Endurance athletes know that starting exercise with glycogen stores fully loaded makes for higher-quality workouts and better performances during competitions. But this long-held tenet of sports nutrition is now being challenged. New research points to benefits of periodically training with low glycogen stores. Dubbed “train low, compete high” — meaning train in a glycogen-depleted state and compete with high glycogen stores — this new twist on training has more than a few athletes and coaches scratching their heads wondering what to make of it all. In this article, we explain the “train low” concept, explore its scientific merits and gaps, and provide some important practical tips in case you want to give it a try.
Glycogen Still Rules
The fact that a carbohydrate-based diet fills glycogen stores and leads to higher-quality and longer training sessions, along with better performances, has been known for over 50 years. This approach — we’ll call it “train high, compete high” — has a proven track record of success, and the reason is simple: As an endurance athlete, you rely primarily on two fuel sources to generate the energy needed for muscle contraction during exercise — carbohydrates and fat. For endurance exercise at a moderate intensity (say, about 70% of VO2 max), 50–60% of the energy required for muscle contraction is obtained from carbohydrates, with the rest coming from fat. Push the pace, and your reliance on carbs increases. The problem is that while fat stores are available in abundance — you’ve got an estimated 80,000 calories worth on board — your carbohydrate reserves weigh in at a paltry 2,000 calories when you total what’s available from glucose in the bloodstream and glycogen in the muscles and liver. So a single, long exercise session can severely deplete your very limited carbohydrate fuel reserves.
Hence the importance of starting exercise with adequate glycogen stores, taking in carbs during long-duration exercise (more than 60–90 minutes), and replenishing stores in between or after training sessions. Athletes who have “hit the wall” — or “bonked” — know firsthand the price to be paid for running low on muscle glycogen. When this occurs, you are forced to rely on your meager supply of glucose circulating in the bloodstream. Liver glycogen stores can be tapped to resupply blood glucose for a short while, but when that energy reserve runs dry, blood sugar drops — and that’s when you slam into that wall of pain. Your pace slows dramatically, because you’ve switched from relying on fast-burning carbs for muscle power, to much slower-burning fat. Fat simply can’t be metabolized fast enough to support your pace, so you slow down or even stop.
Surprising Benefits from Training Low
While glycogen is clearly the premium endurance fuel, this fact hasn’t stopped sports scientists from trying to figure out ways to tap into the vast reserves of body fat as an energy source — and, in doing so, to spare those precious few carbohydrate stores. And that’s where the idea of training in a glycogen-depleted state took shape.
In a laboratory in Denmark, researchers came up with a brilliant idea for comparing the effects of training in a low-glycogen state versus a high-glycogen state. They took a group of men and had them perform leg extensor exercises — a kicking motion — 5 days a week for 10 consecutive weeks while consuming a predominantly carbohydrate-based diet. For each participant, one leg was exercised every day for an hour, while the other leg was exercised every other day in two separate one-hour sessions. In that way, the total workload for each leg was the same, but half the exercise with the leg that was trained twice a day every other day was done in a glycogen-depleted state.
Fast-forward 10 weeks. Researchers reported that the two training regimens produced equally strong legs. But where they differed dramatically was in how long it took to exercise to exhaustion. Legs that had trained in a glycogen-depleted state for 50% of the workouts were able to go nearly twice as long as those that started each session with high levels of glycogen!
What could possibly explain this unexpected response?
It turns out that periodically exercising in a glycogen-depleted state literally changed the metabolic machinery in muscle cells. Muscle cells naturally contain microscopic energy-producing power plants called
mitochondria. These micro power plants are what fuel muscle contraction, and with glycogen stores in short supply for half the workouts, they were able to retool or adapt their metabolic machinery to be more effective at burning fat as a fuel.
Gaps in the Research
Before you completely deep-six your carb-loaded workouts in favor of the minimal-carb variety, consider some of the shortcomings of this study. Although intrigued, naturally skeptical scientists also wasted no time in pointing out the many limitations of the research. Chief among them was the fact that the subjects in the Danish study were untrained, and therefore the study provided no evidence that a comparable performance benefit would occur in trained athletes. Also, the leg exercises that were employed were not comparable to the whole-body exercise that a marathon runner, cyclist, or triathlete faces when training or competing.
It was clear that this research would need to be repeated with trained athletes, and utilizing whole-body exercise similar to “real-world” training. In fact, such a study was subsequently carried out using trained cyclists and triathletes. Here again, training low — i.e., in a glycogen-depleted state — produced metabolic changes consistent with a greater ability to burn fat as a fuel source during exercise.
Theory Fails to Deliver on Performance
In theory, these metabolic changes toward greater fat burning should have enhanced exercise capacity. But in this follow-up study with trained cyclists and triathletes doing real-world exercise, no measurable performance benefit was seen. In addition, as would be expected when training in a glycogen-depleted state, high-intensity-exercise training for these athletes suffered.
Subsequent studies of exercise in a fasted state and withholding carbs during exercise have fairly consistently reported a boosting of the metabolic machinery for utilizing fat
as a fuel source. But, again, these metabolic changes have failed to translate to improved performance during high-intensity exercise. Most of the studies conducted so far have been relatively short term — spanning up to 45 training sessions. It may be that training low periodically over a longer period of time — say, over many months — will lead to measurable performance benefits.
The Train-Low Conundrum
It is clear that much more research is needed in order to understand the potential benefits, if any, of train-low strategies. That said, training in a glycogen-depleted state is not necessarily new or quite so radical for endurance athletes. In fact, some athletes gravitate toward this practice either intentionally or for practical reasons. For example, it’s common for athletes to engage in early-morning endurance workouts after an overnight fast. Also, triathletes who train two or three times a day are probably doing some of their workouts in a glycogen-depleted state, simply because there is too little time or opportunity to take in adequate carbs between workouts. Five-time Tour de France winner Miguel Indurain is said to have regularly trained in a fasted state. And highly successful African distance runners are known to train early in the morning in a fasted state.
So, what’s the verdict on this “train low” concept?
It’s pretty clear that incorporating some amount of training in a glycogen-depleted state increases the ability to utilize fat as an energy source during exercise. Unfortunately, that hasn’t translated into performance benefits as of yet. Therefore, you could simply take a pass on this notion of “train low” until more research is done and training protocols are developed showing clear performance benefits.
But if you have early-adopter tendencies and are interested in tinkering with train low, or if you find that you are already doing this type of training, the following practical tips may be helpful to consider.
Practical Tips to Train Low Effectively
Know when to train low and when to train high. Training in a glycogen-depleted state is not appropriate for high-intensity workouts and competitions. In these situations, train high and compete high. Train low during lower-intensity, longer workouts.
Don’t make the mistake of shifting to an overall diet of minimal carb intake. Yes, those incredible distance runners from Africa do early-morning training runs after fasting overnight, but they consume an overall diet that is predominantly carbohydrate-based.
If some is good, it doesn’t mean that more is better. If every workout is a train-low session, your ability to train and compete at a high intensity will suffer — you simply won’t have the required glycogen reserves. You could also suffer from a loss of muscle mass, increase your risk of getting colds, and put a serious damper on your mood. Balance periodic train-low workouts with your usual, train-high sessions.
Train low can be achieved in a couple of ways — both may be helpful. There are essentially two different types of train-low strategies. The first is to work out in the morning after an overnight fast, while also avoiding the intake of carbs as you exercise. In this situation, your muscle glycogen stores will likely be high, but liver glycogen stores will be low. The second method involves working out twice in one day and not consuming many carbs between workouts. In this scenario, the second workout is done in a muscle-glycogen-depleted state. The two approaches appear to lead to somewhat different training adaptations, so incorporating both into your workouts may have benefits.
Practical ways to incorporate train low into your workouts:
- Early in your training cycle, when you are establishing your endurance base and the exercise intensity is low, train low periodically (about 1–2 times per week) to help maximize aerobic adaptation. You can accomplish this by doing morning workouts after fasting overnight and by avoiding the intake of carbs during these workouts. Start with a short, 30-minute workout while fasted — slowly, over time, extend the duration of this workout to about 90 minutes. And because recovery is very important after these training sessions, you might consider not training hard again for at least 48 hours following a prolonged fasted training session.
- As you build up and prepare for increased speed with short speed workouts: While maintaining high mileage, train high, i.e., have high glycogen stores, before your high-intensity speed work; train low, i.e., in a glycogen-depleted state, periodically before endurance workouts.
- In the consolidation phase of your training — that time when you transition toward longer-duration speed workouts and decreasing mileage: Train high, i.e., have high glycogen stores, in the morning before your speed work; incorporate a same-day second endurance workout in a glycogen-depleted state a few days per week.
Train Low, Compete High
If you want to incorporate train low into your workouts, the bottom line is to do it wisely. That means periodically and for lower-intensity workouts.
Finally, during competitions and recovery, a carbohydrate-based intake is needed. You need carbs to load glycogen stores before a competition in order to maximize performance — i.e., to compete high — and within 30 minutes after exercise when rapid recovery is your goal.
References:
Hansen AK, Fischer CP, Plomgaard P, Andersen JL, Saltin B, Pedersen BK. Skeletal Muscle Adaptation: Training Twice Every Second Day vs. Training Once Daily. J Apply Physiol 2005; 98: 93–99.
Yeo WK, Paton CD, Garnham AP, Burke LM, Carey AL, Hawley JA. Skeletal Muscle Adaptation and Performance Responses to Once a Day Versus Twice Every Second Day Endurance Training Regimens. J Apply Physiol 2008; 105: 1,462–1,470.
Drust B, Morton JP. Promoting Endurance Training Adaptations with Nutritional Interventions: The Potential Benefits of “Low Carbohydrate” Training. Kinesiology 2009; 41: 19–24.
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