When Under-Eating Looks Like Overtraining

Your sessions have gone flat. The pace that used to feel easy now takes work, your top sets have stopped moving, you wake up tired, and you are starting to wonder whether you are doing too much.

That is the point where most people back off training. Sometimes that is the right call. But under-fueling and accumulated training fatigue produce many of the same signals, and they can happen together, so backing off without also looking at the food often solves half of it at best. What follows is how to work out whether food is part of yours.

What under-eating actually means here

Not every calorie deficit creates this problem. A modest, well-managed deficit with good training and enough protein is a normal thing to do. It starts to matter when intake is too low relative to training demand to support health, recovery and performance. Dieting is one of the common routes in, which is why a long cut running alongside rising training volume is worth a closer look.

Sports nutrition calls that low energy availability. Take what you eat, subtract what the training cost, and what remains has to cover your hormones, your immune system, your bone turnover, your recovery and your brain. Two people can eat the same 2,400 calories and be in completely different situations, because one of them spent 300 calories on a walk and the other spent 900 on a long run and a lift.

The 2023 IOC consensus statement on Relative Energy Deficiency in Sport (Mountjoy et al.) describes this as a syndrome of health and performance problems in female and male athletes exposed to low energy availability. Note both sexes. The older framing treated this as something that happened to female endurance runners, and that framing has cost a lot of men several bad seasons.

Why you cannot settle this with a number

You will see a threshold quoted online: 30 calories per kilogram of fat-free mass per day, below which things go wrong. It is worth knowing where that number came from. Loucks and Thuma (2003) studied 29 regularly menstruating, habitually sedentary young women in a lab and found that luteinizing hormone pulsatility was undisturbed at and above 30 kcal/kg of lean body mass per day, and disrupted below it. That is a real finding. It is also a specific hormonal marker, in one population, under controlled conditions.

It was never meant to be a pass-fail line you calculate on yourself after a rough week, which is roughly why the 2023 IOC statement moved toward assessing this clinically across a range of health and performance indicators rather than hanging it on a single figure. Your fat-free mass, your food logging and your training expenditure are all estimates, the last one roughest of all. Combine three fuzzy inputs and you do not get a diagnosis.

So stop trying to compute your way to an answer. Read the pattern instead.

The pattern in a hybrid week

What tends to happen is everything sliding at once, gradually, while nothing in particular is wrong.

Same run, same route, higher heart rate and a worse feel. Lifts that stall or slowly regress despite sensible programming. Soreness that hangs around a day longer than it used to. Sleep that breaks up in the early hours. Getting sick more often than your friends. Hands and feet cold. Mood flattened out. For women, cycle changes or a cycle that goes missing. For men, libido dropping off a cliff for no obvious reason.

Appetite is the confusing one, because it goes both directions. Some people get relentlessly hungry. Others lose appetite entirely, which feels like proof they do not need more food. A loss of appetite does not rule under-fueling out. Logue et al. (2018) make the point that low energy availability does not require an eating disorder to show up. Plenty of it is simply athletes not eating enough for the volume they are doing, often without realizing the training load crept up. That is the failure mode the hybrid athlete macros piece keeps circling: training moves week to week, and intake quietly stays where it was in January.

Hitting your daily total may not be the whole picture

You can land on your calorie target every day and still spend most of it in a hole, because the food arrives after the work instead of around it.

The 2020 follow-up review from Logue and colleagues collects what little evidence exists on within-day energy balance. In one study of female endurance athletes, those with menstrual dysfunction spent 22 hours of the day in a deficit greater than 300 calories, against 18 hours for athletes with normal cycles. In another, male athletes with suppressed resting metabolic rate spent 21 hours in a deficit over 400 calories, against 11 hours for the comparison group. Similar estimated daily energy availability, different within-day patterns.

Hold that loosely. It rests on two small observational studies, and the review is candid that the viability of measuring within-day energy deficiency, and its ability to identify athletes at risk, is unclear. Treat it as a suggestion rather than a finding: long stretches in a large deficit may matter even when the daily estimate looks the same. Not settled, but enough to support the practical case for not following a hard session with hours of minimal food, which is the argument for putting real meals around your workouts rather than treating the daily number as the only thing that counts.

Why a deload muddies the picture

Worth being precise about the words first. Overtraining syndrome is a specific clinical diagnosis and it is rare. What most people mean when they say they are overtrained is accumulated fatigue, or overreaching, which is common and far less dramatic. The distinction matters here because the two problems are genuinely tangled together rather than sitting on opposite sides of a line.

Stellingwerff et al. (2021) went through the training-overload literature looking for exactly this overlap. They found substantial shared symptoms between overtraining syndrome and RED-S, with both running through the same hypothalamic-pituitary pathways, and in 18 of the 21 studies they analyzed there were signs that energy or carbohydrate availability had fallen during the training-overload period. Their conclusion is blunt: a good share of what gets attributed to training overload may be misdiagnosed under-fueling.

Which is also why a deload is a poor test. Take a down week and you feel better, and that reads as confirmation that training was the problem. But you also burned less that week while eating the same, so energy availability went up too. The week moved both variables at once, so it cannot tell you which one mattered. Training resumes at the old volume with the old intake, four weeks later you are back in the same place, and the conclusion is that you need another deload. That loop can run for a year.

The more useful move is to change the other variable on purpose and watch what happens.

What to change first

Give it two to three weeks, in this order.

Start by feeding the sessions. Carbohydrate before the hard work and a real meal with protein and carbs within a couple of hours after it. On days you run and lift, the meal between the two is the one that matters most, which the piece on running and lifting on the same day covers in more detail. This alone fixes a surprising number of cases, because it is a distribution problem as often as a total problem.

Then raise the total on your hardest days. Not every day. Carbs are the lever, since that is what the work is actually spending. As a first trial, add roughly 300 to 500 calories, mostly from carbohydrate, on those days. Treat that as a starting point to adjust for the session, your body size and your response, not a universal prescription. Keep protein steady across the week rather than moving it around.

If you have been dieting for a long stretch, consider that a few weeks at maintenance is the honest next move rather than another push.

A macro calculator gives you a starting point for what those days should look like, a first guess to adjust from rather than a prescription. If your training load has grown since you last ran it, the old number was answering a question about a different week.

Then judge it on the things that were sliding. Heart rate at your usual easy pace, bar speed on your working sets, how you sleep, how you feel by Thursday. Not the scale. The scale will not tell you anything useful for a couple of weeks.

When this needs more than a food fix

If you add food properly and nothing improves, or if there are harder signals in play, this stops being a macros question. A menstrual cycle that has stopped, repeated bone stress injuries, or a relationship with food that feels like it is running you rather than the other way around all warrant a sports dietitian or a doctor. Those are treatable, and they are treated faster when someone looks at them properly.

The short version

Under-fueling and accumulated training fatigue look nearly identical from the inside, and they often travel together. Do not try to settle it with a threshold calculation, because the inputs are too rough to trust. Feed the sessions first, add carbohydrate to your hardest days, and give it three weeks. If the flatness lifts, under-fueling was likely part of the problem. That is a useful clue, not a diagnosis.

If you are not sure what your training week actually costs, start there. Run the numbers, then let three weeks of honest sessions tell you whether they were right.