The facts about running in a carbohydrate depleted state

‘Fasted training’ is a term that has been in the vocabularies of most runners for several years. But how many of us fully understand the term and its specific purpose? For some runners, fasting training is a matter of comfort. They want to train first and it’s too early to eat anything before going for a run. For others, it’s a way of trying to improve adaptation from training. Whatever field you are in, it is important to understand the process and whether it can actually have negative consequences for your performance.


There is evidence to suggest that doing some training in a carbohydrate-deficient state can help use more fat for fuel. This could be an advantage, particularly in endurance events where glycogen stores are depleted and a limiting factor. With moderate intensity, glycogen stores can last from 90 to 120 minutes. The theory is that if we help our bodies adapt to using more fat as fuel, we can save supplies for longer. This concept, known as “low training”, was first used in long distance road cycling. But as is often the case, the scientific processes and research behind this theory are misinterpreted: so many runners believe that “low training” doesn’t just mean occasionally training in a state of carbohydrate depletion, but that carbohydrates should be kept at a low level. a very low level always.

How to ‘train low’

While “low training” involves exercising in a carbohydrate-depleted state (many prefer to do this first thing in the morning), there are a few key protocols to observe:

01. These sessions should only be done a maximum of two or three times a week.

02. Ideally they should last up to 90 minutes, with an intensity of no more than 60% of your VO2 max or a perceived exertion level of 6/10.

03. Above all, you should still consume your overall carbohydrate requirement after training, evenly distributed throughout the day; essential to get the adaptation from training. A meta-analysis of studies showed that low carbohydrate consumption for three weeks or more began to have negative consequences for health and performance.

Risks of fasting training

Studies from Loughborough University reveal that low carbohydrate availability often results in a depressed immune system.

Research led by Mike Gleeson, professor of exercise biochemistry at Loughborough and one of the leading voices in athlete health, showed that “depression of post-exercise immune function is more pronounced when exercise is continuous, prolonged ( sessions longer than 90 minutes), of moderate to high intensity (55-75 percent of aerobic capacity) and performed without food ‘.

Additionally, other research has shown that fasting exercise can increase the stress hormone cortisol. If the cortisol level is chronically high, this can lead to hormone downregulation, resulting in poor metabolism, poor training adaptation, storing more fat, and presenting a greater risk of injury and disease.

Another large area of ​​research over the past 18 months has looked at bone health. A recent literature review, by Craig Sale and Kirsty Elliot-Sale at NottinghamTrent University’s Sports, Health and Performance Enhancement Research Center, confirms strong links between low energy availability and deteriorating bone health. Furthermore, they have shown that regardless of energy, carbohydrate availability before, during and after exercise has a positive impact on the bone response after a hard workout. Although more research is needed in this area, many animal models support these findings.

From my observations and experiences in the clinic, I would add that fasted training is not an option for those recovering from RED (relative energy deficit in sports syndrome) or overtraining syndrome.

The advantages of refueling

Feeding your morning workout is beneficial for your health, performance, and body composition. Taking in a few carbohydrates before training, and then recovering later with a good combination of protein and carbohydrates, ideally within 30 minutes, will ensure optimal adaptation and progression and encourage a consistent approach to training.

Refueling options before the morning run

  • Hot crisscross bun
  • Glass of juice and a banana
  • Slice of toast
  • 1 Weetabix with milk
  • Fruit yogurt
  • Sports gel

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