months during the trial there were significant differences with regards to carbohydrate and fat intake between the groups.
The difference in this study compared to the one we initially looked at was that the authors monitored overall caloric intake and reported that overall intake between groups was similar with both groups reducing their total calories from the beginning of the trial by around 500-600 kcals.
So if energy intake is similar between groups it can be expected that weight loss will also be similar, as demonstrated here. This has been supported by other trials that have compared low-carbohydrate to low-fat diets while also attempting to ensure similar calorie intakes between groups (Tay et al. 2008, Morgan et al. 2008, Dansinger et al. 2005, Stern et al. 2004). So there is nothing special about diets such as the low-carbohydrate diet, it is just that due to certain limitations in food choices it can make it easier for someone unknowingly to reduce their total calorie intake, and hence promote more weight loss.
Calorie restriction is the number one factor for weight loss. There is no evidence to support the superiority of a low-carbohydrate diet over low-fat IF calorie intake is similar for both. However, adherence to diet is crucial for obtaining results. There is no one size fits all; some people just find it easier to follow a certain diet over another. The important thing is to find the diet that will work for YOU, and allow you to create the calorie deficit that will lead to your weight loss.
Dansinger, M. L., Gleason, J. A., Griffith, J. L., Selker, H. P., & Schaefer, E. J. (2005). Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial. Jama, 293(1), 43-53.
Gardner, C. D., Trepanowski, J. F., Del Gobbo, L. C., Hauser, M. E., Rigdon, J., Ioannidis, J. P., … & King, A. C. (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(7), 667-679.
Mansoor, N., Vinknes, K. J., Veierød, M. B., & Retterstøl, K. (2016). Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials. British Journal of Nutrition, 115(3), 466-479.
Morgan, L. M., Griffin, B. A., Millward, D. J., DeLooy, A., Fox, K. R., Baic, S., … & Truby, H. (2009). Comparison of the effects of four commercially available weight-loss programmes on lipid-based cardiovascular risk factors. Public health nutrition, 12(6), 799-807.
Stern, L., Iqbal, N., Seshadri, P., Chicano, K. L., Daily, D. A., McGrory, J., … & Samaha, F. F. (2004). The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: one-year follow-up of a randomized trial. Annals of internal medicine, 140(10), 778-785.
Tay, J., Brinkworth, G. D., Noakes, M., Keogh, J., & Clifton, P. M. (2008). Metabolic effects of weight loss on a very-low-carbohydrate diet compared with an isocaloric high-carbohydrate diet in abdominally obese subjects. Journal of the American College of Cardiology, 51(1), 59-67.