Matching proper food products to the after-workout time is still controversial. This aspect seems to be analysed on many levels from which the most frequently mentioned is the influence of consumption of particular kinds of products on the speed of re-synthesis of muscle glycogen. In this article I want to write about this issue concerning the consumption of fruits.
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What do theories state?
According to the common knowledge, the consumption of fruits after workout is not the most sensible solution, because they mainly contain fructose – a totally useless sugar when it comes to the losses of glycogen caused by physical activity. Another disadvantage is “high content of fiber” which slows down the speed of emptying stomach, which makes the time of energy supply recovery longer. You can also hear statements that high consumption of fruits after workout fosters gathering body fat (and again, fructose is to be blamed here).
It's worth wondering if the “accusations” mentioned above are justified. If so – how?
Before I start reviewing this issue, I would like to clarify some misunderstandings connected with the matter of the pace of replenishing glycogen. As it occurs, this topic is often presented in an imprecise way, which leads to shifting attention from very important matters to those much less significant for the development of sports shape on the recreational and professional level.
Meaning of glycogen – slight misunderstanding
There's no doubt that glycogen is an important source of energy for working muscles, and its high reserves let the body improve its efficiency, which is used in professional sports. The misunderstanding concerns the fact that during recreational sports there's no need to pay attention to the pace of replenishing glycogen. What's more, also in case of exhausting training done by sports people preparing to competitions this aspect is not the key matter, as long as workouts are not more often than once a day. And this aspect matters.
What does it mean?
If a contestant trains twice a day, the speed of replenishing glycogen becomes much more important and s/he should take care to do it fast (although, such solution when a workout is done on rinsed glycogen is sometimes introduced on purpose). The level of muscle glycogen also gets important before competitions, when there is one long-lasting effort or many short ones. In that situation the overcompensation of glycogen is induced. In other cases, this aspect is not that meaningful. Therefore, people who train at the gym 3- 4 times a week should definitely not worry about that.
The consumption of fruits and the pace of replenishing glycogen
Fruits are commonly perceived as a condense source of fructose. Whereas, fructose, as it is known, is not so good for replenishing muscle glycogen. It was proven in the research done in the middle of the 90s by Van den Bergh. The problem is that this research (very often given as evidence not in favour of fruits) concerned pure fructose. Whereas, fruits also contain a lot of glucose, saccharose (containing glucose and fructose), and sometimes – starch (it concerns e.g. bananas).
Does it change anything? Yes, it does!
The experiment done in the year 2000 showed, for a change, that the speed of replenishment muscle glycogen in case of supplying saccharose (as I mentioned – the connection of glucose and fructose), is as fast as in case of glucose only. Additionally, Dr Casey and other authors of this research observed that the consumption of table sugar replenished the level of liver glycogen significantly faster. Therefore, taking under consideration the muscle glycogen and liver glycogen, the connection of glucose and fructose occurred to be better than glucose only. On the basis of this research one can risk stating that fruits (containing glucose and fructose) may be even a more advantageous option than products containing only glucose (or probably – only starch which is the glucose polymer).
Explanation of the “puzzle”
The curiosity mentioned above can be easily justified. The speed of absorbing glucose from the digestive tract is limited. It means that the body can absorb only particular amount of this sugar in time. Fructose uses other transport tracts and it doesn't compete with glucose – it is absorbed independently. Thanks to the presence of fructose, it is possible to replenish energy reserves of the body much faster, whereas, this sugar reaches mainly liver and it is metabolized there, as well as stored in a form of glycogen. However, the available data show, that some (small) amount of fructose, after proper changes, may be also used to replenish muscle glycogen.
Against common belief, fruits may be a good source of carbs after workout, as it is not true that sugars found in them are useless in the matter of replenishing glycogen. There are reasons to believe that fruits are a really good choice in that case. However, you should remember, that total content of saccharides in fruits is quite low and it rarely crosses 20%. It means, that you should eat a lot of fruits to supply large dose of carbs. Therefore, fruits are a good addition (but very valuable one) to other carbohydrate products eaten after workout.
Sources: Van Den Bergh i wsp. Muscle glycogen recovery after exercise during glucose and fructose intake monitored by 13C-NMR. J Appl Physiol. 81: 1495-500. Casey i wsp. Effect of carbohydrate ingestion on glycogen resynthesis in human liver and skeletal muscle, measured by (13)C MRS. Am J Physiol Endocrinol Metab. 278: E65-75.