While we often hear messages about cutting back on calories for weight loss, that’s not great advice for athletes—cyclists in particular. Highly active individuals need to eat more, not less, to take in an adequate amount of calories in order to fuel performance and maintain health. In other words, cyclists need to have sufficient energy (calories) available for the body to support training needs.

Longstanding calorie shortfalls, which is now known as low energy availability (LEA), can cause unfavorable physiological and psychological outcomes, which have the potential to impair an athlete’s health and sports performance.

We also shouldn’t think that the off-season is a time when athletes don’t have to fret about getting adequate nutrition. A 2022 study published in the International Journal of Sports Nutrition and Exercise Metabolism assessed the energy-in/energy-out balance among 88 athletes involved in various sports. Researchers found that a higher percentage of athletes had low energy availability during the preparatory phase of their training—the time of year when they’re preparing for game time—rather than during the competitive phase of the athletic season.

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So a cyclist might be more conscious of the importance of ramping up their intake of pasta and oatmeal during their peak riding season for better performance and recovery, and then let things slide during the off-season even though all those Zwift sessions are still burning up heaps of energy.

To help you keep your body energized and fueled enough to take on every workout, here’s what you need to know about low energy availability and why it’s so important to give your body what the calories it needs.

What is low energy availability?

In simple terms, Alexandra Coates, Ph.D., a postdoctoral fellow in The Human Performance Laboratory in the Department of Kinesiology at McMaster University, tells Bicycling that low energy availability represents a state where there is inadequate energy (calorie) intake relative to exercising energy expenditure.

In other words, Coates explains that there is a mismatch between an athlete’s calorie intake and the calories expended as a result of training, leaving inadequate energy available to recover and for your body to function normally. “Essentially, your body requires a certain amount of energy to maintain normal function. If you are expending a lot of energy in training, and aren’t eating enough, your body reacts as it would under times of starvation,” she says.

When you’re not actively training, the body’s energy balance is skewed heavily toward things other than exercise, such as supporting brain functioning and hormone production. But when there is not enough energy to support these vital functions you end up with something known as energy compensation—that’s when our bodies partially compensate for low energy by cutting the energy spent on properly performing physiological processes.

Low energy availability is the driving factor of the recently coined syndrome “Relative Energy Deficiency in Sport” a.k.a. RED-S. In 2014, after a thorough review of the medical literature, the International Olympic Committee officially renamed the Female Athlete Triad as RED-S and expanded the definition to recognize that consuming too few calories is also problematic for male athletes and the symptoms are more expansive in females than menstrual cycle irregularities and poor bone mass.

“While there are immediate symptoms of low energy availability, it will take a certain amount of time (weeks to months) of LEA before you’ll develop full-blown RED-S,” says Coates.

In research circles, energy availability is based on energy intake minus estimated energy expenditure divided by kilogram of fat-free mass (FFM)—unlike your organs, bones and muscles, your fat mass does not need any energy support. In general, experts define three levels of energy availability: Clinically low energy availability (<30kcal/kg FFM), subclinical low energy availability (30-40 kcal/kg FFM for males or 30-45 kcal/kg FFM for females) and optimal energy availability (>45 kcal/kg FFM for males and >45 kcal/kg FFM for females).

What are the symptoms of low energy availability?

There is growing evidence that female and male endurance athletes with insufficient energy intake may experience a variety of consequences that are not conducive to good health and podium finishes. Recent research suggests that the symptoms of overtraining syndrome may be most often caused by low energy availability. Subjects in many previous overtraining studies weren’t eating enough to fuel their increased training load. Eat enough and the chances of overtraining are diminished.

Coates says the symptoms of low energy availability and RED-S can be varied and have various effects on an athlete’s well-being. They can include:

  • Reduced testosterone levels (male athletes)
  • Reduced estrogen levels (female athletes)
  • Suppression of resting heart rate and blood pressure
  • Low libido
  • Weakened bones (increased stress fracture risk)
  • Menstrual irregularities
  • Poor mood and bouts of irritability
  • Immune dysfunction
  • Thyroid suppression
  • Increases in upper respiratory infections and other illnesses
  • Increased fatigue
  • Sleep disturbances
  • Constipation
  • Elevated cholesterol

“With LEA and RED-S you will see suppression of resting metabolic rate as the body attempts to preserve energy. This often results in no weight loss (and sometimes weight gain) despite the caloric deficit,” Coates explains. So being underweight is not a prerequisite for low energy availability or RED-S.

An investigation published in the International Journal of Sports Physiology in 2019 looking into the energy requirements and intake of six professional male cyclists throughout a weeklong Classics campaign (four one-day races, four rest days) highlights some of the health risks of under-fueling.

Riders who failed to match their calorie burn with adequate calorie intake over this period had significantly reduced levels of the hormones testosterone and insulin-like growth-factor-1, both markers associated with under-eating. Postrace and midrace carbohydrate intake was determined to be particularly insufficient to meet demands. Comparatively, riders who met their energy needs on race and rest days saw increased levels of the hormones.

This pattern of effort and under-fueling by the pros might be more likely among amateur and recreational athletes during an intensive training block or during activities like bikepacking, where for several days or weeks, you are a calorie-burning machine.

A study published in BMJ Open Sport and Exercise Medicine in 2018 found that of the 50 male road cyclists tested, those who had eating habits that were restrictive enough to put them in a low energy availability category had lower bone mineral density and reduced average power output during riding tests—both of which are signs of RED-S.

Furthermore, because low energy availability can negatively affect recovery, muscle mass and strength, and neuromuscular functioning, there can be a noticeable drop in exercise performance. “The main reason why performance is impaired is that athletes are unhealthy and unable to recover and adapt properly from training,” notes Coates. “Recurrent stress fractures or illnesses and suboptimal carbohydrate intake are obviously not performance-enhancing.” Muscle protein synthesis takes energy, so if that is in short supply, the body may not be unable to rebuild stronger after a workout, as well.

Research conducted by Coates shows that performance drops from low energy availability can take several weeks to manifest, likely because this is mainly driven by poor health and adaptation to the training.

What’s more though: Energy deficits may reduce iron levels by driving up inflammation, and therefore, increasing the activity of an iron-regulating hormone that can reduce iron absorption from food. This could be another way that low energy availability leads to impaired exercise performance, considering iron makes it possible for red blood cells to deliver oxygen to your muscles to power them and also plays a role in helping you metabolize carbohydrates—your working muscles’ primary fuel source.

Knowing the potential symptoms of low energy availability and RED-S is an important step in understanding that it might be a problem for you and for taking the steps to ameliorate it.

Who is most at risk of developing low energy availability?

When compared to the general population, calorie-burning cyclists and other endurance-focused athletes are particularly susceptible to low energy availability and then going on to develop RED-S, along with the health and performance detriments that come with it. Weekend warriors with inconsistent training bouts or those who work out very casually are not going to need the same calorie load as someone who is training nearly daily and, therefore, won’t face the same risk of low energy availability and RED-S.

We still don’t have a good grasp of the extent to which low energy availability and RED-S affects athletes, but Coates stresses that it’s certainly not uncommon. “LEA can occur in 22% to 58% of athletes in various sports,” she says. One research report said that up to 58% of endurance track and field athletes may suffer from low energy availability.

Data on the prevalence in cyclists is still lacking, but the sport is a known risk factor for low energy availability and RED-S. Coates suggests that the high-energy demand of riding and the belief that being trimmer leads to performance benefits can contribute to the condition. “As cyclists perform such long training days, it makes it hard to fuel adequately,” she explains.

What’s important to keep in mind is that you don’t have to be riding on the pro circuit to be susceptible to low energy availability and RED-S. “Anyone can be at risk for RED-S, and as amateur athletes often have less knowledge about it and may pay less attention to their nutrition habits, they are sometimes at more risk than the elites,” notes Coates. “Think [of the population] working all day and then coming home to do a hard Zwift workout with not much thought to fueling.”

Elites are susceptible but are often surrounded by a roster of physicians, coaches, and nutritionists who can spot problems and implement appropriate nutrition programs. It can be easier for amateurs to misjudge things.

Coates stresses that low energy availability and RED-S are also not restricted to underweight athletes. “You can never tell whether someone has RED-S or not based on their physique,” she says.

What can contribute to low energy availability and RED-S?

The causes of low energy availability and subsequently RED-S is manifold and may include intentional calorie restriction to achieve a certain body composition, exercise-induced appetite changes, time constraints, disordered eating behavior, and food insecurity (rising food costs can make it hard for some people to afford to fuel themselves adequately).

Coates points out that training camps can be problematic by presenting a situation where training volume is increased, yet a person does not change their fueling strategy. Or consider the high-energy cost of multi-day events or cycle touring trips where it is a challenge to properly match calorie intake to calorie expenditure. Coates believes that fasted training can also present scenarios where overall energy intake is below what is necessary.

“Often people think low energy availability or RED-s is occurring because people are intentionally trying to cut calories; however, it is often unintentional,” Coates says. Someone may not be aware of just how many calories they need to support training efforts and then end up eating too little.

Food consumption can also be influenced by hormonal factors. For example, female endurance athletes may experience variations in their appetite across a menstrual cycle. Some people find that their appetite wanes after a hard training session and then fail to eat enough when it returns.

Appetite cues don’t necessarily give you an accurate sense of much you need to eat during heavy training. Sports dietitians and coaches who work with cyclists and other endurance athletes are aware that many of them are prone to overestimating their calorie intake.

Coates adds that low energy availability and RED-S may also occur in athletes who are eliminating food groups from their diet, like grains, or who are attempting to only eat “clean” resulting in a higher intake of low-calorie foods, like raw vegetables.

How you can prevent low energy availability and RED-S?

Among cyclists and other athletes, preventive educational programs and screening to identify those with low energy availability and RED-S are important to sidestep long-term health and performance consequences. “I think the easiest way to prevent and screen for RED-S is through semi-frequent blood testing where items like sex and thyroid hormones and ferritin are measured to spot possible low energy availability,” says Coates. “Some doctors don’t like to give blood tests to seemingly healthy athletes, but luckily there are new companies like Inside Tracker that can get blood work done for you.”

Treatment of low energy availability and RED-S involves increasing energy intake and/or decreasing exercise energy expenditure to address the underlying energy shortfall. Without cutting back on your rides, there are a few dietary steps you can take to help your body get the fuel it needs.

1. Go bigger on calories

If you’re frequently taking part in long, hard rides be sure to better match your calorie expenditure with calorie intake consistently. An extra helping of spaghetti and a larger handful of nuts can do the body good. This can be hard to judge as different sports and different body types use and require different amounts of energy. So it’s always a good idea to work with a professional, such as a sports dietitian, who can more accurately make an assessment of your energy intake in relation to body composition and activity energy expenditure.

A 2021 study in the journal Human Reproduction found that female athletes who had menstrual disorders attributed to insufficient energy intake were able to get their system back on track by increasing their food intake by an additional 300 to 400 calories a day.

When in the throes of high volumes of training, consider going easy on eating too many low energy-dense foods, like raw salads, that and instead opt for more energy-dense foods, like nuts and seeds.

2. Be consistent

It’s also important to eat enough throughout the day, instead of just loading up your calorie intake at one or two meals. A pair of studies conducted by researchers in Norway and Denmark—one involving male endurance athletes and the other focusing on fit females—found that individuals who spent more time each day in a catabolic state where calorie intake at certain times was not enough to match the metabolic demands of the body (in other words they had larger single-hour energy deficits) were more likely to have a suppressed resting metabolic rate, along with higher levels of the stress hormone, cortisol.

Males showed signs of reduced testosterone production when they had within-day energy deficiencies, whereas the production of estrogen in females was suppressed. This is despite overall 24-hour calorie intake not being significantly different among all the participants. (Intermittent fasting, a popular eating approach where people only eat during a short window of time, can place endurance athletes at increased risk for low energy availability and its metabolic and hormone consequences, as suggested by this study.)

Waiting too long between meals or snacks can put your body into an off-and-on starvation mode that contributes to low energy availability and raises the risk for RED-S. Simply changing the timing of when you eat to ensure that you get what you need when you need it, including soon after a big workout, might be all it takes to get the total calories you need to support training.

3. Seek out carbs

When you are burning heaps of calories, it’s not the time to eat carb-light. “Recent research suggests that a lot of the symptoms of LEA and RED-S are driven by low-carbohydrate intake rather than low-caloric intake in general,” Coates says. For example, a 2022 study in Medicine and Science in Sports and Exercise involving 28 elite male race walkers discovered that the negative effects of training on athletes’ iron levels and immune system were more pronounced when insufficient carbs were consumed compared to when they ate a diet with insufficient calories but higher amounts of carbs.

Another 2022 investigation showed that eating inadequate amounts of carbohydrate calories is more detrimental to bone health than eating a diet with low energy availability (or low calorie). So, yes, overall calories still matter, but it’s the calories from carbs that count most. “Low-carbohydrate intake is a stressor on the body, so if you add training stress and low-carbohydrate stress, it can be too much for your body to handle,” Coates explains.

That means carbohydrate-dense foods like pasta, bread, and potatoes should be a fixture of an endurance athlete’s plate. And don’t be stingy with the toppings for an added calorie bonus.

4. Fuel your rides

Remember that calories on the bike count, too. If you consistently finish rides with lion-sized hunger it’s a sign you are underfueling during exercise, which also contributes to low energy availability. These will be more calories that you’ll need to make up for during the rest of your day and that’s not always easy to do. Again, a sports dietitian can help you design a workout fueling plan that keeps you out of the red zone.

The bottom line on low energy availability

Knowing the negative acute and long-term physiological and performance consequences of low energy availability and RED-S should deter cyclists and other athletes from depriving their bodies of much-needed calories and nutrients.

If your health, performance, mood, or overall energy is in decline it’s time to pay more attention to your nutrition and what you’re putting in your body (as well as how much). For many cyclists who are spending plenty of time on the saddle, eating more calories—and definitely those from carbs—is the way to go, even during the indoor riding season.



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