Signs you are under-fueling your training (REDs explained)

The clearest signs of under-fueling are the ones the 2023 IOC consensus treats as primary indicators: missed or absent periods in women, low testosterone in men, low bone density, and high-risk or repeated bone stress injuries [1][2]. Less specific signs include low libido, fewer morning erections, stalled training, poor concentration, irritability and gastrointestinal problems [1][3]. The syndrome is called Relative Energy Deficiency in Sport (REDs), and a diagnosis requires clinical assessment by a physician [1][2].

reload gives training and nutrition guidance and is not medical advice. If anything below sounds familiar, the right next step is a doctor or a registered sports dietitian, not a new diet.

What is REDs?

The IOC describes REDs as a syndrome of impaired physiological and/or psychological functioning in female and male athletes, caused by exposure to problematic (prolonged and/or severe) low energy availability, meaning energy intake that is inadequate relative to the energy spent in exercise [1]. The most well-documented effects are impaired reproductive function and bone health, in both sexes [1].

Not every period of low energy availability causes harm. The consensus separates “adaptable” low energy availability, a short reduction with benign effects, from “problematic” low energy availability, which is linked to greater and potentially persistent disruption of body systems [1]. It also notes that low carbohydrate availability can magnify these effects [1].

What is low energy availability, and is there a threshold?

Energy availability (EA) is energy intake minus exercise energy expenditure, divided by fat-free mass (FFM) [1]. The IOC uses these bands as a guide [1]:

The 30 figure comes from short, tightly controlled studies. In one, 29 regularly menstruating, sedentary women held EA at set levels for 5 days, and luteinizing hormone pulsatility was unaffected at 30 kcal/kg lean body mass per day but disrupted below it [4]. The caveats are important:

• The consensus calls EA a guide “rather than a diagnostic end-point” and says a universal 30 kcal/kg FFM cut-off in females is debated [1].

• In males, the threshold appears to be lower, with the consensus citing a range of about 9 to 25 kcal/kg FFM/day [1]. The evidence is thin. Four days at 15 kcal/kg lean body mass did not change resting testosterone in one study, while 3 days at 19 kcal/kg lean body mass lowered it in a group of athletes in another [5].

• Most controlled studies lasted 5 days or less [5].

• Estimating EA in real life is error-prone, and in elite distance athletes reproductive function was a more objective marker than diet and training logs [6].

Do not treat 30 as a target to sit just above. It is a research reference point, not a safe line.

Signs of under-fueling in female athletes

In a study of national and world-class distance athletes, 37% of the women were amenorrheic, and bone injuries were about 4.5 times more prevalent in the amenorrheic group [6]. Losing your period is a signal to see a doctor, not a normal sign of fitness.

Signs of under-fueling in male athletes

Men get REDs too, though the research base is smaller and men may be more resilient to short periods of low energy availability [5].

• Low testosterone. In the CAT2, total or free testosterone in the lowest 25% of the reference range is a primary indicator, and testosterone below the reference range is a severe primary indicator that counts as two [1][2]. In the same elite distance cohort, 40% of men had testosterone in the lowest quartile, and bone injuries were again about 4.5 times more prevalent in that group [6].

• Low libido and fewer morning erections. The consensus names these as two emerging potential indicators in males [1].

• Low T3, low bone density and bone stress injuries, as in women [2].

Cyclists are not exempt. In a small study of 10 competitive cyclists selected for lower than expected bone density, 70% had low energy availability across the season, 70% were classed as restrained eaters, and mean carbohydrate intake was 3.7 to 4.3 g/kg/day, below sports nutrition recommendations [7].

Performance and everyday signs in both sexes

In a survey of 1,000 female athletes, low energy availability was associated with [3]:

• decreased training response and decreased endurance performance

• impaired judgment, decreased coordination and decreased concentration

• irritability and depression

• gastrointestinal, cardiovascular, metabolic and hematological problems

That study was cross-sectional and self-reported, so it shows association, not cause [3]. The IOC also lists training and performance plateaus or declines, mood disturbances and increased injury risk among REDs outcomes [1]. Resting metabolic rate, low heart rate and blood pressure are recognized only as potential indicators in the CAT2 and are not scored [2].

A stable body weight may not rule under-fueling out, because low energy availability can lower resting metabolic rate, by as much as about 10 to 20% in some studies [5].

What causes under-fueling in endurance athletes?

Low energy availability can come from increased exercise, reduced energy intake, or a combination of both [5][8], whether or not the restriction is deliberate. Low carbohydrate intake is a specific risk, and the IOC consensus reports several studies since 2019 showing that low carbohydrate availability has an energy-independent or magnifying impact [1]. For how intake should change with training, see Rest day vs training day nutrition for endurance athletes and How many carbs per hour on the bike? Ranges by ride length.

When should you see a doctor or sports dietitian?

Book an appointment if any of these apply:

• Your periods have stopped, or your cycle has become irregular.

• You have had a bone stress injury or stress fracture.

• You have a persistently low libido or fewer morning erections.

• Performance has stalled or declined and fatigue does not clear with rest.

• Food, body weight or training has started to feel compulsive or stressful.

The IOC process has three steps: screening, severity and risk assessment based on indicators, and a physician-led diagnosis and treatment plan built with the athlete, coach and health team [2]. Treatment starts with restoring energy availability through non-pharmacological means, supported by a team that includes sports medicine, nutrition and psychology [1]. The consensus says this team approach is especially important when disordered eating is involved [1].

A registered dietitian can build a personalized plan [8]. In the US and Canada, the Certified Specialist in Sports Dietetics (CSSD) credential identifies registered dietitians with sports nutrition expertise [8].

Finding this early is the point. Short, mild periods of low energy availability do not always cause lasting harm [1], and asking for help is a normal part of managing a heavy training load.

How reload handles this

reload sets daily calorie, protein, carbohydrate and fat targets from the training you have done and the training you have planned, so a long ride day and a rest day get different targets. Each macronutrient gets a target range, and the app shows how the day’s targets were built. Energy burned is split into resting and active. The fuel tank estimates stored glycogen from training load and food intake and shows it as Topped up, Adequate, Underfueled or On reserve. It is a model estimate, not a measurement, and it cannot detect or diagnose REDs. If you notice the signs above, talk to a doctor or registered dietitian. reload gives training and nutrition guidance and is not medical advice. See plans at /pricing.

How reload handles this

Daily targets and a fuel tank estimate

1

Connect

Link Garmin, Wahoo, WHOOP or Apple Health so reload reads your training.

2

Check the fuel tank

A model estimate of stored glycogen. It cannot detect or diagnose REDs.

3

Log your meals

Search, scan a barcode, or describe a meal with reload Max.

See plans→

How reload handles this

Daily targets and a fuel tank estimate

1

Connect

Link Garmin, Wahoo, WHOOP or Apple Health so reload reads your training.

2

Check the fuel tank

A model estimate of stored glycogen. It cannot detect or diagnose REDs.

3

Log your meals

Search, scan a barcode, or describe a meal with reload Max.

See plans→

How reload handles this

Daily targets and a fuel tank estimate

1

Connect

Link Garmin, Wahoo, WHOOP or Apple Health so reload reads your training.

2

Check the fuel tank

A model estimate of stored glycogen. It cannot detect or diagnose REDs.

3

Log your meals

Search, scan a barcode, or describe a meal with reload Max.

See plans→

FAQ

Can men get REDs?
Yes. The IOC consensus covers female and male athletes, and low testosterone is a primary indicator in the clinical assessment tool [1][2].

Is losing my period a normal effect of hard training?
No. Functional hypothalamic amenorrhea is a primary REDs indicator, and in elite distance athletes it was linked to about 4.5 times more bone injuries [2][6]. See a doctor.

Can an app or wearable tell me if I have REDs?
No. A REDs diagnosis comes from clinical assessment by a physician [1][2]. Apps and wearables estimate energy expenditure, and those estimates carry error (see How accurate are Garmin, WHOOP and Apple Watch calories?).

Is 30 kcal/kg of fat-free mass a safe minimum?
No single number is safe for everyone. The consensus says a universal cut-off is debated in females, and the threshold in males appears to be lower [1].