By Teemu Virtanen

This month, you have been able to read about carbohydrates and protein as our pivotal sources of fuel and muscle power. Now it’s time to focus on the source of energy that we all have an almost infinite store, and that is fat.

As stated earlier, carbohydrates are most quickly converted into energy. But because the body can only store about 2,000 calories of carbohydrates (in the form of glycogen), this energy source is depleted by 2-3 hours of moderate exercise. In contrast, fats are more slowly converted into energy, and the body has tens of thousands of calories of fat, even on a lean endurance athlete.

Metabolic efficiency is the measure of how well the body utilizes fat as an energy source. Its efficiency can be improved through exercise and nutrition, specifically by exercising more at lower intensities, especially early in a training cycle; and by supporting stable blood sugars by eating more lean proteins, healthy fats, fruits, and vegetables instead of high carbohydrate foods.

Metabolic efficiency training aims to manipulate your macronutrients (carbohydrates, fats and proteins) and exercise adaptation. When you manipulate your exercise with correct aerobic training, it allows you to reap the benefits of increased fat utilization. By effectively manipulating your aerobic training, you will increase mitochondrial capacity (the number of mitochondrial enzymes).

By manipulating your diet to control blood sugar and your insulin response, you will be able to fuel with fewer calories during training and racing, thereby reducing your reliance on consuming simple sugars and increasing utilization of your internal body fat. This becomes extremely important when racing long distances such as Vasaloppet and even longer races.

Even though we burn a mix of both carbohydrate and fat to fuel exercise up to maximal intensities, as the intensity of your exercise increases, your body prefers to use more carbohydrates for fuel. There are several biochemical mechanisms contributing to this increased use of carbohydrates, and these mechanisms decrease your fat oxidation or usage.

The crossover point is that intensity at which you start to burn more carbohydrate than fat, and it can be measured or defined in terms of speed, pace, watts or heart rate. This gives you a pace or heart rate to train below in order to increase your fat-oxidizing capacity, which ultimately makes you a better fat burner.

So, metabolic efficiency training lets your body use fats and carbohydrates more efficiently. It manipulates cellular processes through aerobic training by increasing the size and number of mitochondria in your cells. If you continue this aerobic exercise without a lot of available carbohydrates, your body adapts by increasing its workforce of enzymes that metabolize fat for energy.

This is not to say that interval training or hill repeats do not produce positive changes, but they just don’t improve fat burning. You will be more metabolically efficient if you work on fat-burning (aerobic exercise) first, and then add in the other training regimes. Frequency is the key to this type of training. You will need at least six to seven hours per week in this zone for results, but it is critical to have most of your training in this zone if your aim is to increase your ability to use fat as your energy source.