The Nitty-Gritty of Nutrition, Part 2 – Calories In, Calories Out

[This is Part 2 of the “Nitty-Gritty of Nutrition” Series. Click here to read “Part 1 – The Macro on Nutrients“]

Welcome back! In Part 1 we covered what the various macronutrients used by the body are, and how they are used. Today, we’re going to go over the loss of and replenishment of calories: the rate they’re burned, how many calories we need, and how fast we can replace them.

The Numbers Don’t Lie To Us, But We Do


Before we figure out how much we need to put in the tank, we need to figure out how much we’re removing. Many cyclists use activity trackers that track power and heart rate to give us a pretty good idea of the calories expended on a ride. When we get home, we see those burned calories and think, “Yes! I can eat a whole extra day’s worth of food!” Slow down. Yes, you burned those calories, but one thing a lot of cyclists forget is that some of those calories would’ve been burned even if you were just sitting around watching the Tour de France.

Everyone has a natural burn rate (your metabolism, or what’s known as your BMR – basal metabolic rate). Your body expends energy to do the things you normally do throughout the day: process food, running the organs, keeping the body working. There are plenty of BMR calculators, and these might get you in range of your base caloric needs, multiplied by an activity factor.

For example, let’s say you have a BMR of 1,740. If you’re sedentary you’d multiply that BMR by 1.2 (an low activity factor), which would give you a total of 2,088 Calories/day to maintain your current weight. As cyclists – even excluding our cycling activities – we tend to be more active than most people, so we might want to bump that up a bit. Humans also vary in efficiency (from about 20-25%), and those on the less efficient side use more calories in maintenance (we call this having a “high metabolism”).

To keep the math easy, let’s say my daily caloric burn rate is about 2,400 Cal/day. That’s a little on the high-side, but not unusual for an active person.

  • Daily rate: 2,400 Calories
  • Hourly rate: 100 Calories
  • Bike ride length: 4 hours
  • Calories I would’ve burned not riding: 4 hr * 100 Cal/hr = 400 Cal
  • Calories my Garmin says I burned: 2,500 Cal
  • Calories I burned beyond my normal rate: 2,500 Cal – 400 Cal = 2,100

So here we see first the issue with caloric replacement. If I eat an extra 2,500 Calories today to make up for my four hour ride, I would’ve actually eaten 400 Calories more than I needed to replace (about 19% more), since my normal levels of eating would’ve been enough to cover those four hours of Calories I would’ve burned even if I wasn’t riding. When you put more calories into the body then it needs, it’s stored as fat. That’s an additional 44.4 grams of fat to store (about 0.1 lbs).
But there’s a second part to this: we’re taught to eat on the ride. In fact, we have to, since our body only stores enough readily available energy for 2 hours of intense effort. Even moderate efforts will eventually deplete our glycogen stores (just more slowly). So now let’s say I’m eating about 150 Calories/hr while on the bike.

  • 150 Cal/hr * 4 hr = 600 Calories
  • Calories I burned beyond my normal rate, and that I didn’t ingest while riding: 1,500

Now compare that 2,500 that my Garmin said to the 1,500 that we get when we exclude the food I ate on the ride and my normal burn rate. Suddenly, eating that 2,500 Calories puts me more than 1,000 Calories over what I needed to make up (67% more Calories than I actually needed). That’s 111 grams of stored energy, which is stored as fat (or a quarter of a pound).

So you see, it’s easy to overshoot the mark when it comes to refueling. But at the same time, don’t leave the tank empty. If you under-eat, then some of your energy requirements will be pulled from fat, and you start to lose weight. Under-eating too much however leaves you feeling empty, because your glycogen reserves won’t be topped up, so they’ll drain faster and you’ll have to pull from the slower-burning fat.

Fill’er Up

"Food is fuel. If you get picky about what you put in the tank, your engine's gonna die." Remy's Dad, Ratatouille.
Rest stops are good because you get rest, but they’re bad because you have to stop. Find things you can easily carry with you and eat on the go.

Unlike water, which the body absorbs very rapidly, it takes energy to process food. About 10% of the consumed Calories goes into food digestion (called thermogenesis). So 1/10th of everything you eat goes into processing everything you eat. That also means everything you eat doesn’t go straight into energy you can use for exercise. More than that, there quite a bit that will affect how fast we digest food. For example:

  • Liquids empty from the stomach much faster than solid foods. So foods with higher water content are easier to process.
  • Fat slows the digestive process, which means it’ll take longer for those calories to reach the muscles.
  • High concentrations of sugar (especially in liquid) can slow the emptying of the stomach.
  • Hard exercise – starting at about 70% VO2 max – will also slow digestion (this is more an issue for runners than for cyclists, since they are affected by the sport itself).

The first three are the most important: what you eat has a direct effect on how fast you’ll get energy from it. But before you start pounding some water-dense, high-carb foods thinking to replace that 800 Calories you burned in the last hour, you have to understand that there’s a maximum amount that the body can absorb per hour.

On average, the body can only absorb about 1.2 grams of carbohydrates per kilogram (that you weigh) per hour. So for example:

  • I weigh 165 lbs, or about 74.8 kg
  • 1.2 * 74.8 = 112.2 grams carbohydrates / hour
  • 112.2 grams carbohydrates is about 450 Calories

Anything more than that, and it’ll be sitting in my gut waiting to process, which can lead to gastric distress (i.e. “gut rot”). NOTE: You don’t get to use all 450 Calories towards your glycogen stores. Remember, about 10% of what you eat goes into processing what you eat, so right off the bat you’ve lost 45 Calories. Also, 450 is a theoretical limit; the actual limit – without sending me sprinting for the nearest port-a-potty – is probably closer to 350.

The maximum caloric and carbohydrate intake varies greatly from person to person (from 1-2 grams CHO/kg body weight/hr). On average, it is recommended that for events lasting more than 2 hours, you consider ingesting 200-300 Calories per hour, and approx. 60-70 grams of carbs per hour. Some people may be able to absorb more without digestion problems; others who have slower gastric emptying may have to eat less.

That’s it for now. In part 3, I’ll dig into the caloric debt issue that plagues cyclists and endurance athletes.

[NOTE: Sources include McKinley Health Center (University of Illinois), Cycling Performance Tips, many Wikipedia articles and subsequent related sources,, and several reports from the National Center for Biotechnology Information (NCBI)]