top of page

A problem with the reporting of effects of ketogenic diet?

There is a lot of talk about the so called ketogenic diet. This diet will require an extremely low intake of carbohydrates. This will starve the brain of its main fuel (glucose) and the adaptive mechanism of the body is to turn on ketogenesis stimulating the production of ketone bodies from fats. These ketones are an excellent alternative source of energy for the brain. These ketone bodies are also an excellent source of energy for the muscle.

Ketogenic diet

The ketogenic diet

The theory for athletes is that if you train your body to rely less on carbohydrate by extreme restriction of carbohydrate intake, the body will respond with an adaptation that is referred to as keto-adaptation. This adaptation will allow greater fat burning at the same exercise intensity. It is important to note that formation of high levels of ketones will only happen if carbohydrate intake (relative to carbohydrate use) is extremely low. The claims are that this will help performance. Although it is obvious and known for a very long time that if you carbohydrate restrict, you will increase fat metabolism, the evidence that this keto-adaptation has performance enhancing effects is lacking. Proponents have to resort to anecdotes or they refer to a study that was published in the 1983 (1). This study, however, had quite serious limitations and did NOT show any performance benefits.

The evidence that this keto-adaptation has performance enhancing effects is lacking

A graph used to show benefits ketogenic diet

I have seen versions of the figure above in several places, including a scientific publication (3). The graphic is used to show that a ketogenic diet results in much higher rates of fat oxidation than a “normal” diet. As I mentioned before, there is no debate about whether a low carbohydrate diet results in lower rates of carbohydrate oxidation and higher rates of fat oxidation. However, the numbers and the comparison made between two completely different studies in this graph are pretty meaningless and are not an example of good science in my opinion. A comparison like this would still be somewhat acceptable if the limitations of the comparison were acknowledged, but it is presented as a straightforward comparison with no mention at all of the fact that the studies are vastly different.

As mentioned above, the graphic compares two completely different studies. One of the studies (on the right) is the keto-adaptation study in 1983 by Dr Stephen Phinney (1), the second study (on the left) is from a study we performed at the University of Birmingham 10 years ago (2). The comparison of the two studies is problematic though because, they are so different: different populations, studied during different durations and modalities of exercise etc etc. Virtually everything is different between the studies and thus any direct comparison is pretty meaningless.

Comparison of studies

To begin with, the study by Phinney et al was in 5 male trained cyclists (a very small number even for these type of studies). The Venables et al study was in 150 men and 150 women and included many untrained and overweight individuals (as well as some very trained cyclists). In the study we performed, we have no details of the diet of these individuals and thus it is difficult to compare this to a controlled diet for 4 weeks. Because exercise was controlled the day before and participants likely had a reasonable carbohydrate intake, it is also reasonable to assume that their glycogen stores were at least normal. The glycogen stores in the Phinney study were low. Thus the effects would have been the effects of 1. Keto-adaptation and 2. Glycogen depletion, with no way to distinguish between the two. Therefore the effects can never be attributed exclusively to keto-adaptation.

The effects can never be attributed exclusively to keto-adaptation

More problems

In addition, different pieces of equipment were used to measure oxygen uptake and carbon dioxide production and different calculations were applied to estimate fat oxidation. Then there is the exercise test. Comparing a treadmill test with cycling exercise. It is known that these tests will give different results. The graph compares a 15-35 min graded exercise test on the left with prolonged moderate intensity exercise on the right. It is known that fat oxidation increases over time and thus comparing different exercise modalities at different intensities and different durations is meaningless. I could go on, but it should be obvious that no firm conclusions can be drawn from this comparison and that the use of the numbers presented in the graph is meaningless. So please do not accept these numbers as something meaningful and wait for someone to conduct a controlled study so we have some real data to talk about. This study may show similar results, but the results may also be quite different.

A final note

What always struck me in the 1983 Phinney study is that the 5 subjects were called well-trained endurance cyclists. Yet when they were asked to exercise to exhaustion at a submaximal intensity of 62-64%VO2max they lasted 147 min on average. You would expect trained cyclists to go AT LEAST 4-5 hours at that intensity, not 2.5 hours! I have never heard an explanation for this very short time to exhaustion.


  1. Phinney SD, Bistrian BR, Evans WJ, Gervino E, Blackburn GL. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism. 1983: 32(8):769-76.

  2. Venables MC, Achten J, Jeukendrup AE. Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. J Appl Physiol 2005: 98(1):160-7.

  3. Volek JS1, Noakes T, Phinney SD. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15(1):13-20.


Recent Posts

See All