It’s no secret that many athletes or active individuals feel pressure to look a certain way or hit a specific number on the scale. But this mindset can often lead to overtraining or underfueling, both of which are harmful to an individual's mental and physical health. And though the occasional skipped snack or gel won't substantially affect long-term physical health, consistent underfueling, particularly during bouts of intense training, can impact performance, recovery, and overall health in both men and women. Learn the signs and risks associated with underfueling, and the appropriate measures to take to ensure optimal performance.
Learning the signs and risks of underfuelingIt’s easy to get caught up in daily training or race day goals, but underfueling can impede your efforts to maximize performance. Like most things with training and nutrition, individual responses to underfueling differs between people, but here are some of the general signs and risks associated with underfueling (note: there can be some crossover between the two): 
Signs of underfueling
- Decreased performance
- Decreased muscle strength
- Decreased glycogen stores
- Decreased coordination
- Difficulty concentrating
Long-term risks of underfueling
- Decreased response to training
- Increased irritability
- Increased risk of injury and illness
- Muscle loss
- Poor/inadequate intake of nutrients, leading to risk of deficiency
- Increased risk for disordered eating
- Increased emotional stress
- RED-S (Relative Energy Deficiency in Sport)
Underfueling can lead to metabolic dysfunction in womenRED-S is defined by the International Olympic Committee (IOC) as "impaired physiological functioning caused by relative energy deficiency and includes, but is not limited to, impairments of metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health." According to the IOC, the major factor influencing the development of RED-S is low energy availability (LEA). And though it might seem intuitively that LEA is only possible in underweight people or elite athletes (who quickly burn away all the food they eat), it's actually also seen in recreationally-active individuals—your average Joes. [7,10]
Here is the mathematical breakdown of energy availability:
EA (energy availability)=
(EI (energy intake) (kcal)- EEE (exercise energy expenditure) (kcal)) / FFM (fat free mass)(kg)
In other words, EA is the amount of energy you burn subtracted from the amount of energy you consume per kilogram of non-fat bodyweight. This number will vary between individuals, but research has shown that an EA of 30 kcal/kg lean body mass/day is the threshold for usual reproductive and osteogenic (bone) function in females.  It is important to note that this cut-off does not predict amenorrhea (the loss of menstrual function) in all women. 
To prevent LEA and RED-S, active individuals should maintain the optimal 45 calories per kilogram of lean body mass per day. For example, a 60kg woman with 20% body fat (equal to 48kg of lean body mass) should consume 2,160 calories per day. If a woman falls below the caloric optimal intake consistently, risk of developing RED-S and any components of the Female Athlete Triad (osteoporosis, amenorrhea, and disordered eating) increases. If any of these symptoms occur, energy intake will likely need to be higher than 'optimal' to restore normal metabolic function.
Male athletes can be affected by underfueling and RED-S, tooThough most of the research available on LEA has been conducted in females, there is growing evidence that males may also experience negative outcomes related to LEA.  This is particularly true in men who participate in weight-sensitive sports like cycling, rowing, and running. More research is needed to better understand effects and biological response to RED-S in men, though a reduction of testosterone is likely an important factor. 
Weight loss for improved performance must be done properly to avoid negative side effectsWeight loss can be a controversial topic for athletes, but is appropriate in some cases when centered around nutrient-rich foods, proper fueling for sport, and avoids emphasis on caloric restriction. A 2015 study, for example, found that 21% of Division 1 college football players were obese (≥25 % body fat) and had insulin resistance, and 9% had metabolic syndrome. In cases like these, weight loss can both reduce chronic disease risk and improve athletic performance.
If an athlete or active individual is considering weight loss, it is strongly recommended that they work with a supportive, professional team who understands the physiological, psychological and environmental factors that an athlete may face. It is also important to understand that weight is only one metric of health and performance—an athlete's main focus should always be fueling performance, training and health goals, not a specific body type. Weight loss can be a beneficial side effect of these things.
In fact, research shows that drastic calorie restriction can have substantially harmful effects on performance. A 2011 study found that slower and more reasonable weight loss (defined as ~0.7% body weight lost/week) preserved more lean tissue and saw more improvements in strength than individuals who experienced more severe weight loss (defined as~1.4% body weight lost/week). A separate study from 2009 found that drastic energy restriction paired with physical activity may result in metabolic adaptations that can actually slow down weight loss.
Biomarkers to watch to avoid underfuelingLooking for an objective way to see if you’re properly fueling your fitness and performance goals? Blood biomarkers can be exactly that. Here are a few that can be affected by underfueling to pay attention:
Vitamin DBone health is negatively impacted by both short and long term LEA. This is well documented in active female athletes, but it can also occur in males. And though underfueling on its own can negatively impact bone health, this is especially true when paired with a vitamin D deficiency or insufficiency. Adequate vitamin D is also critical for preventing injury, illness, and delayed muscle recovery. And inadequate vitamin D intake is incredibly common—a 2016 study of vitamin D in professional athletes found that 32% of professional basketball players were deficient and 47% were insufficient of this important vitamin.
CortisolLEA can lead to elevated measures of of this stress hormone as part of hormonal pathway disruptions seen in RED-S. There are also a number of cortisol-related psychological factors that can result in or contribute to LEA or disordered eating, including stress, anxiety, depression, and other mood disruptions.[4,7]
Full iron panelIron deficiency is a common issue in the endurance and running community and is connected to well-known factors like diet, activity level, and gender. Iron deficiency can also be linked to excess inflammation (particularly that caused by exercise), as it can increase levels of a compound called hepcidin, a regulator of iron absorption. When there are higher levels of hepcidin in the body, less iron is absorbed. In these cases, short-term iron markers like blood iron would decrease as absorption slowed. However, chronic inflammation, like that caused by long bouts of intense training, can actually cause ferritin levels to rise as the body stores iron away for the future. These opposing changes in iron markers can be difficult to interpret in isolation, and warrant a full iron panel, like that found in InsideTracker tests. To learn more about how inflammation affects your iron levels, check out our blog on the topic.
It's important to note that inadequate energy availability may also increase hepcidin levels and thus play a role in iron status. A 2016 study of inflammatory response in individuals participating in military training (a combination of load carriage and endurance exercise) found that hepcidin levels were inversely related to energy balance. This indicates that maintaining proper energy balance and optimal caloric intake can potentially lessen the inflammatory response to exercise.
TestosteroneCurrent research indicates that males can also develop suppression of reproductive function (think of it as the male version of amenorrhea) known as exercise hypogonadal male condition (EHMC).[6,7] The mechanism of EHMC is still unclear, and further research in male athletes is needed to determine the role of testosterone levels as a reliable indicator of LEA.[4,7]
DHEASDHEAS is a lipid that plays an important role in the formation of hormones in females, including testosterone and estradiol. A low reading of this biomarker can be indicative of low calorie intake (particularly from fats), high levels of stress, particularly when paired with high activity levels.
A summary of the impact and prevention of underfueling
- Athletes and active individuals of all levels, recreational to professional, are at risk for the negative outcomes of underfueling
- Each person may experience different outcomes of underfueling and it is important to be aware of the potential negative long term effects these behaviors can have. This includes immune function, bone health, reproductive health, and mental health.
- In general, optimal caloric intake is equal to 45kcal/kg of lean body mass/day
- Improper approaches to weight loss can result in underfueling and destructive metabolic side effects
- Your weight does not define your health or performance!
- If you’re unsure of whether you’re fueling your activity/sport properly, work with a sports dietitian to ensure you’re meeting your calorie and nutrient needs
- Monitoring your biomarkers can help to provide an objective measure to prevent underfueling and overtraining
Stevie Lyn Smith, MS, RDN, CSSD, CDNStevie Lyn is a Content Strategist and Team Nutritionist at InsideTracker. As a Registered Dietitian and Ironman triathlete, she enjoys combining her passions to help educate others on how to fuel for overall health and performance. When she’s not swimming, biking, or running with her dog, you’ll find her in the kitchen working on a new recipe to improve her biomarkers.