Energy availability: a different way of planning your nutrition

This something I’ve been meaning to contribute to this forum for a while. I’m laid up on the couch sick without the brain power to do work, so here we are. Without explicitly stating my credentials, I’ve been a researcher in this field for 6+ years and an MD. The following is not medical advice.

This forum is home to a lot of discussion about nutrition both on the bike and off the bike. Primarily, the discussion seems to be (1) how to best fuel work to perform better and (2) how to “win in the kitchen”. For some, 1 and 2 are intrinsically related (e.g., starve yourself on the bike so you can eat more off the bike). For others, they are completely separated. For my money, the answer lives somewhere in the middle, but that’s beside the point.

One of the basic questions that arises is “how much should I eat?” (followed by “what should I eat?”). To help answer this for athletes, I am going to introduce a new-to-this-forum concept: energy availability (EA).

Energy availability is NOT energy balance (EB) (i.e., calories in vs. calories out). EB is actually really hard to assess. Here are the components:

  • How much you’re eating – easy enough
  • Basal metabolic rate – easy to estimate with an equation, annoying to measure in a lab for a civilian
  • The thermic effect of food – how much energy your body requires to breakdown what you’re eating…good luck without being in a whole room direct calorimeter
  • Total energy expenditure throughout the day – easy to estimate for cyclists while on the bike with a power meter, but what about everything else you do?

Each of these components have measurement error in the free living setting, and those errors can add up. However, for the sake of argument, let’s say you could accurately get a handle on your EB. To my knowledge, goal EBs for different adaptations in athletes have not been established. We can intuit that a negative EB will cause weight loss, but how negative does one have to be before running into performance decrements? On the other side, how positive do you have to be to cause muscle protein synthesis without adipose deposition?

Enter energy availability. EA is the preferred marker of nutritional status in athletes (source). EA is defined as your energy intake (EI) minus your exercise energy expenditure (EEE) normalized to your fat free mass (FFM) per day:
Let’s break down the components and then discuss why this is useful:

  • EI – probably the hardest to estimate, but doable
  • EEE – again, this is easy for us cyclists with power meters. Please note that EEE is different from energy expenditure in EB: we only care about what you’re doing while exercising. How do we define exercise? Call it anything where your HR is elevated or you’re engaging in purposeful movement for >30 minutes. Running up and down the stairs 5 times while you’re trying to find your keys on the way to work is not exercise; taking your dog for a long walk after dinner is. That said, for well-trained cyclists, the contribution of these components will be minimal.
  • FFM – why not just body mass? Adipose tissue (fat) is not metabolically active. It doesn’t cost anything (calorically) to maintain, so it’s irrelevant to understanding how to adequately fuel athletes. Traditionally, DXA has been viewed as the gold standard for measuring FFM; however, this has recently come under scrutiny. As far as I’m concerned, you can ballpark your figure with a scale—we don’t need research-grade measurements (or get a skinfold assessment!).

Okay, so you do some math and get an EA in kcal/kg FFM/day. What does it mean? EA quantifies the amount of energy left over for normal bodily function after accounting for the energy lost to exercise. I think the concept is easy to understand. The number you get, however, is weird. What does an EA of 20 kcal/kg FFM/day mean? What does an EA of 40 mean? From a numbers perspective, EB is easier to understand.

Luckily for us, this has been studied! Traditionally, EA has been studied in (white) adolescent and young adult female athletes and the ranges that have been developed reflect those populations. Much of this work defined the female athlete triad and relative energy deficiency in sport (REDS). See below:


Importantly, an EA <30 kcal/kg FFM/day has traditionally been defined as the threshold at which athletes experience the negative changes associated with low EA. This “threshold” has been debated for the past several years and is not particularly germane to this discussion—obviously there is individual variability. What is germane is that this threshold(ish) was defined in female athletes and recent work suggests that male athletes are more resilient to low EA—they start to see health/performance decrements at EAs of 20-25 kcal/kg FFM/day.

Let’s do an example. I am a 76 kg cyclist and let’s say I’m 12% body fat (I’ve had estimates from 5%-18%, lol). The thing that I care about on a day-to-day basis is how much should I eat? We can rearrange our EA equation to calculate EI:
Say my target EA is 45 kcal/kg FFM/day. Last week, I did a 2 hour sweetspot workout + added Z2 for a total of 2.5 hours, putting out 2370 kJ (kcal). Plugging these numbers in:
EI = 45 • (76 • (1–0.12)) + 2370
EI = 5379.6 kcal

Yeehaw, that’s a lot of food! Even if we put me at 18% body fat, we’re still looking at 5174.4 kcal.

So what does this all mean? Obviously if you have day where you’re low, you aren’t going to fall off a cliff; if you have one day where you’re high, you won’t turn into a blimp. It’s all about trends. Short exposures to slightly low EA are not inherently bad. Prolonged periods of low EA or periods of severe low EA can lead to REDS (it’s your total exposure/integral of severity of energy deficiency and time). See this case report and review.

What are some of the signs and symptoms of REDS? Here are the health domains that are affected:

The performance domains:


For what it’s worth, I want to highlight that what we often attribute to “overtraining” is actually under-nutrition. See this review. Furthermore, in both popular discourse and scientific literature, there is a sexist bias: female athletes undereat and male athletes overtrain. That obviously isn’t the case, and we all need to do a better job checking our bravado at the door when discussing aberrant nutrition in men.

I hope someone finds this helpful and can inform future discussion on this forum. I’m happy to answer any questions here or via DM and can provide a boatload of literature for those who would be interested in reading more.


Had a go at the formula. At 94kg and 20% body fat- if I targeted 30 kcal/kg FFM/day I would NOT lose weight :man_shrugging:t2:
If I targeted 45 I would put on at least 2lb fat a week :rofl:

It’s out by around 35% :thinking:

In practical terms you can dial out the errors using MFP by lowering/raising your daily target until you lose/maintain/gain the weight you desire. It doesn’t really matter if it’s metabolically the right number?

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Is this a spam post? That’s too confusing and too much work for me to figure out what I should eat during the day.

Fuel the workouts, count calories if you want. If you gain weight; you’re eating too much or if you’re losing weight; you’re not eating enough. Failing workouts? Maybe you didn’t eat enough before the workout.


I did a cursory read (it’s a long post, even for me!). Very much appreciate you adding to the nutrition discussion on this forum.

First thoughts: those numbers seem really high. 45*FFM(kg) for me puts me at approaching 4000kcal on a non-training day. That’s crazy-talk. Maybe I’m missing something here.


Hmmmm…. A target of 45 kcal/kg of FFM seems crazy high to me too. I’m 84kg, and likely 20% body fat. So the formula would put me at 3024 kcal on non training days. Add an average daily EEE of 1000-1200 kcal, and that puts me at 4024-4224 kcal/day.

If I were to eat that much, I’d be at 90+kg and rising in no time. It’s how I got to the 84 I’m at now; Reduced training in the last couple of years without reduced eating. And I average around 3000-3200 kcal/day with training, which roughly works out to 30 kcal/kg FFM.

My complicating variable is age. I’m in the 50+ male camp, so likely need fewer EA calories than an adolescent girl. Have there been any studies with older and more diverse athletes?

That said, I am on a body recomposition plan for the next few months. Time to ditch the COVID kilos and get back to my preferred 77-78 kilos. I’ll see how my kcal intake/expenditure correlates with these numbers, and how training is affected.

Thanks for the info,


Much of the information on the net about this, including the OPs, seems to come from (or was collected) here:

45 kcal / fat-free kg does seem pretty large, but it is what it says, though looking closer, 40 is all it can support for men.

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I’ve been maintaining my weight arround 75 kg for years using the classic way. This approach would up my ‘‘maintenance’’ (which really is a moving target tbh) by a few 100 calories, which is dfntly not something negligible.

I wouldn’t mind though… because I can eat all day ; p

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Surely you can answer that question with

“How stable is my weight?”

I’d be interested in knowing which responders in this thread actually count their calories. Not just estimating, but actually weighing food and keeping a journal and whatnot.

According to this formula, my maintenance EI would be around 3000. This seems high based on my estimates for my current intake. However, I could very easily be underestimating by 300-400 calories. I also tend to estimate my daily intake based on my most typical day of eating and ignore the higher calorie weekend days.

Here are some numbers to it:

Estimated EI on rest days = 2300
Estimated EI on workout days = 3000-3300, depending on the workout

I workout 5 days a week. This would put me at 30005 + 23002 = 19,600 calories per week.

19,600/7 = 2800 calories per day, on average, putting me at a 32.

Now, suppose I am off 400 on both rest days and workout days. This would add an additional 2800 calories over the week, putting me at 22,400 calories for the week.

Now, because my estimates are based off my weekday eating, assume I am also missing 1000 calories from beer, wine, and pizza. This would put me at 23,4000 for the week.

23,400/7 = 3342 calories per day, putting me at 39.

This is very close to the 40 that was quoted here:

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I’ve been tracking pretty diligently every day (including the weekends) through the MacroFactor app for a while now. MF comes from the team at Stronger By Science and MASS and it takes your logged food and logged bodyweight and calculates a TDEE. My TDEE on a rest day from MF is 2650 which following this math is equal to an EA of 41. My weight has had a very slight trend down (intended) so it’s definitely in the ballpark on my n=1 anecdotal level

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I don’t count calories, no need.

Apologies, I’m tired and I haven’t read the links…but isn’t this still just an energy balance equation, just with the assumption that BMR, thermal efficiance and general life activity scale with FFM? And by normalising by FFM, they become this convenient EA constant?

I sort of agree with those saying that the EA targets probably depend on age, which is why they seem high for those posting in here, I think.

I count calories but realise its theoritical and much more complex in reality, that’s why im very intrueged by this theory.

(1) Low Energy Availability and Weight Management in Ultrarunning | Koopcast Episode 40 - YouTube

On this podcast, they actually mention that this is something most athletes have difficulty wrapping their head around. Same goes for me but I still want to try out this approach. 45*FFM(kg) seems quite high though indeed

That was my first conclusion too. It just seems like another energy balance calc, but far more complicated.

Using the above equation it gives me 2,601 calories BEFORE including any EEE. I would get FAT pretty quickly if I ate that much!

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If you set EEE as 0 then it gives calorie recommendations for daily intake, which seem high compared to “normal” recommendations. But those recommendations aren’t for athletes.

There’s no difference between men and women in this calculation, but I guess that is handled by using FFM - if I remember correctly, healthy women have a slightly higher body fat percentage compared to men, so the equation should give lower calorie needs for women.

A lot of people are saying that the recommended calorie intakes would make them gain weight, but I’m not sure. The recommendation for daily intake seems high, but on days with exercise it only increases by calories burnt during exercise… it doesn’t take into account the extra calories that would be burnt after exercise (due to increased metabolic rate). So the overall effect should be to smooth out calorie consumption over a longer period, consuming slightly more on non-exercise days, and maybe slightly less on exercise days. Not sure if that’s a good plan or not, as I have no expertise in this field.

The thing that is missing for me is how this strategy would be affected by frequency of exercise. Is it just for people training 20+ hours a week, or would it work out for someone who is only doing one or two 30-minute sessions a week?

It’s also going to suffer from the simplification of food as calories, there’s no attempt to take into account the different effects of different food components - although this would be horribly complex and is avoided by saying that the EB calculation is for setting long term trends rather than precise goals.

I also find it a bit odd how in the OP the thermic effect of food is partly used to justify the difficulty of calculating daily energy balance, but there’s no problem for ignoring it in the EB calculation.

But overall, I like it as a concept - it quantifies (and thus limits) acceptable energy deficits, includes body composition instead of “weight” and seems to be a safer way of thinking about nutrition.


What a great explanation of things. Thank you for taking the time to write all of this.

I’ve not read this review; it was not consulted in my writing.

Here’s the key part: this has been researched. Basically the research says that we can ignore accounting for all this other stuff by thinking in these terms, because EA is quantifying the amount of energy needed for cellular processes after accounting for exercise. In some ways, I’d say don’t ask how the sausage is made–shooting for an EA of 40-45 means that you’re in the right ballpark for ensuring you have the requisite energy for the body to do it’s thing.

I’m not sure it’s been studied, but in the example of 1-2 sessions of 30 minutes per week, I’d say live your life and have a recovery drink after those sessions.

Women have higher body fat percentage and, on average, lower body mass.

More germane to the discussion on sexes is that most of the work in this field was, for once, done in women. As I said, it is likely that men are more resilient in tolerating lower EAs than women. It is probable that men are optimized at a lower EA (I’d say 40ish) than women. Simply put, men have a much less energetically expensive hypothalamic-pituitary-gonadal axis to maintain than women.

To people saying “well I can just weigh myself” or “I’m going to gain a ton of weight on that!”: having a static weight does not mean that you are fueling sufficiently. All that means is you are giving yourself the bare minimum to not enter catabolism. This is easier to illustrate in a female example: a female athlete loses 5 pounds for championship season and doesn’t put it back on. Upon losing 5 lbs, her previously monthly periods become irregular, and over the next year, she menstruates 6 times. Her weight does not change further. Is she eating enough? Her weight isn’t changing! The answer clearly is NO because she’s experiencing physiologic dysfunction demonstrated by her persistent menstrual dysfunction. As the body experiences energy deficit, it preferentially shunts energy to physiologically more important systems (e.g., the heart) at the expense of those less vital to life (e.g., muscle synthesis).


In that case, should a daily 60 min easy walk (~250kcal) still be added to the equation (through EEE) for well trained athletes (20 hours of training a week) ?

I’d say yes. That’s ~1500 kcal throughout the week that aren’t being accounted for. Have a cookie, you earned it! Though if you’re training 20 hours a week, it’s hard to imagine that you need to be concerned about overeating.