The Dangers of Under Fueling

This article is from Monique Ryan, MS, RD, LDN, CSSD, author of Sports Nutrition for Endurance Athletes.

It starts well intentioned and innocently enough: You want to drop a few pounds and get leaner for the upcoming triathlon season. You are focused—keeping an eye on your calorie intake and planning to continue on this path as you approach longer and harder workouts.

But then a funny thing happens and you start to like your new, extra-lean physique, and you begin to consistently under-eat. Experts contend that pushing the calorie-cutting envelope can potentially backfire and actually lead to negative effects on both your health and performance.

Recently, an expert panel brought together by the International Olympic Committee, examined this exact topic. A new term: Relative Energy Deficiency in Sport, or RED-S, was coined to describe health and performance issues that arise when athletes don’t eat enough to cover both training and daily life activities. The RED-S concept builds and expands upon the condition known for the past decade as the Female Athlete Triad. While the Triad describes the relationship between low energy availability, menstrual dysfunction, and poor bone health in female athletes, RED-S includes this triad and other related consequences. RED-S includes male athletes and proposes endurance athletes as a unique group of athletes at risk of relative energy deficiency.

So what exactly is RED-S? The condition refers to having inadequate energy available to support a range of bodily functions. It is the lack of energy (or calories) “left over” for daily functioning after accounting for the energy cost of training. While this cost of training can be significant in triathlon training, you shouldn’t underestimate what your body requires in order to recover from such training and continue to function optimally in daily life.

It might seem that cutting back on calories leads to minor problems—a poor workout here and there—the actual backlash of not consuming enough calories can often be even more complex than athletes realize. The consequences can range from mildly disruptive to serious. Beyond affecting hormonal function in women (and with emerging hormonal data on men), this deficiency can also impair metabolic rate, the immune system, gastrointestinal function, heart health, protein synthesis, and psychological health in both female and male triathletes.

For example, a triathlete may reduce their calorie intake to reduce weight and body fat, but find that at some point their weight loss efforts come to a halt. This is the downside of under-eating and the effect it has on your metabolism. Other signs of RED-S could be frequent injuries, recurring colds and other illnesses, and poor recovery from training—nothing a triathlete should even get close to!

Below are some highlights from the RED-S committee’s risk assessment model, which can help identify athletes who may be at risk.

Possible signs of RED-S:

  • Prolonged abnormally low body fat
  • Substantial weight loss (5 to 10-percent of body mass in 30 days)
  • Low energy availability of prolonged or severe nature Abnormal menstrual cycle in women
  • History of one or more stress fractures associated with hormonal dysfunction and low energy availability
  • Reduced bone mineral density

Possible outcomes of RED-S:

  • Decreased endurance performance Increased injury risk
  • Decreased training response
  • Decreased glycogen stores
  • Decreased muscle strength
  • Depression
  • Irritability
  • Decreased concentration and coordination
  • Impaired judgment

While determining energy needs can be complex and left to the expertise of a sports dietitian to zero in on your target calories for specific training days, you can make some estimates based on established formulas. Your total energy needs for the day are a combination of your:

  • Resting metabolic rate (RMR)
  • Daily physical activity
  • Training expenditure
  • Thermic effect of food

It is not practical or likely that all of these factors will be measured directly in each triathlete, some estimations can be made using calories per pound (or kilogram) based on that day’s training. Current guidelines are:

  • Rest day or very mild activity: 12-14 calories/lb (26-31 kcal/kg)
  • Up to one hour of moderate exercise: 15-17 calories/lb (33-37 kcal/kg)
  • 1-2 hours moderate intensity exercise: 18-24 kcal/lb (40-53 kcal/kg)
  • Several hours of activity: 24-29 kcal/lb (53-63 kcal/Kg)

Let’s look at a male triathlete who weighs 165 lb., has a desk job, and on a given day does a 60-minute high intensity swim in the morning and a 90-minute run in the evening. His total energy requirements for that day are 4,200 calories.

Keep in mind that these guidelines should be interpreted for each individual. For example, when in the throes of full IRONMAN training, you may want to eat more energy than outlined above on a rest day to better replenish fuel stores and prepare for the coming training week. If you would like to decrease body fat or body weight you may implement a mild calorie restriction—about 300 calories daily—on some training days, but not others. If you are trying to recover from RED-S and your performance has been suffering, then consuming the full calorie amount is advised. Lastly, these calorie amounts can include energy consumed during exercise.

Although you might keep a food journal and meticulously log every calorie you consume, RED-S can begin unintentionally. Triathlon training varies in caloric demands from day to day, and often an athlete who isn’t trying to meet a weight loss goal will accidentally under-eat because they aren’t taking their workouts into consideration while planning their diet—or planning their diet at all. Athletes need to remember that in addition to fueling training sessions, the calories they consume go toward their total energy needs. Finding the proper balance can be tricky, but remember that skipping fuel during long workouts can lead to a relative energy deficit. Trust me, it’s important to get it right.

See what to eat and when with Sports Nutrition for Endurance Athletes. Ryan demystifies optimum daily nutrition and shows simple steps to make the best decisions about what you eat and drink.

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