Weight Training on an Empty Stomach for Fat Loss

woman raising kettlebell over her head in an empty gym
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Weight training on an empty stomach is generally said to be beneficial for maximum fat loss, but is this just trainer speak or is there a science to back up this all too familiar assertion?

Fasted Exercise and the Fasted State

The idea of a fasted state is that your body has had time to digest and metabolize a good proportion of what you ate at the last meal, thus altering your body's fuel preference from glucose to fat.

Most people reach a fasted state four to six hours after eating a meal.

Hormones like insulin and glucagon change in relation to the amount of glucose in blood and liver. Glucose burning is favored when blood glucose is high, and when this subsides after a time without food, fat burning is given preference to preserve the much lower stored quantities of glucose in muscle and liver and to help maintain that all-important blood glucose level.

The Evidence Against

The advice to train in a fasted state is, with all good intentions, a strategy to increase fat burning with the hope of using up some stored fat. However, the use of fat and glucose as energy fuel not only occurs in relation to your fasting state but also the intensity of the exercise you are doing. The heavy lifts or fast running will use stored muscle glucose (glycogen) more than fat.

If you exercise when fasted, the tendency to overeat will be strong in the post-exercise period, possibly negating any fat-burning advantage.

In addition, fat and glucose fuels get prioritized over 24 hours in relation to the fasted state and all energy demands. The extra fat you might burn during a fasted exercise session is unlikely to be critical in helping you to mobilize that stubborn stored fat when considered overall. And the risks of exercising too intensely in a fasting state are muscle degradation as your system pulls apart amino acids to help preserve critical blood glucose and immune system depression that can be triggered by chronic low blood glucose and the rise of cortisol.

The Evidence For

Conversely, there is evidence that supports the idea that fasted exercise may promote fat loss. There are two things that happen to the body in the fasted state: increased insulin sensitivity and increases in growth hormone(GH). Both of these, insulin sensitivity and growth hormone are known to increase fat loss. When you consider that these are more pronounced in a fasted state, the case could be made that fasted exercise would result in more fat loss.

When to Eat Pre- and Post-Exercise

The best strategy to burn maximum fat, and lose the weight if that's your aim is to eat two hours before exercise or weight training, or, if you wake early and like to exercise first thing, have a snack like a piece of honey toast or an energy bar, or a diluted glass of juice or small sports drink on the move. That way you can meet the requirements of not being in full food complement and still encourage the fat burning process without getting into negative territory. And importantly, don't overeat after exercise if you want to lose some fat weight.

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