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.