Are You Running On Empty?

Last week I reposted an Instagram post from the Sports Nutrition Association around calculating what is called your “energy availability”. A couple of people reached out because they were a bit confused as to how to perform the calculation. Before I elaborate on how to calculate your energy availability I think it is important to understand why knowing how much energy you have available is important.

Why Is Knowing Your Energy Availability Important?

One’s energy availability refers to the energy that is available for your body to perform its basic functions to keep you alive as well as fuel your exercise and movement. When your energy availability gets too low you will start to experience symptoms of low energy availability. This can include a drop in exercise performance, disrupted sleep, food cravings, lack of motivation, fatigue, brain fog, and hormone imbalances. Long term low energy availability can lead to what is called Relative Energy Deficiency in Sports or RED-S[1]. It might sound like energy availability is only a concern for athletes or people who are exercising a lot, but actually anyone restricting calories, either on purpose for the goal of losing fat or unintentionally due to a very high activity level, can suffer from low energy availability.

Calculating Your Energy Availability

Now that we know why knowing your energy availability is important how do you figure out your energy availability?

You will need to know three things.

  1. Your average daily caloric intake
  2. You average daily training caloric expenditure
  3. Your lean body mass in kilograms

Your average daily caloric intake can be determined by tracking your food for a week using an app like Cronometer and averaging your caloric intake for a week.

Your average daily training caloric expenditure represents how many calories you burn during exercise over the course of a week (7 days). One way to calculate this value is to look at your fitness tracker and add up all the calories it said you burnt during exercise over the course of a week and divide by 7. If you don’t have a fitness tracker then you can use an exercise calculator to get a rough estimate based on the activity, your weight, and the duration you exercised.

An example can help here. Say my week of exercise looked like this:

Monday: Intervals on the rower for an hour

Tuesday: Weight Training for an hour

Wednesday: Weight Training for an hour

Thursday: 40 minute run

Friday: 30 minutes of yoga

Saturday: Weight Training for an hour

Sunday: Rest day

According this exercise calculator, this is how many calories I burnt during each one of these activities:

Monday: 816 calories

Tuesday: 408 calories

Wednesday: 408 calories

Thursday: 567 calories

Friday: 102 calories

Saturday: 408 calories

Sunday: 0 calories

Over the course of the week I burnt a total of 2709 calories. To the average calories burnt during exercise over the course of a week I would divide 2709 by 7

2709 / 7 = 387 calories

Therefore my average daily training expenditure would be 387 calories.

To calculate your lean body mass you first need to know your body fat percentage. The best way to figure that out is to have it measured via a DEXA scan or via a BodPod, however not everyone has had this done. The most accessible way to calculate your body fat percentage is using another online calculator, I prefer the Navy Body Fat calculator, all you need is a scale and a measuring tape. Once you know your body fat percentage you can calculate your lean body mass by taking your weight and multiplying it by your body fat percentage. This will give you your fat mass. Your lean mass is then your weight minus your fat mass. To put that in a formula it would look like this

Fat Mass = weight * body fat percentage

Lean Body Mass = weight – fat mass

If your body weight is in pounds (lbs) you first need to convert that to kilograms (kgs). To do this take your weight in lbs and divide it by 2.2 or you can use this online conversion tool. For example, I weight 153 lbs so my weight in kgs would be calculated like this

153 / 2.2 = 61.81 kgs

Using the Navy Body Fat calculator I determined my body fat percentage to be 12%. To find my lean body mass in kgs I would then do the following calculations

Fat Mass = 61.81 * 0.12 = 7.41 kgs

Lean Body Mass = 61.81 – 7.41 = 54.4 kgs

So my lean body mass is 54.4 kgs.

Now that you have the three numbers you need you can figure out your energy availability. This is calculated using the following formula

Average Daily Caloric Intake – Average Daily Energy Expenditure / Lean Body Mass = EA

Let’s make it more concrete using real numbers

Average Daily Caloric Intake = 2700 calories

Average Daily Training Expenditure = 387 calories

Lean Body Mass = 54.4 kg

2700 – 387 / 54.4 = 42.5 calories/kg

Energy Availability Cutoffs

Now that you have your energy availability what is optimal and what is not?

Ideally you want your energy availability to be 40 calories/kg or above. When you get into the 30-40 calories/kg you could start to see some negative effects from the amount of energy available to your body. Once you get below 30 calories/kg then you can suffer some serious consequences [2].

What To Do If You Have Low Energy Availability?

There are three variables at play in this equation, caloric intake, exercise output and lean body mass. If you fall below 40 calories/kg, especially if you fall below 30 calories/kg, you are going to need to manipulate one of these variables to increase your energy availability.

Decreasing your lean body mass, which means gaining fat, is probably not something you want to do but it might be beneficial in some cases. If you are trying to live lean all year around it can be quite detrimental, putting on a little body fat might actually be good for you. Obviously this will correlate to the energy intake variable as well. More on that below.

The next variable you can tweak is your caloric expenditure during exercise. Hard charging athletes might not want to hear this, but you might want to drop your training volume. Are you working out 7 days a week? Do most of your training days include 2 exercise sessions? Is all that training necessary? Could you benefit from less volume? If you are not earning a living off your training, then I think the answer might be to reduce your training volume.

The last variable is your caloric intake. This is going to be the most popular variable and easiest variable to tweak. If you increase your caloric intake you will automatically increase your energy availability. It sounds straightforward, but oftentimes it is not as easy as it appears. I find that the reasons people are not eating enough is because they are trying to lose weight or they feel full and satiated and don’t feel hungry.

In the case where someone is purposely trying to lose weight and in a low energy availability state chances are the scale is not moving despite restricting calories. This is because if they are in a low energy availability state, their body does not want to lose weight because they already are not taking in enough energy to support their basic bodily functions. The only solution for this person is to eat more or exercise less.

In the situation where the person feels full and satiated and are unintentionally not eating enough they are more than likely starting to experience symptoms of low energy, decreased performance, disrupted sleep, mood swings, brain fog….their body is screaming at them that they need to eat more but they don’t have the desire. This person is likely eating a very “clean” diet. They are eating all whole foods which fill them up fast but are not calorically dense. For this person they probably need to ease up on the food quality and allow more “processed” calorically dense foods into their diet. Smoothies, nut butters, pancakes, waffles, pizzas, a night out with friends at a restaurant, would help this person get in the calories and increase the drive to eat. You can still eat whole foods, just make them more hyperpalatable.

Obviously increasing your caloric intake can also have an impact on your lean body mass. This brings me to my final point that you can tackle the energy availability problem by manipulating more than one variable. You can eat more and add a bit of body fat in the process, or you can eat more and decrease your exercise activity, you do not need to take a singular approach.

Increasing your energy availability is not as easy as it sounds. Food and exercise are very much habit based, so we are asking someone to change long established habits which is never easy. In addition, the mental aspect of asking someone to change their expectations and beliefs about what they need to do to be successful is a very hard thing to do. One way to overcome both of these challenges is to switch focus from whatever their current goal is (weight loss for example) to performance. Universally, whatever the reason for the low energy availability state is, as someone increases their energy availability their performance in their workouts should improve so switching to focusing on performance can be quite an eye opener to them. This can be due to having more fuel, but also because their sleep, mood, and/or recovery should get better.

Much of the energy availability problem is due to a mismatch between our current environment and our ancestral norms. No one was training for an IronMan, running a marathon, or doing CrossFit hundreds of thousands of years ago. Movement and “exercise” was done out of necessity to survive or for fun (think dancing for example). Furthermore no one was concerned with what they looked like with their shirt off. Whether the environmental mismatch comes from the amount of physical activity we are doing, or our desire to achieve a certain level of leanness, we should not let our health fall to the wayside in order to achieve that goal. To be our best selves we must correct this environmental mismatch and give the body what it needs, and in the case of low energy availability that is more food, and/or less exercise.

Working to correct these environmental mismatches is what the goal of my health coaching practice is all about. I work to align people with ancestral norms in order to achieve their optimal health so they can be the best athlete, parent, co-worker, and human being they can be. The best way to consume the content I produce is by joining my mailing list, you can do that by filling out the form below. When you do you will get a welcome email from me with links to all the past content I have produced. Be sure to hit reply to that email and say hello!

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  1. (n.d.). The IOC consensus statement: beyond the Female Athlete …. Retrieved January 16, 2021, from
  2. (n.d.). IOC consensus statement on relative …. Retrieved January 14, 2021, from

4 thoughts on “Are You Running On Empty?

  1. Thanks for the detailed explanation. Very handy. I’m pretty sure I’m running a bit low, morning fasting leading me to low cals, and then some binge-ing at night that messes up my sleep and then it’s not good.

    Maybe have a diagram with a person at 8%, 12%, 15%, 20%, 25% and some descriptors so folks can get a sense of where they might be quickly? This way folks can do some guesstimate now.

    Thanks RB!

    1. Thanks Kenn!

      Yeah fasting can make it difficult for sure, especially if you are active.

      I think the diagram can be useful, but I also think people have distorted images of their body and might not pick the correct percentage. The tape measure and scale won’t lie 😉

      1. Distorted … that’s why we need some images ;P … and a coach to keep us honest.

      2. Yeah agree the coach part will help but I still think the calculation is going to be better

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