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Metabolism – What is It?

Critical Concepts Series: Metabolism for Athletic Women – Part Three

Metabolism can seem very complicated. But simply put, it’s how your body turns the food you eat into energy.

For athletic women over 40 who train hard, it’s important to understand a more complete picture of metabolism, energy balance and energy flux.

A common trap to fall into is the belief that you only use energy when you are exercising. And by extension, the only way to increase any energy deficit is to exercise more – harder, longer, more frequently. 

Another mistake is thinking that the only thing that matters is the size of a calorie deficit instead of the deficit compared to how much energy comes in and goes out.

This is part three in a series of articles on energy intake for athletic women over 40.

  1. Part one looked at why low daily energy intake has negative effects on everything from motivation to performance in athletic women.
  2. Part two covered why “energy in, energy out” is more nuanced than “calories in, calories out.”
  3. Part three – this article – will focus on metabolism. You’ll also learn the factors of total daily energy expenditure, including non-exercise activity thermogenesis and purposeful physical activity. You’ll also get an introduction to energy flux.
  4. And in part four, you’ll learn more about how to improve your metabolism as an athletic woman over 40.

metabolism for women athletes over 40 - woman with brown hair sits on a gym floor leaning against a loaded barbell

Metabolism and Total Daily Energy Expenditure (TDEE)

The truth is that until you die, you never stop expending energy. 

When you say your metabolism “is broken,” that’s not accurate. Your metabolism never stops while you’re alive, and it doesn’t break.

It simply adapts.

If you’re interested in how to “boost” your metabolism, stay tuned for part 4 in this series.

To understand how you’re constantly expending energy, let’s look at a four-part model of total daily energy expenditure (TDEE) (1): 

1. Resting Energy Expenditure (REE)

The amount of energy you use across 24 hours from all non-activity, such as sleeping, sitting quietly, breathing, etc. This is also called Basal Metabolic Rate (BMR). REE usually accounts for about 70% of your TDEE.

2. Thermic Effect of Food (TEF)

The energy required to digest and absorb food. Protein requires the most energy to digest and has a higher TEF compared to carbohydrate and fat. This is approximately 10% of TDEE.

3. Non-Exercise Activity Thermogenesis* (NEAT)

This is all the movement and activity you do which isn’t intentional or structured activity. NEAT is generally greater than the energy you expend through exercise. This is about 15% of TDEE.

4. Exercise Activity Thermogenesis (EAT)

This is the energy you expend when engaging in intentional exercise. Along with eating less, it’s one aspect people try to manipulate to create a calorie deficit…aka the “move more” in “eat less, move more.” This is approximately 5% of TDEE.

graph of total daily energy expenditure for understanding metabolism and how does it work

*Thermogenesis is the production of heat which occurs when your body is involved in processes such as digestion and sustained physical activity.

What is the Energy Balance Equation?

By now, you’re making progress toward understanding that energy balance is a bit more nuanced than the “calories in, calories out” model.

Energy in is influenced by (among other factors):

  • the balance of protein energy and non-protein energy.

And energy out is influenced by (among other factors):

  • energy burned at rest
  • energy burned during the metabolism of food
  • energy burned during non-exercise activity, and
  • energy burned during formal exercise. 

Each one of these factors can influence the other.

It’s worth noting: The importance of nonexercise activity thermogenesis is often far underestimated. More about increasing NEAT in part four.

metabolism is how the body uses food like this container of chicken and sweet potato

What is Energy Flux?

Which brings us, finally (mops sweat from brow!), to the concept of energy flux

Flux is the magnitude and direction of flow through a system. In this case, it refers to the magnitude and direction of the flow of energy through your metabolic systems

Energy intake is the influx of energy into the body. Energy expenditure is the efflux of energy from the body. (2) In other words:

The energy you take in = energy influx.

The energy you use = energy efflux.

How we commonly refer to energy balance (or imbalance) largely ignores the magnitude of the energy flux through the body. 

This concept is perhaps best illustrated with some numbers.

all the chemical reactions your body uses to turn food into energy and building blocks is metabolism - a woman holds a glass container of food

High Energy Flux vs Low Energy Flux Effects on Metabolism

Let’s say you’re trying to achieve a daily energy deficit of 500kcal for fat loss – a commonly made recommendation, even to women athletes over 40. 

You could cut your energy intake to 1000kcal per day and make sure your energy expenditure is 1500kcal per day. 

-1500 + 1000 = 500 calorie deficit

Or you could eat 2000kcal per day and expend 2500kcal. 

-2500 + 2000 = 500 calorie deficit

Each of these energy in minus energy out examples gives you a deficit of 500kcal. And you might expect these to have an equal effect…except they don’t.

You’ll look, feel, and perform completely differently with one level of energy intake versus the other. 

woman eats a bite of food from a container of chicken and sweet potato

Eating 1000kcal per day will leave you depleted, struggling to maintain body composition even when you’re not eating much, and at risk for Low Energy Availability and RED-S.

Eating 2000kcal per day means you have plenty of energy to train hard, build muscle, stay active, and perform well.

This is where understanding the concept of energy flux is more useful than straight up deficits or surpluses.

Summary of Metabolism – How Does it Work?

Part four will look at how energy flux affects metabolic adaptation and practical ways to “improve metabolism.”

In the meantime, here’s a quick summary:

  • When considering your total daily energy expenditure, it’s better to understand it from a complete point of view. You use energy to exercise. But you also use energy at rest, to digest and absorb food, and through all the non-exercise activity you do. In a day, the energy used via non-exercise activity is often greater than what you expend during exercise.
  • Energy flux takes into account both the magnitude and direction of any energy flow through your body rather than simply whether you’re in a surplus or deficit. It’s a more nuanced way to consider your overall energy balance. 


  1. Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: implications for the athlete. J Int Soc Sports Nutr. 2014 Feb 27;11(1):7. doi: 10.1186/1550-2783-11-7. PMID: 24571926; PMCID: PMC3943438.
  2. Melby, C.L.; Paris, H.L.; Sayer, R.D.; Bell, C.; Hill, J.O. Increasing Energy Flux to Maintain Diet-Induced Weight Loss. Nutrients 2019, 11, 2533. https://doi.org/10.3390/nu11102533

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Steph Gaudreau

Hi, I'm Steph Gaudreau (CISSN, NASM-CPT)!

Nutrition and fitness coach for women, Lord of the Rings nerd, and depending on who you ask, crazy cat lady. My mission is to help you fuel for more: bigger muscles, strength, energy, and possibilities. We’ll do it with my signature blend of science, strategy…and a little bit of sass.


Purpose built for strength, fitness, and athleticism. This is a templated, app-based 12-month progressive strength program for women over 40.


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