The Low Down on Training Low – Depletion Training

Training Low is a strategy whose goal is to enhance fat metabolism or use of fat as energy during training. The mechanism is to purposely withhold carbohydrate prior to exercise in order to force the body to work out with low carbohydrate stores (glycogen). The assumption is that the body will have to use more fat to make up for the energy deficit.

Some of the runners who subscribe to this theory go on low intensity medium distance runs first thing in the morning with no breakfast; they do not break the fast that they have been on while sleeping through the night.

Here is the science behind the myth.

Assumption 1: Fat will be preferentially used if carbohydrate is not available.

FALSE. Actually, the body’s go-to energy source during a short fast is protein. Because of the amino acid structure making up proteins, they can more readily produce new glucose and provide rapid energy. The primary costs are:

  • nitrogen buildup which is toxic and impairs performance by causing fatigue and dehydration
  • muscle depletion and no chance for muscle growth

Not what the average runner has in mind.

Assumption 2: Low intensity runs are effective in forcing fat to be used as energy.

FALSE. After a short period of time, your body will shift to using a higher percentage of fat as energy to avoid the complications of using protein as energy listed above; but this can take up to 20 minutes of running before fat breakdown reaches maximum activity. This delay is due to the relatively long time it takes to remove fat from storage. This process is controlled by 3 hormones depending on the conditions-

  • glucagon (stimulated by low blood sugar)
  • epinephrine (stimulated by exercise, stress)
  • cortisol (stimulated by stress, sickness)

Each one of these hormone’s activities is induced when your body is lacking something, is under stress or is sick. These same hormones also induce glucose release from glycogen for the identical reasons. But if glycogen is depleted due to fasting, this hormonal activity reaches a higher level of energy desperation and may result in increased risk of illness or injury.

Assumption 3: Low intensity workouts use more fat than high intensity ones.

FALSE. While it is true that as exercise intensity increases the percentage of fat used for energy decreases, the absolute amount of fat used increases with intensity. In other words, when driving a car cross country you may get better gas mileage as compared to city driving, but the long distance drive is still going to burn more fuel overall.

Assumption 4: Depletion training has been shown to improve performance.

FALSE. Very little reliable research has been conducted using this training strategy. The few studies that are review worthy show no improvements in performance without detrimental effects on the athlete (Hawley and Burke, 2010). Most of this research has been conducted with cyclists in a laboratory with no real evidence from ‘the road’.

  • Even the studies that do show some metabolic training enhancements, report that the athletes choose lower intensity workouts following these training low sessions due to fatigue and stress.

Overall, most of the support for this training regimen comes from anecdotal evidence, i.e. stories from one runner to another and should not be applied to the running population as a whole without some evidence based data.

You will not go wrong by eating carbohydrates (60% of calories) and most especially breaking the fast before you run!

Kathleen Deegan, PhD, MS, RD
Sports Nutritionist, California State University, Sacramento
SRA Fueling Specialist

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