We work out. We eat. We work out. We eat some more. What is it that we’re eating? How do the things we eat ultimately drive our legs? And how many calories are we really burning? I’m not going to give you specific eating guidelines since everyone has their own dietary needs, and needs to adjust them to their activity and personal requirements. But it’s good to understand what food really is to the body, how it’s used, and how fast it’s used.
Notes on Notation
Before I delve too deeply, I want to clarify something regarding calories. There are two notations for calories:
- c (lower-case “c”): this is the scientific notation for a “calorie”, and is equivalent to 4.184 Joules of energy – the energy needed to increase the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
- C (upper-case “C”): known a a food, nutritional, or dietary calorie, this is equivalent to 1,000 calories (lower-case “c”), and is also called a kilocalorie. A calorie is very small amount of energy, so when working with food, it’s easier to use Calories (1,000 calories) instead.
For the purpose of this article, all references to calories will mean food calories (upper-case “C”).
The Macro on Nutrients
There are three macronutrients that our body uses:
- Carbohydrates (Carbs, CHO): contains 4 Calories / gram
- Fat: contains 9 Calories / gram
- Protein: contains 4 Calories / gram
Contrary to the untold number of diet claims, our body actually needs all of these in some form. They key – like everything else in life – is moderation, and understanding what they’re used for. So let’s learn some more about these macronutrients.
- The main source of fuel for the body.
- It’s the easiest and fastest macronutrient to convert into energy.
- All tissues and cells in our body can use glucose for energy.
- Carbs are stored in the muscles and liver (in the form of glycogen) for readily available energy.
- Unusued glucose not stored in the muscles and liver is stored in fatty tissues as triglycerides (stored for long-term energy needs).
Fats (yes, we actually need fats):
- Needed for normal growth and development of the body.
- Long-term (slow-burn) energy reserves (our most concentrated source of energy is in our fat stores).
- Provides cushion/protection for the organs.
- Maintains the cell membranes.
- Needed for growth.
- Used in tissue repair (muscles, hair, skin, organs, etc.), antibodies, and the creation of essential hormones and enzymes.
- This is the last source of energy for the body, since the body has many other uses for protein, and is the hardest for our body to extract energy from.
A Place for Everything, and Everything In It’s Place
Carbohydrates are quickly broken down into glucose, which is stored as glycogen. Don’t confuse glucose (a simple sugar) with glycogen. Glucose is what the body uses to generate glycogen (a polysaccharide of glucose) that is then stored in the muscles and liver. When energy is needed quickly (immediate and/or high-intensity work), glycogen is broken back down to glucose, which is then used to power the body (through a process called glycolysis). NOTE: glycogen stored in muscle tissue is used directly for that muscle, whereas glycogen stored in the liver is converted into glucose which is delivered into the blood and then to the rest of the body for fuel.
The body stores enough glycogen to support approx. 2 hours of high level exercise before being depleted. It’s about 300-600 grams in muscles (1,200-2,400 Calories) and another 80-110 grams in the liver (300-400 Calories). Once all that’s gone, if you haven’t kept up with your nutritional intake, you hit the dreaded wall or “bonk.” Our body will shift to fat metabolism, which has been credit in giving us that sense of a “second wind,” but you won’t be able to support your initial levels of exertion, and it won’t last long.
At lower intensities of exercise, the body doesn’t pull all it’s energy from glycogen. It’s smart. At levels less than 50% VO2 max, the body will pull upwards of 50% of its needed energy from fats instead of stored glycogen. This is because the body naturally optimizes its energy stores for your exertion levels. If you’re not spending your energy as quickly, then there’s time to convert fat triglycerides into glucose to supplement what’s being pulled from glycogen stores. So a note for anyone wanting to shed a few pounds: pull back on the throttle. Spend more time in lower zones and you’ll actually burn more fat relative to glycogen, and be able to do this more days in a row.
Don’t knock fat though. Fat is important as a long-term, energy storage system for the body because it’s far more energy-dense, and requires less water than glycogen (about a sixth). Though our glycogen stores are typically enough to get us through the energy requirements of a single day (or 2 hours or hard cycling), our fat stores have enough energy to last for nearly a month. In fact, fat is such an efficient way to store energy in the body that if we had to store that same amount of fat as glycogen, our body weight would increase on average by about 60 pounds!
Protein really isn’t used by our body for energy except in cases where fats and carbohydrates aren’t available (meaning, in malnourished states). Instead, the body uses protein for building/rebuilding, including muscle repair and cell injuries, like the tiny tears/damage that occur during exercise.
Now that we know what these things are and how they’re used, in the next part we’ll get into how we burn and replenish our energy levels.
[NOTE: Sources include McKinley Health Center (University of Illinois), Cycling Performance Tips, many Wikipedia articles and subsequent related sources, Nature.com, and several reports from the National Center for Biotechnology Information (NCBI)]