Fuel 101: Calories
Fuel 101: Calories
Hit Your Target. Don't Just Count Down.
Most nutrition apps treat calories like a budget you're trying to cut. Log your food, stay under your number, lose weight. Simple enough if your main goal is to sit less and eat less. But if you're training four, five, six days a week, that framing is working against you from the start.
Calories are energy. Not good energy or bad energy. Just energy. And athletes need a lot of it.
What a calorie actually is
A calorie is a unit of heat. One kilocalorie (what we call a calorie on food labels) is the amount of energy needed to raise one kilogram of water by one degree Celsius. Your body uses this energy constantly, whether you're asleep, working at a desk, or two hours into a long ride. The total amount you burn across a day is your Total Daily Energy Expenditure, or TDEE.
TDEE has four components. Your basal metabolic rate, which is the energy your body uses just to keep you alive, makes up the largest chunk at roughly 60 to 70 percent. Then there's the thermic effect of food (digesting your meals actually burns calories), non-exercise activity like walking and standing, and finally your actual training.
That last number varies enormously from one day to the next, which is the core problem with a fixed daily calorie target.
Why your target needs to move
A sedentary person burns roughly the same number of calories every day. Give them a fixed target and it works reasonably well. Give that same fixed target to someone who rides 90 minutes on Tuesday, does nothing on Wednesday and runs 20km on Saturday, and the number is wrong two days out of three.
Under-fuelling training days means going into sessions already behind. You'll fatigue earlier, recover more slowly and adapt less effectively to the training stimulus. Do it consistently and you're not just leaving performance on the table, you're building a cumulative deficit that eventually catches up with you.
The correct approach is to let your calorie target flex with your training. More on hard days. A little less on rest days. Across the week, the average looks sensible. Day to day, it reflects what your body is actually doing.
The risk most athletes don't know about
Relative Energy Deficiency in Sport, known as RED-S, is what happens when energy intake consistently falls short of what training demands. It used to be called the Female Athlete Triad, but it affects athletes of all genders across all sports.
The consequences go well beyond feeling tired. RED-S impairs bone density, hormonal function, immune health, concentration and mood. It increases injury risk and slows recovery. A lot of athletes who train hard but stall, get ill repeatedly or pick up niggling injuries are dealing with some degree of energy deficiency without realising it.
The fix sounds obvious: eat more. But it only works reliably if your calorie target actually reflects what your training demands.
What this looks like in practice
A useful starting point for most endurance athletes is somewhere between 1.4 and 1.7 times their basal metabolic rate on training days, depending on session length and intensity. A two-hour Zone 2 ride might add 600 to 900 calories to your daily requirement. A 90-minute threshold run can be similar.
Think of your calorie target as a floor on training days, not a ceiling. You are trying to match your intake to your output. That is a fundamentally different mindset to tracking calories to lose weight, and it changes everything about how you approach food.
Calories are the starting point
Hitting your calorie target matters. So does what those calories are made of. Two athletes eating the same total can have very different energy levels, recovery rates and body composition depending on how those calories are split between carbohydrate, protein and fat.
Calories set the stage. The rest of the Fuel 101 series covers what to fill them with.
Next in the series: Fuel 101: Carbohydrates — Your Engine, Not Your Enemy