Quick answer: Under-fueling (low energy availability) means you eat too little to cover both your training and your body's basic work. Over time it can disrupt periods, weaken bone, and cause fatigue, poor sleep, low mood, and stalled training, a syndrome the IOC calls REDs. Women over 40 who diet while training more are at real risk, and the symptoms overlap with perimenopause, so it is easily missed.
Under-fueling is not about being thin. It means eating too little to cover both your training and everything else your body does, from repairing muscle to running your hormones. When the gap lasts, the body cuts back on non-essential work, and reproductive hormones and bone are first in line. For active women over 40, that can look a lot like perimenopause, which is why it is so often missed.
What under-fueling and REDs actually are
Low energy availability is when the energy you eat, minus the energy you spend training, leaves too little for your body’s basic functions. Relative Energy Deficiency in Sport (REDs) is the set of health and performance problems that follow when that gap is large or lasts too long. It affects recreational athletes, not just elite ones.
The International Olympic Committee updated its consensus on REDs in 2023, published in the British Journal of Sports Medicine (Mountjoy and colleagues). It describes REDs as a syndrome that can affect hormones, periods, bone, metabolism, immunity, mood, heart health, and performance. Two updates matter for everyday runners and lifters. First, low carbohydrate availability (not just low total calories) is a growing concern. Second, not all low energy availability is harmful. Short, deliberate periods can be adaptable, while long or severe ones become problematic.
Researchers often quote a figure of 30 kcal per kilogram of fat-free mass per day as a line for low energy availability. The consensus stresses that there is no single cut-off that applies to everyone, so the warning signs matter more than a calculation.
The signs to watch for
The signs of under-fueling are mostly vague on their own: tiredness, feeling cold, poor sleep, low mood, low libido, frequent colds, slow recovery, and stalled training. The more specific red flags are changes to your periods and bone stress injuries. Several signs together, especially alongside more training or less eating, is the pattern to take seriously.
- Periods change. Cycles get longer, lighter, or stop.
- Bones complain. Shin, foot, or hip pain that worsens with impact can be a bone stress injury.
- Energy drops. You feel drained, and rest days do not help.
- Training stalls. Easy runs feel hard and lifts stop progressing, even though you train consistently.
- You are always cold, get dizzy when you stand up, or catch every bug.
- Mood shifts. Irritability, anxiety, or flatness, sometimes with food preoccupation.
The 2023 consensus introduced a clinical assessment tool (REDs CAT2) for clinicians to grade risk using signs like these alongside blood tests and bone scans. It is a clinical tool, not a self-test, but it reflects how seriously the field now takes these symptoms.
Why midlife women who train and diet are at risk
Midlife women are at risk because two pressures often arrive together: perimenopause brings shifts in body composition that prompt dieting, and the same women often step up training to compensate. Eating less while training more is exactly how energy gaps open. Appetite does not reliably rise to match training, so the gap can grow without you noticing.
Common routes in:
- Dieting for midlife weight gain. Cutting calories hard while adding workouts.
- Short eating windows. Intermittent fasting that leaves little room to fuel hard sessions.
- Low-carb eating with high training volume. The consensus flags low carbohydrate availability specifically.
- Training fasted. Morning runs or lifts on nothing, followed by a small breakfast.
- Busy lives. Skipped lunches and long gaps that add up over weeks.
- More and more mileage. Adding long runs or extra hard sessions to “burn it off” raises energy needs faster than appetite follows.
Most REDs research has been done in younger athletes, and women in their 40s and 50s are under-studied. The mechanisms do not stop at 40, though, and bone is already under pressure in this decade as estrogen falls. That makes protecting energy intake more important, not less.
How it masks as perimenopause
Under-fueling and perimenopause share many symptoms: irregular periods, poor sleep, fatigue, low mood, low libido, and bone loss. Both involve lower estrogen, through different routes. So a woman in her 40s who stops having regular periods while training hard and eating little may be told it is “just perimenopause” when energy availability is part of the story.
| Symptom | Under-fueling | Perimenopause | Clue that points to fueling |
|---|---|---|---|
| Irregular or missing periods | Yes | Yes | Changes started with a diet or training increase |
| Fatigue | Yes | Yes | Improves noticeably when you eat more |
| Poor sleep | Yes | Yes | No night sweats, but waking hungry or wired |
| Low mood | Yes | Yes | Food preoccupation or anxiety around eating |
| Bone stress injury | Yes | Rarely on its own | Any stress fracture warrants a fueling review |
| Hot flashes and night sweats | Not typical | Yes | Points more to perimenopause |
| Feeling cold, dizziness | Common | Less typical | Points more to fueling |
They also overlap. You can be in perimenopause and under-fueling at the same time. In the UK, NICE guidance says perimenopause in women over 45 can usually be diagnosed from symptoms without blood tests, which is sensible, but it means the energy question has to be asked on purpose. Tracking symptoms against training and eating, rather than in isolation, is what makes the pattern visible. Our guide to irregular periods covers the other common causes.
When to see a doctor
See a doctor if your periods stop for three months or more before menopause, you have a stress fracture, you feel faint or dizzy during exercise, or you are exhausted despite rest. Seek help too if food, weight, or eating feels distressing or out of control. Ask for an assessment that includes energy availability, not only hormones.
A clinician may check thyroid function, iron stores, and other bloods, and may suggest a bone density scan if you have had a stress injury or long gaps between periods. Low iron is common in women who run and causes similar fatigue, so it is worth checking at the same time (see our guide to iron and ferritin for female runners). A sports dietitian can help you work out how much to eat around training. If eating has become a source of anxiety, a clinician experienced in eating disorders is the right starting point, not a training plan.
How to fuel better
Fixing under-fueling usually means eating more, especially around training, rather than training less. Eat some carbohydrate before hard or long sessions, get protein and carbohydrate within a couple of hours after, and avoid long gaps on training days. Hormone and bone recovery takes months, so consistency matters more than one good week.
The way you train matters too. A “less is more” plan, the approach Phaes uses (heavy lifting, one short sprint session, a short HIIT circuit, truly easy days, a weekly rest day, and a reset week every fourth week), burns less energy than piling on long runs and repeated hard intervals. That makes the energy gap harder to open in the first place.
Practical starting points:
- Eat before hard sessions. A banana, toast, or yogurt is enough before a sprint session or heavy lift.
- Hit a protein target. Active women over 40 generally need 1.6 to 2.2 grams per kilogram of body weight a day. Our guide to protein for women over 40 breaks it down, and the protein calculator gives you a number.
- Hit your macros, including healthy fats. Build meals around protein, add healthy fats (olive oil, nuts, seeds, avocado, oily fish), and do not cut fat to save calories. Fat is energy-dense and helps you reach your needs.
- Do not fear carbohydrate. It is the main fuel for hard training.
- Hydrate. Drink enough through the day, more on hot or hard training days.
- Match food to training. Bigger training days need bigger eating days.
- Pause aggressive dieting while you are training hard and symptoms are unexplained.
Where Phaes fits
Phaes asks about sleep, energy, symptoms, and your cycle in one daily check-in, and reads them against your training load. When fatigue climbs, periods shift, and training stalls together during a heavy block, Phaes flags the pattern and eases the plan instead of piling on more. We do not just track your symptoms. We read what they might mean and adjust the plan, and we point you to a clinician when the pattern needs one.
The bottom line
Under-fueling happens when you eat too little for the training you do, and it can disrupt periods, weaken bone, and drain energy. Midlife women who diet and train more are at real risk, and the symptoms are easy to blame on perimenopause. Eat enough around training, take period changes and bone injuries seriously, and see a doctor if the warning signs stack up.
Frequently asked questions
What is RED-S?
RED-S (now usually written REDs) stands for Relative Energy Deficiency in Sport. It is a syndrome described by the International Olympic Committee in which too little energy, relative to training, harms health and performance. It can affect hormones, periods, bone, metabolism, immunity, mood, and heart health. It affects recreational athletes as well as elite ones, and men as well as women.
What are the signs of under-fueling in women?
Common signs include irregular or missing periods, constant tiredness, feeling cold, poor sleep, low mood or irritability, low libido, frequent colds, slow recovery, stalled or falling performance, and stress fractures or bone stress injuries. Some women also notice hair thinning, dizziness, or digestive problems. No single sign is diagnostic, which is why a clinician should look at the whole picture.
Can under-eating make perimenopause symptoms worse?
It can make them harder to read and may add to them. Low energy availability suppresses the hormone signals that run the menstrual cycle, so it can cause skipped periods, poor sleep, low mood, and fatigue, which are also classic perimenopause symptoms. Both also put pressure on bone, so having both at once is a particular concern.
How many calories do I need if I run and lift?
There is no single number. Your needs depend on your size, muscle mass, and how much you train. Researchers measure energy availability as intake minus exercise energy, per kilogram of fat-free mass, but that is hard to do at home. Practically, if you train most days and have any of the warning signs, you are probably eating too little and should get dietitian support.
Is intermittent fasting bad for women who train?
Not automatically, but it makes under-fueling easier. A short eating window can make it hard to eat enough on training days, especially before and after hard sessions. The 2023 IOC consensus also highlights low carbohydrate availability as a contributor to problems. If you fast and train hard, watch for the warning signs closely.
When should I see a doctor about under-fueling?
See a doctor if your periods stop for three months or more before menopause, you have a stress fracture or bone stress injury, you feel dizzy or faint during exercise, you are exhausted despite rest, or your relationship with food is causing distress. Ask for an assessment that considers energy availability, not just hormones.