Review points to effectiveness of keto diet, but also cites wide range of evidence quality

Researchers associated with the US Army Research Institute of Environmental Medicine gathered the evidence for the effectiveness of ketogenic diets in a systematic review.  The research was published in the Journal of the International Society of Sports Nutrition​.

Shedding fat while keeping muscle the Holy Grail of sports nutrition

The authors noted that maintaining muscle while cutting fat has been a goal of many athletes over the years.  This is especially true in sports that rely on power to weight ratio as a key metric, such as endurance running or cycling, and is of interest in physique and appearance sports such as bodybuilding and ballet.

Recent attention has focused on trying to metabolically shift the body more toward fat utilization with less reliance on carbohydrate stores during exercise as a way to achieve this goal, the authors said.  To test whether this strategy has been working the authors conducted a literature search in mid April 2020 using search terms ‘ketogenic diet’ and ‘ketosis,’ along with some other additional qualifiers.  The researchers specifically limited their search to human studies done on physically active adults.

Authors pore through data from 13 studies

In addition to eschewing animal studies, the authors excluded studies on sedentary populations and those in which the carbohydrate intake for subjects was greater than 50 grams a day.  The authors found 13 studies that met those criteria. Nine of those were parallel design and four were of the crossover variety.  The duration of the studies ranged from 21 to 84 days, with a mean of 61 days.  The researchers were able to extract data for 243 subjects in the 13 studies.

The mean diet makeup for the ketogenic diet (KD) groups was 2,318 Kcals per day, made up 8% carbohydrate, 70% fat and 22% protein.  The control diet mean values were 2,433 calories and  52% carbs, 29% fat and 19% protein.

Ketogenic diets found to cut fat, maintain muscle

The primary finding was that body mass declined in the KD groups, which the researchers said was significant because only one of the 13 studies placed subjects in a low calories diet.  For the other studies, the subjects ate what they wanted, while still adhering to the macronutrient profiles of each diet.

“Therefore, the mechanism by which consuming a KD results in loss of body mass is not obvious,”​ the authors wrote.

Many subjects find keto diet unpalatable over the long haul

One possible explanation was that subjects in the KD groups were in fact eating less than the prescribed amount.  This speaks to one criticism of the KD eating style, that being that many subjects find it unpalatable over the long haul.

“The highly restrictive nature of KD may have limited the overall energy consumption of participants due to reductions in food choice, or a potential dislike in foods available to consume,”​ the authors wrote.

Another possible reason for this finding was that KD diets increase satiety and decrease appetite, the authors said.

The authors found that the overall body mass declines could be attributed to reductions in fat mass.  So in that sense the ketogenic diets were delivering on the promise of cutting fat while maintaining lead body mass.  The studies that measured substrate oxidation found that KD increased the utilization of fat both at rest and during exercise, both in a fed and fasted state.

So while all that seems positive for proponents of ketogenic diets, the authors said potential significant biases could have affected the results.

“Bias across these studies is the result of allowing participants to self-select diet interventions, and inability to blind participants to their treatment group, even with randomization​,” the authors wrote.

Limitations of studies cloud conclusion

The authors also noted that some of the studies did not report complete data, which prevented them from conducting a full blown meta analysis of the research.  And the body composition data was gathered via bio-electrical impedance analysis, which is reliable in some settings but less so in a longitudinal design featuring repeated measurements.  Hence their ‘Yes-but’ conclusion.

“The overall results from individual studies indicated that body mass is reduced when consuming KD. Reductions in body mass appear to be due to negative energy balance, whether intentional or not, as 12 of the 13 studies included in this review aimed to maintain energy balance while participants consumed KD,”​ they wrote.

“However, there is some risk-of-bias and limitations in the literature due to self-selection of diet intervention, lack of controlled feeding, and use of BIA to measure composition. To avoid these potential biases and limitations, future investigations should randomize participants into treatment groups, provide all food and drinks to ensure desired energy and macronutrient intakes, and use more sensitive techniques to measure body composition,” ​the authors concluded.

Source:​ Journal of the International Society of Sports Nutrition​​​
18​, Article number: 41 (2021)
Body composition changes in physically active individuals consuming ketogenic diets: a systematic review​
Authors: Coleman JL, Carrington CT, Margolis LM