Can RED-S cause weight gain?

October 24, 2025 •

4 min read

Recent research indicates that up to 45% of female athletes and 19% of male athletes experience disordered eating behaviors, which can lead to Relative Energy Deficiency in Sport (RED-S). Contrary to the expectation of weight loss, this syndrome can surprisingly cause weight gain, not just weight loss, due to complex physiological adaptations.

Quick Summary

Investigate how Relative Energy Deficiency in Sport (RED-S) triggers metabolic and hormonal changes in athletes, resulting in unexpected weight gain. Unintended weight changes are a key indicator of this complex condition, revealing a paradox in energy balance.

Key Points

Metabolic Slowdown: In response to low energy availability, the body slows its resting metabolic rate to conserve energy, a primary mechanism that can cause weight gain in RED-S.
Hormonal Chaos: RED-S causes a rise in the stress hormone cortisol and a drop in thyroid hormones, which both contribute to metabolic changes and increased fat storage.
Visceral Fat Accumulation: High cortisol and low testosterone (in males) are linked to an increase in visceral fat, often while muscle mass is being broken down for fuel.
Normal Weight is Not Protective: An athlete does not need to be underweight to experience RED-S. Metabolic and hormonal changes can lead to weight gain even at a normal or higher body mass index.
Weight Fluctuations are a Red Flag: Unexpected weight changes—either loss or gain—are a key sign of RED-S, indicating the body's disrupted energy balance.
Recovery Reverses the Effects: Restoring adequate energy availability through increased nutrition can reverse metabolic and hormonal damage, normalizing weight and improving health.

Understanding the Paradox: Why Under-fueling Leads to Weight Gain

Relative Energy Deficiency in Sport (RED-S) is a condition that arises from a chronic mismatch between energy intake and expenditure, leading to low energy availability (LEA). While the logical assumption is that a calorie deficit will lead to weight loss, the human body is remarkably complex. When faced with prolonged LEA, the body enters a protective 'starvation mode' to conserve energy for essential functions, such as breathing and circulation. This survival mechanism involves a significant slowing of the resting metabolic rate (RMR), the amount of energy the body burns at rest. As a result, the body becomes highly efficient at storing any incoming calories, often as fat, making weight gain a possible and frustrating side effect for athletes.

The Hormonal Impact of Relative Energy Deficiency in Sport

The metabolic slowdown seen in RED-S is not a simple switch but a cascade of complex hormonal changes. These hormonal disruptions play a central role in driving weight gain and altering body composition.

Elevated Cortisol: Chronic energy deficit is a stressor for the body, causing the adrenal glands to produce more cortisol. Persistently high cortisol levels are associated with increased appetite, cravings for high-calorie foods, and the storage of visceral (deep abdominal) fat.
Suppressed Thyroid Hormones: The body conserves energy by down-regulating the thyroid gland's function. This results in lower levels of thyroid hormones, which are responsible for regulating metabolism. This further slows RMR and can lead to symptoms like persistent fatigue, feeling cold, and unexplained weight changes.
Reduced Sex Hormones: LEA suppresses the production of sex hormones like estrogen and testosterone. In men, this can lead to decreased muscle mass and an increase in visceral fat. In women, low estrogen not only disrupts menstrual cycles but also impacts metabolism and bone health.

Comparison of Metabolic States in Healthy vs. RED-S Athletes


Feature	Healthy Athlete	RED-S Athlete
Energy Availability	High	Low
Metabolic Rate	Optimal for performance	Suppressed/Low
Hormonal Balance	Balanced cortisol, thyroid, and sex hormones	High cortisol, low thyroid, low sex hormones
Appetite Regulation	Functioning hunger/satiety signals	Dysregulated; potential cravings or loss of appetite
Weight Trajectory	Stable or intentional changes	Unexplained fluctuations (loss or gain)
Body Composition	Optimal ratio of lean mass to fat mass	Decreased lean mass, potential increase in visceral fat

Recognizing the Signs Beyond the Scale

Athletes and coaches must look beyond weight to spot the signs of RED-S. Weight gain is just one of many indicators of underlying physiological dysfunction. Some key non-weight related signs include:

Declining performance: A noticeable drop in athletic ability, endurance, or strength.
Recurrent injuries: An increase in injuries, especially stress fractures, due to compromised bone health.
Menstrual dysfunction: Irregular or absent periods in females (amenorrhea).
Chronic fatigue: Feeling persistently tired and having poor recovery from training.
Frequent illness: A weakened immune system leading to more colds and infections.
Mood changes: Increased irritability, depression, or anxiety.

Recovery and Weight Restoration in RED-S

Successfully treating RED-S involves more than just increasing caloric intake; it requires a multi-disciplinary approach. For many, weight restoration is a necessary part of the process, and it's essential for athletes to overcome the fear of weight gain, which is often tied to their athletic identity and body image. The goal is to restore energy availability, which will allow the body to reverse the harmful metabolic and hormonal adaptations. The recovery journey requires patience and professional guidance from sports physicians, dietitians, and psychologists.

Key aspects of recovery include:

Increasing Energy Intake: A sports dietitian will create a nutrition plan to meet the athlete's energy requirements, reversing the low energy availability that caused the metabolic suppression.
Adjusting Training Load: In some cases, reducing the volume or intensity of exercise is necessary to reduce energy expenditure and allow the body to heal.
Hormonal Monitoring: Blood tests can track key markers like cortisol and thyroid hormones to monitor physiological recovery.
Addressing Psychological Factors: Many athletes with RED-S also have body image issues or disordered eating patterns that must be addressed by a mental health professional.

For more comprehensive information on the diagnosis and management of RED-S, a seminal paper was published in the British Journal of Sports Medicine.

Conclusion: Navigating the RED-S Weight Paradox

In conclusion, the answer to the question, "Can RED-S cause weight gain?" is a definitive yes. The syndrome's effect on metabolism and hormonal regulation can trigger a counterintuitive weight increase, especially in athletes under-fueling for their high energy expenditure. Recognizing this surprising symptom is crucial for early detection and intervention. Athletes should focus on proper fueling, not just aesthetics or weight goals, to prevent the serious health consequences of RED-S and achieve sustainable, optimal performance.

Frequently Asked Questions

Under-eating, in the context of high energy expenditure, triggers the body’s survival response. This causes the resting metabolic rate to slow down, making the body more efficient at storing incoming calories as fat to conserve energy, which can lead to weight gain.

Yes. An athlete's body composition, not just their weight, can be affected by RED-S. Many individuals with the syndrome have a normal or even above-normal BMI but experience metabolic dysfunction and unintended weight changes.

Key hormones include increased cortisol, a stress hormone that promotes fat storage and cravings, and decreased thyroid hormones, which regulate metabolism.

The weight gained is often fat, and specifically visceral fat. In a state of low energy availability, the body is more likely to break down muscle tissue for energy while storing any excess calories as fat.

The primary goal of recovery is to restore energy balance, which will normalize metabolic and hormonal function. While this often leads to a healthier body composition, weight restoration (or gain) may be necessary in some cases for full health recovery, especially for bone and hormonal health.

RED-S-related weight gain is typically accompanied by other symptoms like declining performance, persistent fatigue, frequent illness, mood changes, and for females, menstrual irregularities. It is not an isolated event but part of a larger pattern of physiological stress.

The best approach is to work with a sports dietitian and medical team. The focus should be on increasing overall energy intake to repair the metabolism and address the underlying energy deficit, rather than focusing on weight metrics.