In the past, when we have discussed nutrition on this blog, it was usually to make a point about wellness. See, for example, my post “Diet and Your Immune System.” But diet means more than wellness. It can dramatically affect athletic performance, too. So this time I want to look at calories for performance.
Considering calories for performance means looking at food as fuel. Fortunately, we measure both food and exercise in terms of energy: kilocalories, or calories for short. Your daily food allowance depends on your metabolism and your activity level. The conventional recommendation for sedentary people is 2,000-2,600 calories for a male and 1,600-2,000 calories for a female. The ubiquitous “Nutrition Facts” label on American food is based on 2,000 calories per day, which is obviously a compromise between the supposed needs of males and females.
Your Metabolic Rate
The body burns energy even at rest. Your minimal daily calorie requirement — the amount of energy you need to take in just to stay alive — is called the basal metabolic rate (BMR). BMR varies with age, gender, and body size. But even after accounting for those variables, your BMR can still vary with the idiosyncratic processes of your unique body.
Say your BMR is 1500 calories, which is fairly typical for an adult male. Then you need 500 calories’ worth of activity if you want to base your diet on the 2,000 calories of the Nutrition Facts label. Harvard Medical School offers charts that show the calorie burn of various activities for three different body weights (you tend to burn more calories, whatever you do, when you’re heavier). Those charts show that somebody in the middle range burns about 60 calories every 30 minutes while doing desk work. That means a little over half a day’s work (4.17 hours) would take care of your 500-calorie allowance.
How Many Calories a Day?
A daily diet of 2,000 calories, in other words, is probably close to the needs of a typical sedentary man. But I think it’s fairly clear that an average man cannot support athletic performance on that diet. Even a one-hour bike ride at moderate speed (15 mph) will burn nearly 750 calories. And if you’re a serious athlete, a one-hour moderate speed bike ride is just the day’s first activity. After that, you might run 6-7 miles (850 calories) or swim intervals for an hour (500 calories).
That means if you have a desk job, and you’re training 8-10 hours a week in your spare time, you may need about 3,000 calories per day to keep going. Anything less than that, and you will likely lose weight fairly steadily, which is probably why triathletes are so often slim.
Calories for Performance
How, then, do you calculate your personal daily calorie requirement? First, you need to know your BMR. There are various calculators on the web you can use to estimate it, but bear in mind that your actual BMR is as unique as you are. All the numbers I’ve been tossing around in this post are back-of-the-envelope estimates.
If you know your BMR, you can try to figure out how many calories you burn when you exercise. The Harvard Medical School table I linked to above is a good starting point. Or, if you use a wearable, it will likely estimate your calories for an activity based on weight and your heart rate. Neither of these is likely to be precisely accurate for you, so your best bet is to start with estimates (either from the table or your wearable) and track them over time. Eventually, you should be able to tell how close the estimates are, based on what your weight is doing over time.
But you can be a lot more precise than that. Peak’s nutrition planning service can measure your metabolic rate, scan your body to determine its composition, and plan your calorie intake of carbohydrates, protein, and fat based on dietary restrictions, food preferences, and even travel schedule. This is the science of calories for performance.