Obesity and Overweight

Overweight and obesity mean that a person is in a weight range, given his or her height, that can cause health problems. Obesity causes even greater health risks than being overweight. The NICHD is one of many federal agencies and NIH Institutes working to understand overweight and obesity. The NICHD supports and conducts research on the causes of excess weight, how to prevent and treat obesity, and related topics, including conditions caused by obesity.

What are obesity, overweight, and body composition?

Body composition is a term that describes how much of a person’s body is made up of bone, fat, water, and other substances. “Overweight” and “obesity” both mean that a person is at an unhealthy weight. An obese person has too much body fat. An overweight person weighs too much, but the extra weight may come from muscle, bone, body water, and/or fat.1

Health care providers often use body mass index (BMI) to determine if a person is overweight or obese. Adults who have a BMI between 25 and 29.9 are generally considered overweight. Adults who have a BMI of 30 or higher are considered obese.2 A child’s weight status is determined using a special chart that takes into account the child’s age and gender.3 

How many people are affected by/at risk for obesity & overweight?

More than 35% of U.S. adults are obese,1 and more than 34% are overweight.2 Obesity affects 17% of all children and adolescents in the United States, which is three times the prevalence from just one generation ago.3 Nearly 32% of children and adolescents are either overweight or obese.4


How are obesity & overweight diagnosed?

Using Body Mass Index

The most common way to determine if a person is overweight or obese is to calculate body mass index (BMI). BMI is an estimate of body fat based on comparing a person’s weight to his or her height.

Health care providers also look at BMI, along with information about additional risk factors, to determine a person’s risk for developing weight-related diseases. Usually, the higher a person’s BMI, the higher the risk of disease.

BMI for Adults

An adult’s BMI can be determined using a BMI calculator. Health care providers use BMI ranges to indicate a person’s weight status. For adults, a BMI of:

  • 18.5 to 24.9 is considered normal weight
  • 25.0 to 29.9 is considered overweight
  • 30.0 to 39.9 is considered obese
  • 40.0 and higher is considered extremely obese1

It is important to remember that although BMI is generally a good way to estimate how much body fat a person has, it does not measure body fat directly and therefore is not reliable in all cases. For example, a person may weigh extra because he or she is athletic and has a lot of muscle, and not because he or she has excess body fat.

BMI for Children and Teens

For children age 2 and older and for teens, BMI uses weight and height, but adds sex and age into the calculation. Instead of using a specific number like the BMI charts for adults, the BMI for children and teens is listed as a percent. This percentage indicates a child’s BMI in relation to the BMIs of other children of the same sex and age. A child and teen BMI calculator can provide a BMI.

Children age 2 and older are considered:

  • At a healthy weight if their BMI falls between the 5th and the 85th percentiles
  • Overweight if their BMI is between the 85th and 95th percentiles
  • Obese if their BMI is at or higher than the 95th percentile2

Other Ways to Measure Body Fat

Body fat can be measured in other ways in addition to BMI, including waist circumference, calculation of waist-to-hip circumference ratios, measuring the thickness of a skinfold (a pinch of skin and fat), and techniques such as ultrasound that are more precise than BMI. A health care provider can help determine if such tests are necessary.


What are the treatments for obesity & overweight?

To maintain a healthy weight, it is important to create a balance between the calories eaten and the calories burned through physical activity and bodily functions. To lose weight, a person usually needs to reduce calories and increase physical activity.

If lifestyle changes are not enough, medicines and weight-loss surgery also are options for some people who need to lose weight.1

What are some tips for healthy eating?

  • Emphasize fruits, vegetables, whole grains, and fat-free or low-fat milk and dairy products.
  • Include lean meats, poultry, fish, beans, eggs, and nuts.
  • Eat a diet low in saturated fats, trans fats, cholesterol, salt (sodium), and added sugars.
  • Drink more water instead of sugary drinks.

How much physical activity does an adult need?

Getting enough physical activity is an important way to help prevent or reduce overweight and obesity and related health problems.

  • Adults age 18 and older need at least 30 minutes of physical activity on 5 or more days of the week to be healthy.
  • Children and teens need at least 60 minutes of activity a day for their health.

What causes obesity & overweight?

A number of factors can play a role in weight gain. These include diet, lack of exercise, factors in a person’s environment, and genetics. Some of these factors are discussed briefly below.

People gain weight when they eat more calories than they burn through activity. This imbalance is the greatest contributor to weight gain.


The world around us influences our ability to maintain a healthy weight. For example:

  • Not having area parks, sidewalks, and affordable gyms makes it hard for people to be physically active.
  • Oversized food portions increase Americans’ calorie intake, making even more physical activity necessary to maintain a healthy weight.
  • Some people don’t have access to supermarkets that sell affordable healthy foods, such as fresh fruits and vegetables.
  • Food advertising encourages people to buy unhealthy foods, such as high-fat snacks and sugary drinks.1


Research shows that genetics plays a role in obesity. Genes can directly cause obesity in such disorders as Prader-Willi syndrome.

Genes also may contribute to a person’s susceptibility to weight gain. Scientists believe that genes may increase a person’s likelihood of becoming obese but that outside factors, such as an abundant food supply or little physical activity, also may be required for a person to put on excess weight.2

Health Conditions and Medications

Some hormone problems may cause overweight and obesity, such as underactive thyroid, Cushing syndrome and polycystic ovary syndrome (PCOS).

Certain medicines also may cause weight gain, including some corticosteroids, antidepressants, and seizure medicines.1

Stress, Emotional Factors, and Poor Sleep

Some people eat more than usual when they are bored, angry, upset, or stressed.

Studies also have found that the less people sleep, the more likely they are to be overweight or obese. This is partly because hormones that are released during sleep control appetite and the body’s use of energy.1


Are there disorders or conditions associated with obesity & overweight?

In adults, being overweight or obese increases the risks for several problems, including1,2:

  • Coronary heart disease
  • Type 2 diabetes
  • Certain types of cancer
  • High blood pressure (hypertension)
  • Unhealthy levels of certain types of blood fats, such as high total cholesterol or high levels of triglycerides
  • Stroke
  • Liver and gallbladder disease
  • Sleep apnea and respiratory problems
  • Osteoarthritis, a breaking down of cartilage and its underlying bone within a joint
  • Gynecological problems, such as infertility.

Overweight and obesity also increase the health risks for children and teens. For example, type 2 diabetes once was rare in American children, but the number of children with it today has grown rapidly. Early onset of type 2 diabetes can cause the early onset of complications such as vision loss, nerve damage, and cardiovascular disease.3 Researchers express concern that because of obesity, today’s children will be the first generation that may not live as long as their parents.4

Some of the possible effects of obesity in children include:5

  • High blood pressure and cholesterol
  • Liver disease
  • Sleep apnea and other lung-related problems
  • Problems related to bone development
  • Early onset of menstruation for girls
  • Increased risk of cardiovascular disease in adulthood

In addition, overweight children are more likely to become overweight or obese as adults.

Can obesity & overweight affect fertility?

Many studies have highlighted the link between obesity and infertility in women. For example, obesity can contribute to problems with ovulation and to irregular menstrual periods. It also contributes to a lowered response to fertility treatment and to miscarriages. Research indicates that reducing obesity improves women’s reproductive health.1

Women with a condition called polycystic (pronounced pah-lee-SIS-tik) ovary syndrome, or PCOS, face a higher risk of both obesity and infertility. Lifestyle changes, such as losing weight, can trigger body changes that facilitate conception in women with PCOS.2,3 Learn more about PCOS.

Men’s obesity also is associated with a higher risk of infertility. There are several ways that excess weight may affect a man’s fertility, including changes in his hormone and semen production.4


Obesity and Overweight: NICHD Research Goals

NICHD goals related to overweight and obesity cover a broad range of areas, including:

  • Preventing obesity through a better understanding of the genetic, molecular, and organ-system factors related to adiposity
  • Exploring social and psychological antecedents of behaviors related to nutrition and physical activity
  • Understanding the role of obesity in pregnancy and perinatal health
  • Promoting the health of particular populations that are at increased risk for obesity
  • Creating and testing interventions for various settings, such as in homes, schools, community centers, and camps

Do obesity & overweight affect pregnancy?

How much a woman weighs when she gets pregnant and how much weight she gains during pregnancy can affect her health and that of her baby. Entering pregnancy with a normal body mass index (BMI) and gaining weight within the recommended levels during pregnancy are important ways to protect a mother’s and a child’s health.1

The Institute of Medicine recommends the following ranges of weight gain during pregnancy for American women:

  • Pregnant women who are underweight (BMI of less than 18.5) should gain 28 to 40 pounds.
  • Pregnant women at a normal weight (BMI of 18.5 to 24.9) should gain 25 to 35 pounds.
  • Overweight pregnant women (BMI of 25 to 29.9) should gain 15 to 25 pounds.
  • Obese pregnant women (BMI greater than 30) should limit weight gain to 11 to 20 pounds.1

Recent NICHD research shows that gaining more weight during pregnancy than recommended increases the risk for complications.

Obesity-related Health Risks for Mothers

Women who are overweight or obese during pregnancy face several possible health risks, including high blood pressure, gestational diabetes, and an increased chance of needing a Cesarean delivery.1,2

Gestational diabetes is diabetes that begins during pregnancy. Women who have had gestational diabetes are at a higher lifetime risk for obesity and type 2 diabetes.

Obesity-related Health Risks for Fetuses

The developing fetuses of obese women also are at increased risk for health problems. For example, researchers found a connection between maternal obesity and neural tube defects, in which the brain or spinal column does not form properly in early development. Also, research suggests that obesity increases a woman’s chance of giving birth to a child with a heart defect by around 15%.2

Gestational diabetes also can cause problems for a newborn, including dangerously low blood sugar, large body size that may cause injuries at birth, and high bilirubin levels, which can cause other health problems.2

Children whose mothers had gestational diabetes also are at a higher lifetime risk for obesity and type 2 diabetes.2

Preventing Obesity and Overweight in Pregnancy

In light of the rise in rates of obesity in the United States, the American Congress of Obstetricians and Gynecologists encourages women to seek guidance about nutrition and weight reduction from a health care provider if they are overweight and considering getting pregnant.3

Good nutrition, staying active, and gaining the right amount of weight are important ways to promote a healthy pregnancy. The Weight-control Information Network provides tips on maintaining a healthy pregnancy.


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