When we recommend carbohydrate intake for the day, it is often expressed as grams per kg bodyweight. For example, for most sports this will be between 5 and 8 g/kg with values up to 12 g/kg on some days, in some sports. Protein intake is also expressed per kg body weight or per kg fat-free mass and this makes sense. However, when it comes to carbohydrate intake during exercise, recommendations are provided in grams per hour. A recent study challenged this view. Should we really change the recommendations?

Carbohydrate recommendations during exercise
In a previous blog I explained the reason for the grams per hour recommendations. In a large number of studies there seemed to be only small differences between individuals in terms of the maximum amount of glucose someone could oxidise after about 90 min of exercise; this was always up to 60 grams per hour. Sometimes less than 60 grams per hour, but we hardly ever saw values above 60 grams per hour.
A few years later, when we had discovered multiple transportable carbohydrates, we compiled the studies that had used multiple transportable carbohydrates. When we plotted body weight against maximal exogenous carbohydrate oxidation (which were much higher than 60 rams per hour), there really did not seem to be any correlation.
Recent study challenges recommendations
However, a recent study challenged the view that carbohydrate recommendations during exercise should be expressed as grams per hour. The study was performed by well-respected researchers and the study was generally well designed and well performed. The authors argued that the lack of correlation previously reported between body weight and exogenous carbohydrate oxidation was caused mainly by 2 factors:
The studies we used were not specifically designed to study this very question
The range in body weights was relatively small in our studies making it more difficult to find differences. In the authors words: “That conclusion, however, was based on secondary analyses of data from studies where the role of body size was not an aim, and therefore, the range of body size was somewhat limited.”
The first point is correct. We never designed a study to look at the effects of body weight on exogenous carbohydrate oxidation. We only looked at all of the data we had collected over many years and many cyclists that had gone through the lab, and we never published a paper specifically in this topic.
The second point is probably not as strong. In all the previous studies we never had exclusion criteria for body weight, and we just recruited people with all sorts of body weights into our studies. This could be looked at as a strength not a weakness. The body weights of participants in the studies, probably reflected body weights of the “endurance population” quite well. The range was about 56 kg to 93 kg. Female athletes were included in this sample as well. But we will come back to this range of body weights in a minute.
The new study on carbohydrate recommendations during exercise
So, what did the researchers do in this new study? They recruited cyclists with varying body weights and divided them into 2 categories: A group with lower body weights and a group with higher body weights. They asked the participants to cycle at a low to moderate intensity and they gave them glucose. The intake was 90 grams per hour. They then measured the oxidation of the ingested carbohydrate and plotted the oxidation rate against body weight. The authors report that there was a significant correlation and in their conclusion they state that the results justify rethinking the recommendations.
Intuitively it makes sense that someone who is heavier uses more fuel and also more fuel from ingested carbohydrate. The authors argue that a larger person will have a larger intestine and therefore a greater capacity to absorb. However, this argument is not as solid as it appears because most absorption of carbohydrate take place in only a very small part of the intestine.

Different interpretation of the same data
When I looked at the data, I interpreted them a bit differently and I will explain why.
Firstly, when I saw the cloud of data my first impression was not very different from the cloud of data that we had demonstrated before; the data seemed all over the place. Some people with low body weight had high oxidation rates, some heavier individuals had low oxidation rates. Then I realised there were only 3 data points outside the range that we presented before. So, although the authors argue that the range in our previous report was too narrow, they really only added 3 data points. But it did appear that those 3 data points indeed had a large effect on the outcome (although the authors argue that this was not the case).
Outliers within the data
One of these data points is the person with the highest body weight. This person not only had the highest weight, but also had the highest oxidation rate. In fact, the reported oxidation rate was so high that it exceeded the intake rate (how can someone oxidise more than they ingest?). So, this data point should probably have been removed from the data analysis. Without access the original data I redrew the figure and recalculated the correlation. Maybe not perfect but it gave a good impression of what would happen if this person was removed. The correlation disappeared.
The other 2 data points were individuals with a lower body weight. When these two lower body weight individuals were removed, there really was nothing left of any correlation. So, it is probably fair to conclude that the entire conclusion was based on these 3 data points (one of which should have been removed).
Some of this is just an academic discussion anyway in my opinion because when we work with athletes we work with individuals. From all studies we can see that individuals with the same body weight can have very different oxidation rates of the carbohydrate. We also know that different individuals have different tolerances and maybe the guidelines are just that: guidelines. Maybe for more precise recommendations, we would need to measure an individual’s exogenous carbohydrate oxidation rates.
Single vs multiple transporter carbohydrates
There is another limitation of the study and that is the carbohydrate source used in the study. If we are interested in the capacity to oxidise carbohydrate (which appears to be the factor that determines prolonged endurance performance >2.5 h), we should always give multiple transportable carbohydrates because these have the potential to increase oxidation rates 2-fold compared with a single carbohydrate. This in turn has been shown to translate into performance benefits over a single carbohydrate source. The new study investigated glucose only and thus the oxidation rates were low, and it does not really tell us much about the “capacity” to oxidise exogenous “carbohydrate”. The reason the investigators used glucose is likely the fact that this is much easier to study than a combination of carbohydrates, so it is an understandable pragmatic reason, that in this case doesn’t help the practical usefulness of the results.
World records in exogenous carbohydrate oxidation
Another reason why it is hard for me to believe that body weight really is an important factor is the fact that I have done these measurements with very elite athletes (runners, cyclists and triathletes) and the highest values I have ever seen were with athletes at the lighter end of the body weight spectrum. If we would have added these data into the cloud presented above, it would be even more difficult to see a correlation.

Conclusions
Probably the most important point is that we should not try to come up with recommendation based on a single study. We should always take into account the totality of evidence and based on that body of evidence we should come up with the best possible recommendations. If we look at the totality of evidence, there is maybe one main conclusion we can all agree on:
We must conclude there may be reason to personalise the advice by doing measurements (which only be possible for a very select few), or we should stick to advice that is easy to follow and can be adjusted based on tolerance.
One factor that might play a role is habitual carbohydrate intake. In a previous blog we discussed “training the gut”. It is clear that the gut is adaptable and someone with a very high carbohydrate intake may have higher oxidation rates compared with someone who has a lower carbohydrate intake. It is also likely that a heavier person generally eats more than a lighter person and, in this light, one could expect differences between lighter and heavier individuals. However, in a group of elite athletes with energy expenditures in excess of 4000 kcal per day and some days up to 8000 kcal (also smaller athletes), one would expect that there is little or no difference in how “trained” the gut is.
References
Jeukendrup Curr Opin Clin Nutr Metab Care. 13(4):452-7, 2010
Ijaz et al International Journal of Sport Nutrition and Exercise Metabolism, (Ahead of Print) 2024