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Sports nutrition: the latest research into low glycogen training
Could training when muscle carbohydrate stores are low be advantageous to athletes?
Andrew Hamilton looks at the very latest research in this area and how it translates into training recommendations for athletes…
When it was first proposed as a useful nutritional approach to training, the ‘train low, race high’ theory ruffled plenty of feathers because it stood conventional wisdom about carbohydrate feeding on its head. To briefly recap, train low, race high is a theory born out of genetic evolution of the human race, and which suggests that training when muscle carbohydrate stores are low might actually be advantageous for performance.
The reasoning behind the theory goes something like this: our gene selection in the Late Palaeolithic era (when our ancestors roamed the plains as hunter-gatherers) would have been strongly influenced by the need to ensure survival during periods of famine, with certain genes evolving to regulate efficient intake and utilisation of fuel stores – so-called ‘thrifty genes’.
These genes would have enabled our forebears to utilise energy more efficiently, enabling them to forage for food and escape predators even when enduring famine conditions. As hunter-gatherers, without agriculture, they wouldn’t have had access to abundant supplies of ‘carbohydrate-dense’ crops and cereals but in order to survive, physical endurance and the occasional high-intensity burst of energy would still have been needed.
Thrifty genes and exercise
What’s fascinating is that there’s convincing evidence that our genetic makeup has remained essentially unchanged over the past 10,000 years and certainly not changed in the past 40-100 years(1), which almost certainly has profound implications for the 21st century athlete. In recent years, a number of ‘exercise genes’ involved in the adaptation to exercise and training have been identified, and some it seems are also affected by the biochemical environment in the muscle – eg how much muscle glycogen is present or circulating levels of hormones and other signalling molecules released when exercise is performed(2-4).
The obvious question, then, is this: given that these genes have evolved to help us maximise our adaptation to and physical capacity in a low-carbohydrate environment, is the almost universally recommended high-carbohydrate diet for athletes disadvantageous in any way? Or to put it another way, could vigorous activity in a carbohydrate-depleted state (as would have been the norm for our ancestors) possibly produce better training adaptations in the modern athlete? A number of scientists are increasingly confident that (thanks to our thrifty genes), lower levels of muscle glycogen during training might stimulate certain metabolic pathways in the body, resulting in better muscular adaptation to training(5).
Twice daily, alternate day endurance training
One of the earliest and most well respected studies to look into the effects of low-glycogen training compared the training adaptations in muscle produced by performing leg extension exercise either twice a day on alternate days, or once a day on consecutive days(6). Exercising twice daily resulted in muscles performing an identical volume and intensity of training, but doing so in a low glycogen state during the second session of the day.
The striking finding was the very significant gain in both time to exhaustion and total work performed in the twice daily, low-glycogen trained muscles compared to daily trained muscles (see table 1). In addition, the Danish researchers discovered that the low-glycogen trained muscles became better at burning fat for energy and soaking up carbohydrate to store muscle glycogen once carbohydrate feeding was resumed.
Table 1: Maximal power output and time until exhaustion at 90% of maximal power output before and after 10 weeks of training and total work before and after 10 weeks of training.
The implications of these findings were startling because they seemed to completely contradict one of the most universally accepted tenets of sports nutrition – that muscle glycogen depletion should be avoided at all costs. In plain English, this research indicated that although low muscle glycogen content is known to blunt performance on the day, when it comes to training adaptation, this might not be a reason to avoid glycogen depletion.
New research, new questions
Very recent human studies have added weight to the notion of train low, race high theory, particularly for producing desirable metabolic effects such as increased fat burning. However, they have also raised important questions because these metabolic effects didn’t seem to translate directly into increased performance (see box 2).
For example, Australian scientists have recently studied the effects of a cycling programme in which selected sessions were performed with low muscle glycogen content on training capacity and subsequent endurance performance(9).
In the three-week study, seven endurance-trained cyclists/triathletes trained once daily, alternating between 100-min steady-state aerobic rides (AT) one day, followed by a high-intensity interval training session (HIT; 8 x 5 minutes at maximum self-selected effort) the next day. Another seven subjects trained twice every second day, first undertaking AT, then 1-2 hours later, the HIT. In this second group of course, the HIT session was completed in a low-glycogen state.
Forty-eight hours before and after the first and last training sessions, all subjects completed a 60-minute steady-state ride followed by a 60-minute performance trial. Muscle samples were taken before and after the steady-state ride and rates of fat and carbohydrate oxidation were measured.
The results showed that, compared to the daily training group, the low-glycogen group experienced favourable metabolic changes, including higher levels of resting muscle glycogen, higher rates of whole body fat oxidation, and higher levels of key enzymes involved in fat oxidation and aerobic energy production. However, unlike the rat study (see box 1)(7), levels of a similar gene transcription activator (PPAR-gamma) remained unchanged. More importantly perhaps, while cycling performance improved by approximately 10% in both groups, there was no additional improvement whatsoever in the twice daily, low-glycogen group.
Meanwhile, similar results were obtained in a study that used running as a training model, carried out by scientists at Liverpool John Moores University earlier this year(10). Although this study did not have subjects following a strict low-glycogen training regime, it did examine the effects of reduced carbohydrate availability, by restricting carbohydrate drink use.
Three groups of recreationally active men performed six weeks of high-intensity intermittent running, four times per week. Groups 1 and 2 consumed a 6.4% glucose or placebo solution respectively. Both groups trained twice a day, two days per week. Drinks were taken immediately before every second training session and at regular intervals throughout exercise. Group 3 meanwhile trained once daily per day, 4 days per week and consumed no beverage throughout training.
Those in group 2 (who were training in a low-glycogen state during their second run) had significantly higher post-training levels of an enzyme called succinate dehydrogenase, a key enzyme in aerobic metabolism and one which indicates that the low-glycogen training had induced a greater level of aerobic adaptation. However, when the researchers looked at performance such as improvements in maximal oxygen uptake and distance covered on the Yo-Yo Intermittent Recovery Test, there were no significant differences between the groups. The researchers concluded that ‘training under conditions of reduced carbohydrate availability provides an enhanced stimulus for inducing oxidative enzyme adaptations of skeletal muscle, but this did not seem to translate into improved performance during high-intensity exercise’.
Low glycogen and strength
Finally, it’s worth reiterating that (as we reported in PP254) there still doesn’t seem to be any evidence that low-glycogen training is beneficial for very high-intensity exercise, such as resistance training. When Australian scientists examined the influence of pre-exercise muscle glycogen content on the activity of several genes involved in the regulation of muscle growth in seven male strength-trained subjects, they found that low muscle glycogen content had variable effects on the activity of these genes involved in glycogen synthesis and importantly, any differences in the activity rates were completely abolished after a single bout of heavy resistance training(11). The scientists concluded that ‘commencing resistance exercise with low muscle glycogen does not enhance the activity of genes implicated in promoting muscle hypertrophy’.
This notion also finds favour with a leading scientist in this field, Dr Keith Baar. He believes that if anything, weight training in a glycogen-depleted state may decrease training adaptations. This is because the transcriptional changes (activating genes) following resistance exercise are no different in a glycogen-depleted state (unlike endurance training) and the greater metabolic stress of training with low glycogen can actually reduce muscle protein synthesis. Therefore, strength training in a glycogen-depleted state should be avoided!
Should you train low and compete high?
If you’re new to the ‘train low, race high’ concept, there’s a lot of information to take in here, so let’s begin by summarising what the current research says about the subject:
- Training with lower levels of glycogen in the muscles appears to elicit greater endurance adaptations in muscles, such as improved aerobic efficiency and increased capacity to burn fat compared to training with high levels of muscle glycogen
- This greater metabolic adaptation almost certainly occurs as a result of enhanced activation of so-called ‘thrifty’ genes;
- There is no such advantage when strength training; indeed, low-glycogen training may actually be disadvantageous for strength and power athletes;
- High levels of muscle glycogen are always recommended for maximum performance on any given day (eg during competition); while training with low glycogen stores may enhance long-term adaptation, actual performance during this training will not be enhanced and may well be diminished;
- It’s still unclear as to the exact performance benefits of low-glycogen training. Although there are undoubtedly favourable metabolic changes after low-glycogen training, the results are rather mixed as to whether these changes translate into performance gains.
The last point is worth reiterating. Although the initial evidence is looking promising, there are a number of questions that we need to answer before we know categorically whether a train low, race high approach offers real performance advantages over conventional training approaches (see box 2).
It’s also worth adding that low-glycogen training carries with it a number of risks and drawbacks (see box 3) and these should be considered carefully before plunging headlong into a train low, race high strategy.
Despite all these caveats, however, a number of exercise physiologists are convinced that some low-glycogen training can yield real benefits for endurance athletes. There’s no doubt that for maximum performance on the day of a competition, you need to start your event with maximum glycogen reserves. However, training is about trying to teach your body to become as efficient as possible at producing energy – your actual performance during training is of lesser importance. So this is the time when it might be worth including some regular low-glycogen workouts. By doing so, you can stimulate your ‘thrifty genes’ to enhance your energy efficiency and production, which when combined at a later date with high-glycogen stores, could help you achieve a PB. Box 4 and table 2 below give some suggestions on how to introduce some low glycogen training into your routine.
Remember, though, to be cautious. If you do decide to experiment with some low-glycogen training, only do so once or twice a week and for limited periods. Be sure, too, to watch very carefully for symptoms of overtraining and fatigue.
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