Understanding Metabolism’s Role in Eating Disorders & Recovery

In a culture preoccupied with body size, body shape, and fitness, the word metabolism is commonly equated to burning calories. Yet, the human metabolism is truly much more complex.

What is metabolism and how does it work?

Human metabolism is the sum of all chemical reactions taking place in each cell of the body.  Through intricate systems of digestion and absorption, energy from food (calories) gets converted into a chemical form of energy (called adenosine triphosphate, or ATP for short). Energy is used by cells in the body to:

• Fuel cellular processes
• Maintain organs and structural tissues
• Regulate body functions, like breathing, circulating blood, growing, and repairing cells, and maintaining a healthy body temperature.

How does metabolism change in a state of starvation?

People with certain eating disorders, such as anorexia nervosa, reduce the amounts and types of foods they eat. This results in deficient energy intake.  Over time, this leads to a state of starvation, and in turn, causes the metabolism to slow.  This means there is not enough energy to maintain all bodily processes, so certain processes are slowed down. 

First, the body slows down processes that are deemed “non-essential.”  For example, when the body is working hard to stay alive, it deprioritizes things like hair, skin and nail nourishment. The reproductive system powers down. (Notably, this leads to other changes in hormone function, bone health, and menstrual cycles in women).  Less energy is used on tasks like temperature regulation, so people tend to become cold easily.  And, even the internal muscles pushing food along the digestive tract are less active, leading to slowed movement of food, constipation, and heightened fullness. People may experience food as “the problem,” causing bloat or discomfort. But, it is actually lack of food that worsens these symptoms.

As malnutrition and energy deprivation continue, the body next turns to life-sustaining processes.  Heart function, for example, becomes compromised as heart rate slows and rhythm and blood pressure change. Eventually, the energy deficit will also impair the body’s ability to repair cellular structures. This causes muscle wasting and loss of vital organ mass, including loss of brain matter.  These consequences can be life-threatening.

Signs of a Slowed Metabolism

People in a state of starvation due to an eating disorder experience noticeable signs of these metabolic changes, like

• cold intolerance,
• menstrual cycle changes,
• hair loss, and
• difficulty concentrating.

Yet, the most serious and dangerous consequences of malnutrition and energy deprivation – changes in heart rhythms, loss of bone and muscle mass – may not be readily apparent to the individual.  Additionally, other populations with low energy availability, such as those with atypical anorexia, relative energy deficiency in sport, or low-weight avoidant restrictive food intake disorder (ARFID) are also at risk for these metabolic changes and related side effects.

Considering these many metabolic changes, patients with anorexia nervosa often have energy usage that is as low as 70% of what would be expected based on age, gender, height, and weight.  Fortunately, consistent and progressive increases in energy intake during recovery stimulate the metabolism and restore bodily functions to a normal rhythm.

What happens to metabolism during treatment?

As weight and energy availability improve, metabolism normalizes incrementally.  As a person gets closer to a healthy weight, the bodily processes that were once slowed down begin to rev up again.  For example, a person might start to feel warmer, and digestion may gradually improve. 

In some cases, metabolism may speed up so much that it exceeds the rate of a healthy person’s metabolism.  This is due to the Thermic Effect of Food, which accounts for the energy utilized by digestion, absorption, transport, and storage of nutrients after eating.  This increase in metabolic rate:

• occurs shortly after food consumption
• peaks in two-to-three hours,
• and lasts six-to-twelve hours.

For healthy adults, the Thermic Effect of Food increases metabolic rate by 10%.  In patients recovering from anorexia nervosa, the Thermic Effect of Food may increase metabolic rate by up to 30%.  This is why people recovering from anorexia nervosa may need to eat more and differently than expected to recover bodily functions and meet weight goals.  

Implications of Metabolic Changes on Treatment

Healthy people need to consume approximately 3500 additional calories to gain one pound of body weight. However, individuals recovering from anorexia nervosa require substantially more calories for a similar amount of weight gain. Also, more energy is needed to maintain higher weights. Thus, as people gain weight during recovery, the number of calories needed to maintain weight is a moving target.  For these reasons, individuals recovering from anorexia nervosa need increased calories to attain and maintain their treatment goal weight during the nutrition rehabilitation process.

Additionally, there are some signs of increased energy utilization.  For example, sometimes energy is inappropriately utilized to excessively heat the body, which leads to night sweats during recovery.

Even after achieving a healthy body weight, those recovering from an eating disorder may utilize energy inefficiently.  Things seem to take 3-6 months, on average, to get back to baseline.  This phenomenon is not unique to recovery from anorexia nervosa.  It is seen in other conditions where the body is damaged, including people recovering from burns, surgery, injury, and sepsis.

Incredibly, the human metabolism can readily adapt to our environments to meet the needs of our bodies.

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