Number-crunching formulas that approximate metabolic rate only work well for average individuals of a normal body composition, height, and weight. Let's face it - most endurance athletes aren’t average. By undercutting their resting metabolic rate by just a few hundred calories, an athlete can completely deplete storage carbohydrate levels in less than a week. Conversely, a daily energy surplus of just a few hundred calories can stretch the spandex by more than a pound a week. Therein lies the danger of fueling based on an approximate mathematical equation for metabolism.

A more precise method of determining resting metabolic rate is via respiratory measurements in an exercise physiology lab or health center. This test costs anywhere from $50 to $300, depending on where you have it performed. In the procedure, your inspired oxygen and expired carbon dioxide are measured to determine your daily caloric expenditure at rest, as well as the percentage of fat and carbohydrates that are being burnt. To account for the thermal effect of physical activity, this test can also be performed as you ride your bike or run on a treadmill, and your caloric expenditure at each speed or heart rate can be determined, allowing you to precisely determine proper caloric intake for energy deficit, energy balance, or energy surplus.

The reason this test works is because the amount of oxygen consumed and carbon dioxide produced directly correlate to the amount of nutrients that are being burned for energy. A specific volume of oxygen is associated with a precise caloric value, and expired carbon dioxide reflects carbohydrate utilization. The leftover caloric consumption is then attributed to fat utilization (protein use as energy, at rest, is negligible, and is actually quite minimal until the 2-3 hour mark of exercise).

So let’s look at an example of how a triathlete can use metabolic calorie determination to their advantage. Joe Triathlete takes a resting metabolic rate test at his local exercise physiology lab and discovers that his body at rest burns 1800 calories. This means that Joe could lie in bed all day and do absolutely nothing, and he would still burn 1800 calories. This also means that if Joe were to follow a 2300 calorie per day diet without exercising, he would gain a pound of fat a week!

Then Joe takes an exercise metabolic test and finds out that 650 calories per hour is the average caloric expenditure associated with the heart rate at which Joe will be performing his 2 hour weekend bike ride. To maintain energy balance, he will need 1800 calories from his resting metabolic rate, plus 650 calories x 2 hours of riding, for a total caloric consumption of 3100 calories.

He can then adjust this number up or down, depending on his energy balance goals. If Joe is trying to lose weight, he could underfeed by 500 calories during the day of his bike ride. If he’s trying to gain weight or carbohydrate load, he could overfuel accordingly.

For athletes attempting to pursue weight loss and performance simultaneously, my coaching recommendation is as follows: eat at least the number of calories associated with your resting metabolic rate, and achieve any desired caloric deficit through exercise. Furthermore, when attempting weight loss, males should not consume more than 1000 calories under their daily energy expenditure (not to be confused with resting metabolic rate), and females not more than 500 calories. If a caloric deficit is too large, the metabolism will naturally decrease, performance will be limited, energy level and mental focus will suffer, and the immune system can be compromised.

For an athlete on a training program that cycles between hard and easy training and includes recovery days, the greatest emphasis on energy balance should take place on the hard days. During easy training and recovery, it is possible to underfeed without significant metabolic damage. In such a scenario, a triathlete who is attempting to lose weight can maintain full storage carbohydrate levels for hard training sessions and races, while allowing for an energy deficit and fat burning on easy and recovery days. This practice is discussed in more detail in my forthcoming book on nutrition for endurance athletes.

In summary, maintenance of energy balance in endurance athletes is crucial to performance, and utilization of metabolic data combined with knowledge of calories consumed is the most precise method to achieve balance.

Until next time, train smart,
Ben Greenfield