Slow carbs are in fashion. There are slow carb diets, slow carb sports nutrition products, and just slow carbs. But do they live up to the hype? In this blog we will first look at the theory and then at the evidence. The spoiler: it doesn’t live up to its expectations when it comes to performance.
Are slow carbs better?
In previous blogs we discussed that there are different types of carbohydrates (Check it out here). Some carbohydrates are classed as complex, others as simple. But, interestingly enough, the terms complex and simple do not necessarily explain what carbohydrates are slow and what carbohydrates are fast. Some complex carbs are fast and some simple carbs (sugars) are slow.
Some complex carbs are fast and some simple carbs (sugars) are slow.
The slow or fast often refers to the ability to change blood glucose concentrations as we discussed in this blog about the glycemic index. A fast carbohydrate is emptied from the stomach rapidly, is digested rapidly, is absorbed quickly and escapes the liver (is not converted into something else other than glucose in the liver). A slow carbohydrate or a slow release carbohydrate is usually digested slower and absorbed slower. In some cases it needs to be converted to glucose in the liver and that can be a process that slows down the delivery. It is therefore said that these carbohydrates deliver energy more gradually and over a longer period of time. Below is an overview how we define slow and fast carbohydrates based on their use as a fuel in muscle:
There are plenty of products on the market that claim benefits for slow carbohydrates. Some of these slow carbohydrates are sugars, others are more starch like carbohydrates. I will focus on those claims that are made in the context of exercise. The most important claims are:
Do not cause a spike in glucose
Do not cause a spike in insulin
Provide energy slowly over a longer period of time
Ideal for sports like football with 90 min of play and limited opportunities to drink or eat
Prevent the rapid drop in blood glucose you see with a high glycemic index carbohydrate
We will discuss these claims below:
Slow carbs do not cause a spike in glucose
This is true, there is plenty of evidence for this. Probably every single study shows this. However, we now make the assumption that this is significantly different than a fast carbohydrate. At rest, there can be large differences between a fast and a slow carb (see article on glycemic index). However, during exercise, unless it is very very low intensity, this is not the case and this is related to the second claim:
Slow carbs do not cause a spike in insulin
Again, this claim is correct and plenty of studies support this. However, a fast carbohydrate does not result in a spike in insulin during exercise either. During exercise, especially high intensity exercise (such as match play, racing or hard training), adrenaline (epinephrine) will cause a drop in insulin. When we try to measure insulin during such activities we need to measure an extra sensitive methods because the insulin concentrations are so low, even when large amounts of a fats carbohydrate are ingested.
In one study (1) we gave either water, an extreme amount of a fast carbohydrate (glucose) that is far beyond anything someone would ever take during exercise (180 grams per hour!!) or a "normal" amount that is along the line of recommendations. The subjects exercised at a moderate intensity (around 60% of their VO2max). We saw that with the normal amount of carbohydrate, insulin stayed low and was not different from ingesting water. With the extreme carbohydrate, insulin was elevated slightly but stayed within the range of resting values. Certainly not the spike that is sometimes mentioned. There are plenty of other studies with similar results during exercise.
Slow carbs provide energy over a longer period of time
Again this claim is correct. But it sits in the intestine longer and it is released from the intestine for longer. The question is if this is desirable or whether you could achieve the same effect by just taking a smaller amount of a fast carbohydrate? This brings me to the next point.
Slow carbs are ideal for sports with limited opportunities to drink or eat
This claim is also correct as stated because of the slower delivery from. The intestine. However, in what sports are there no opportunities to take some carbohydrate for longer periods of time. And what does “longer periods of time“ mean? Is this 30 min, 60 min, 2h? More? If we ingest glucose or another fast carbohydrate just before a football match, this glucose will start to appear in the circulation 5 min later and over the next 45-90 min its availability will increase. In football (soccer), after 45 min there is another opportunity to take carbohydrate. So, a sport where a slow carb could be effective according to this theory would need long period (far in excess of 45 min) with no access to food or drinks.
Essentially what you are doing is storing a carbohydrate in your stomach instead of a bottle. In other words: sipping a fast carbohydrate source from a bottle would have the same effect on energy delivery as a large amount of a slow carbohydrate in your stomach and intestine. But we make one big assumption in this case, and that is that there are no negative side effects of having a larger amount of carbohydrate in your stomach or intestine.
Essentially what you are doing is storing a carbohydrate in your stomach instead of a bottle.
Studies seem to suggest over and over that a higher oxidation efficiency (more carbohydrate is oxidized and less remains in the intestine) is correlated with better tolerance. In most studies we did, with slower carbohydrates we saw more gastro-intestinal problems. For example, in one of the first studies we used a slower starch (a mixture of amylose and amylopectin) and this cased severe gastro-intestinal problems in several of the subjects (2). In the studies with isomaltulose and trehalose we also saw more GI complaints (3, 4). In studies were we give glucose:fructose solutions, which are oxidized more completely than for example glucose, we also see fewer problems (see blog on multiple transportable carbohydrates). The bottomline seems to be that, the more carbohydrate there is in the intestine, that is NOT oxidized, the greater the chances of gastro-intestinal discomfort and slower carbohydrates generally stay longer in the intestine.
Slow carbs prevent the glucose drop of a high glycemic index carbohydrate
Reactive hypoglycaemia is something that we discussed in a previous blog and any low glycemic index carbohydrate can help to prevent this. We also showed that it has no impact on performance, even though hypoglycaemia may develop and if you wanted to develop reactive hypoglycaemia you can also do it by changing the timing of intake.
So although there is some truth in all of these claims, during exercise they don’t hold up or they are simply not relevant.
Are there studies that directly compare slow and fast carbohydrates during exercise? Yes, there are a few. Those studies show that slower carbohydrate are indeed oxidized slower and faster carbohydrates deliver energy faster. In some studies, there is a very small difference in blood glucose but the clinical relevance of this is highly questionable. One study showed clearly more gastro-intestinal problems with a slow carbohydrate (isomaltulose) and that study also showed reduced performance with slow carbs (likely as a result of the gastro-intestinal problems) (5).
Overall I would conclude that there few situations where the theory of slow carbs would bring promise. But even in those situations we do not have any evidence to support this. In the vast majority of situations faster carbohydrates that are rapidly digested and are likely to cause less gastro-intestinal problems are preferred. In any case the same effect as slow carbs could be achieved by simply taking less of a fast carbohydrate. So in brief, slow carbs do not live up to the hype, at least not in the context of exercise.
Jeukendrup AE, Wagenmakers AJ, Stegen JH, Gijsen AP, Brouns F, Saris WH. Carbohydrate ingestion can completely suppress endogenous glucose production during exercise. Am J Physiol. 1999 Apr;276(4):E672-83.
Saris WHM, Goodpaster BH, Jeukendrup AE, et al. Exogenous carbohydrate oxidation from different carbohydrate sources during exercise. J Appl Physiol 1993;75(5):2168-72.
Achten J, Jentjens RL, Brouns F, et al. Exogenous oxidation of isomaltulose is lower than that of sucrose during exercise in men. J Nutr 2007;137(5):1143-8. [published Online First: 2007/04/24]
Venables M, Brouns F, Jeukendrup A. Oxidation of maltose and trehalose during prolonged moderate intensity exercise Med Sci Sports Exerc 2008;In press.
Oosthuyse T, Carstens M, Millen AM. Ingesting Isomaltulose Versus Fructose-Maltodextrin During Prolonged Moderate-Heavy Exercise Increases Fat Oxidation But Impairs Gastrointestinal Comfort and Cycling Performance. Int J Sport Nutr Exerc Metab 2015 doi: 10.1123/ijsnem.2014-0178