A comment on our Eating After the Ride post raised the question of whether the recommendation to consume one half gram of carbohydrate for each pound of body weight during the 30 minutes after you finish a bike ride is contradicted by recommendations to avoid all carbohydrates immediately after exercise because carbs supposedly disrupt a two hour “synergy window” during which the release of human growth hormone (HGH) by the body is claimed to produce a wide range of miraculous effects. The HGH industry is a cesspool of fraud, ignorance, scams and ridiculous claims that are completely unsupported by scientific research (more on this can be found in Human Growth Hormone and Cycling).
One piece of HGH-related nonsense about the so-called “synergy window” that has been copied and reproduced on many, many websites and blogs cites a study published in the Journal of Applied Physiology as evidence in support of its claims. The research that is cited has nothing whatsoever to do with HGH or a two-hour post exercise window but it is relevant to concerns of cyclists who are concerned with what to eat after a ride.
The study was carried out by S.A. Newsom and colleagues. It is titled “Energy deficit after exercise augments lipid mobilization but does not contribute to the exercise-induced increase in insulin sensitivity” and you can read it for yourself here. The investigators were interested in the consequences of replacing the energy stores burned during exercise with either fats or carbohydrates. They carried out a carefully controlled study that involved different groups of experimental subjects engaging in a period of controlled exercise followed by the ingestion of carefully controlled meals and snacks for the rest of the day. The investigators varied whether energy stores were replaced by carbs or fats and measured insulin sensitivity (an indicator of the degree to which the system is primed and ready to process glucose into glycogen) and lipid metabolism (an indicator of the extent to which adipose tissue, i.e., fat, is being mobilized or broken down) the following day.
Here’s what they did.
Nine men between 28 and 30 years of age participated in the study. There were four experimental conditions and each participant took part in each of the four conditions over four different experimental sessions.
The following general procedure was carried out in each of the four experimental sessions:
* Participants fasted over night and were admitted to the hospital where testing would take place the next morning.
* After admission and a 30 minute rest period, oxygen consumption and carbon dioxide production were measured.
* This was followed by approximately 90 minutes of exercise in 3 of the 4 conditions. The 4th condition was a control where participants did not exercise. The exercise was split evenly between a treadmill and an exercise bike and each participant burned approximately 800 kilocalories (kcal) during the exercise period. Oxygen consumption and carbon dioxide production were measured at several points throughout the exercise period to insure participants were exercising at the required rate and expending the required amount of energy.
* The participants ingested meals at periods of 30 minutes, 5 hours, and 10 hours after exercise. The experimental manipulation was in the nutritional make-up of these meals.
* Three hours after the last meal all of the participants ingested an identical snack to control for any effects of the last food eaten.
* The participants then spent the night at the hospital and a variety of physiological measures were taken the following morning.
There were four experimental conditions in the study. These were:
1. A Control condition in which participants did not exercise. Participants were fed meals that maintained their fat and carbohydrate balance.
2. A Balanced condition in which participants were fed enough carbohydrate to replace the glycogen lost during exercise and enough fats to replace the fats lost during exercise.
3. A Low Carbohydrate condition in which participants were fed meals that did not contain enough carbohydrate to replace the glycogen burned during the exercise period. The total energy loss of the exercise (kcal burned from both fats and glycogen stores in the body) was offset by increasing the amount of fat in the meals.
4. A Low Energy condition in which participants were fed enough carbohydrate to replace the glycogen lost during exercise but were not fed enough fat to replace the fat burned during exercise.
There are two important comparisons to consider with regard to the consequences of failing to replace either fats or glycogen (by ingesting carbs) after exercise.
(A) In the Control condition no exercise takes place and energy balance is maintained by replacing energy lost (during rest) to fat metabolism with fat in the diet and energy lost to burning glucose with carbohydrate in the diet. In the Low Carbohydrate condition energy balance is also maintained but it is accomplished by shorting the amount of carbohydrate in the diet and replacing the missing carbohydrates with fats. Testing the next day showed that the amount of glycogen stored in muscle tissue was significantly lower in the Low Carbohydrate condition than in the Control condition. Insulin sensitivity was also higher in the Low Carbohydrate condition than in the Control condition. This means that insulin was more active the following day in the Low Carbohydrate condition. Insulin plays a critical role in converting blood glucose into glycogen that can be stored in the muscles (and liver) and the system would be expected to show higher sensitivity to insulin when it is in glycogen debt and operating to replace lost glycogen stores.
(B) In the Balanced condition exercise take place and energy balance is maintained by replacing energy lost (during exercise) to fat metabolism with fat in the diet and energy lost to burning glucose with carbohydrate in the diet. In the Low Energy condition energy lost to burning glucose is replaced by carbohydrates in the diet (in other words, muscle glycogen lost to exercise is completely replaced) but energy lost to burning fats is not. Testing the next day showed no differences in muscle glycogen between the two groups. However, there was an increase in plasma fatty acid mobilization and oxidation and an increase in plasma triacylglycerol concentration the next day in the Low Energy condition as compared to the Balanced condition. This means that fat metabolism was higher in the Low energy group.
What does this mean for the active cyclist?
The results discussed in (A) above provide another source of evidence that failure to ingest enough carbohydrate following exercise results in lower stores of muscle glycogen the next day. A deficit in muscle glycogen translates into less energy on the bike, lower performance levels, and an increased tendency to bonk on the ride. The critical need to replace muscle glycogen after exercise is the basis for the recommendation made in Eating After the Ride to ingest a heavy carbohydrate load during the first 30 minutes after you get off the bike in order to take advantage of the brief period during which a high-efficiency glycogen storage process take place that allows blood glucose to be stored as muscle glycogen without the use of insulin.
Note that this study by Newsom et. al. provides evidence that muscle glycogen is depleted the day after exercise if carbohydrates are not consumed in sufficient quantity during the 12 hours or so after exercise. Although the investigators made sure that participants were fed a meal withing 30 minutes of exercise, the study is not concerned with the brief period of enhanced, efficient glycogen storage that takes place immediately after exercise and provides no evidence one way or the other about the consequences of ingesting or failing to ingest sufficient carbohydrates immediately after exercise.
The results discussed in (B) provide evidence that fat metabolism is higher the day after exercise if fat intake in the hours after exercise is depressed. This is of interest to cyclists who are concerned with losing weight. Body fat is being burned at a higher rate the day after exercise if the fats burned during exercise are not replaced with fat in the diet. Note that there is no indication here one way or the other whether the increased amount of fat metabolism shown the following day is sufficient to produce noticeable weight loss. However, if you are interested in losing weight through the loss of body fat, increased levels of fat metabolism have to be better than no increase in fat metabolism in the long run. Note also that fat metabolism was increased in the Low Energy condition even though carbohydrate intake was kept high enough to replace the muscle glycogen lost to exercise. Taking in enough carbohydrate after exercise to replenish glycogen stores allows you to be ready for an exercise session the next day and does not stop fat metabolism that is breaking down body fat to supply energy.
In summary, what you eat after a ride makes a difference. It isn’t the case that calories ingested from fats are equivalent to calories ingested from carbohydrates when replacing the calories burned during exercise. Eating carbs after a ride replaces lost muscle glycogen, gets you ready to ride the next day, and does not stop fat metabolism. Refraining from eating fats after a ride increases the burning of body fat the day after the ride and does not interfere with glycogen storage. The take home message seems simple: Replace glycogen by ingesting carbs after a ride; metabolize fat and possibly lose weight by not eating fats after a ride.