Overweight and obesity are a growing concern in the United States and worldwide. Weight gain can contribute to a host of health conditions, ranging from cardiovascular disease and cancer to cognitive decline and COVID-19. Maintaining a healthy weight can help minimize these health problems and maximize overall quality of life.

Fat doesn’t just take up space. It is metabolically active tissue that produces its own hormones, immune-regulating cytokines and chemical messengers known as adipokines. These chemicals play a role in regulating appetite and energy expenditure and contribute to chronic inflammation, which underlies a wide range of health conditions. Visceral fat deep within the abdomen is more strongly associated with health problems than subcutaneous fat under the skin.

Weight gain often goes hand in hand with metabolic abnormalities. Metabolic syndrome—a cluster of conditions including excess abdominal fat, high blood sugar, abnormal cholesterol and triglyceride levels and high blood pressure—raises the risk for type 2 diabetes, heart disease and strokes.

Visceral fat can accumulate around the heart and inside the liver and other organs. Over time, fat buildup in the liver—non-alcoholic fatty liver disease (NAFLD) or its more severe form, non-alcoholic steatohepatitis (NASH)—can lead to cirrhosis, liver cancer and the need for a liver transplant.

Overweight and obesity contribute to other types of cancer as well. According to the Centers for Disease Control and Prevention, excess fat raises the risk for at least 13 different malignancies, including breast, colon, kidney, ovarian, pancreatic and stomach cancers.

Carrying excess weight puts increased pressure on the hip and knee joints and the lower back, which can lead to pain and impaired mobility. People with obesity are more likely to experience sleep apnea. Obesity can also contribute to cognitive problems and pregnancy complications, and a growing body of research suggests that it has a detrimental effect on immune function.

Most recently, overweight and obesity have been linked to worse severity and greater risk of death from COVID-19. One CDC study found that more than three quarters of people hospitalized with COVID-19 in the United States had overweight or obesity.

Finally, excess fat can have a negative effect on self-esteem, worsen depression and make people with HIV less willing to start or stay on antiretroviral treatment because of concern that antiretroviral drugs will cause weight gain.