The facts about running in a carb-depleted state

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


There is evidence to suggest that doing some training in a carbohydrate-depleted state can help use more fat for fuel. This could be an advantage, particularly in endurance events where glycogen stores become depleted and a limiting factor. With moderate intensity, glycogen stores can last 90 to 120 minutes. The theory is that if we help our body adapt to using more fat as fuel, we can spare the stores for longer. This concept, known as ‘training low’, 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 ‘training low’ not only means training occasionally in a carbohydrate-depleted state, but that carbohydrate should be kept at a very low level at all times.

How to ‘train low’

While ‘training low’ does involve exercising in a carbohydrate-depleted state (many prefer to do this first thing in the morning), there are some key protocols to observe:

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

02. They should ideally be up to 90 minutes long, at an intensity no higher than 60% of your VO2maxor a 6/10 perceived exertion level.

03. Most importantly, you should still consume your overall carbohydrate needs after training, distributed evenly over the day; critical to get adaptation from training. One meta-analysis of studies showed that a low carbohydrate intake for three weeks or more began to have negative consequences for health and performance.

Risks of fast training

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

The research led by mike gleesona professor of exercise biochemistry at Loughborough and one of the leading voices in athlete health, showed that ‘post-exercise immune function depression is most pronounced when the exercise is continuous, prolonged (sessions longer than 90 minutes), of moderate-to- high intensity (55-75 per cent of aerobic capacity) and performed without food intake’.

In addition, other research has highlighted that exercising in a fasted state can cause the stress hormone cortisol to rise. If the cortisol level is chronically high, this can lead to a down regulation of hormones, resulting in poor metabolism, bad adaptation from training, storing more fat and presenting a higher risk of injury and illnesses.

Another big area of ​​research in the last 18 months has looked at bone health. A recent review of the literature, by Craig Sale and Kirsty Elliot-Sale at Nottingham Trent University’s Sport, Health and Performance Enhancement Research Centre, confirms strong links between low energy availability and deterioration of bone health. Additionally, they have demonstrated that, independent of energy, carbohydrate availability before, during and after exercise has a positive impact on bone response after hard training. While more research is necessary in this field, many animal models support these findings.

From my own observations and experiences in clinic, I would add that fasted training is not an option for anyone who is recovering from NETWORKS (relative energy deficiency in sport syndrome), or overtraining syndrome.

The benefits of fueling

Fueling morning training is advantageous for health, performance and body composition. Taking on some carbohydrates before training, then recovering afterwards 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.

Pre-morning run fueling options

  • hot cross bun
  • Glass of fruit juice and a banana
  • slice of toast
  • 1 Weetabix with milk
  • fruit yoghurt
  • sports gel

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