Weight fluctuations are common during an illness, and the answer to whether you gain weight is not a simple yes or no. The body’s response to sickness is a complex physiological event that can cause the number on the scale to change quickly. These temporary shifts are usually a result of fluid changes or the immune system’s heightened activity, rather than true changes in body fat or muscle. The effect on weight depends on the specific nature of the illness and how your body balances energy intake against the increased demands of fighting the infection.
Why the Scale Jumps: Fluid and Inflammation
The most immediate reason for a sudden weight increase during sickness is fluid retention. When the immune system activates to fight a pathogen, it triggers systemic inflammation. This inflammatory response causes blood vessels to become more permeable, allowing fluid, immune cells, and proteins to leak into surrounding tissues, leading to swelling or edema.
Hormones also play a large part in fluid management during stress and illness. The stress hormone cortisol, often elevated during sickness, can affect the balance of electrolytes and promote water retention. This fluid shift is a temporary increase in total body water, not a gain of body mass, which contributes to the higher number seen on the scale.
Reduced mobility further contributes to temporary weight gain, especially in the extremities. When a person is ill, they typically spend more time lying down or sitting, which slows the return of fluid from the tissues back into the circulatory system. Swelling may be particularly noticeable in the hands, feet, and ankles.
How Illness Affects Calorie Balance
Illness significantly disrupts the balance between calories consumed and energy expended, which determines long-term weight change. An activated immune system increases the body’s basal metabolic rate (BMR). Fighting an infection, especially with a fever, requires a large amount of energy; the BMR can increase by up to 10% for every one-degree Celsius rise in body temperature.
The body experiences a “sickness metabolism” where energy is redirected to produce immune cells and combat the pathogen. This heightened activity causes the body to burn more calories than usual, often leading to a caloric deficit and weight loss. In severe illnesses, the body may even break down muscle reserves to provide the protein and energy needed for the immune response.
Working against this increased energy demand is the concurrent decrease in physical activity. While the internal metabolic rate rises, a sick person is usually immobile, often confined to bed. This significantly lowers the total daily energy expenditure from movement, which can offset the increased metabolic rate and make it harder to maintain a caloric deficit.
The input side of the balance is also affected by illness, leading to varied outcomes. Many people lose their appetite due to nausea, congestion, or discomfort, which naturally limits caloric intake and promotes weight loss. Conversely, some individuals consume high-calorie comfort foods, like sugary drinks or rich broths, which can quickly lead to a caloric surplus and subsequent weight gain.
When Weight Changes Require Medical Attention
While minor weight fluctuations are normal during illness, certain changes should prompt a conversation with a healthcare provider. Rapid weight gain is often a sign of excessive fluid retention, particularly if the gain is two to three pounds per day or five pounds in a week. This sudden increase, especially with severe swelling or shortness of breath, can indicate an underlying condition related to the heart, liver, or kidneys that needs evaluation.
Weight changes can also be a side effect of medication used to treat the illness. For instance, corticosteroids, sometimes prescribed to manage inflammation, are known to cause fluid retention and increased appetite, contributing to weight gain. Any unexpected weight change after starting a new prescription should be discussed with the prescribing physician.
Unexplained weight loss is also a serious symptom, particularly if it is persistent or dramatic. Losing more than 5% of your total body weight over six to twelve months, without intentional change in diet or exercise, may signal a more significant underlying health issue. A doctor can help determine if the weight change is a temporary side effect of a minor illness or a symptom of a more complex medical condition.