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.

Obesity and Overweight: Condition Information

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 Learn more about BMI and other measurements of overweight and obesity.


  1. National Library of Medicine. (2012). Obesity. Retrieved August 8, 2012, from MedlinePlus: http://www.nlm.nih.gov/medlineplus/obesity.html
  2. Centers for Disease Control and Prevention. (2012). Overweight and obesity: Defining overweight and obesity. Retrieved August 8, 2012, from http://www.cdc.gov/obesity/adult/defining.html
  3. Centers for Disease Control and Prevention. (2012). Overweight and obesity: Basics about childhood obesity. Retrieved August 8, 2012, from https://www.cdc.gov/obesity/childhood/index.html

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


  1. Centers for Disease Control and Prevention. (2012). Overweight and obesity: Adult obesity facts. Retrieved August 8, 2012, from http://www.cdc.gov/obesity/data/adult.html
  2. Centers for Disease Control and Prevention. (2011). FastStats: Overweight and obesity. Retrieved September 6, 2012, from http://www.cdc.gov/nchs/fastats/overwt.htm
  3. Centers for Disease Control and Prevention. (2012). Overweight and obesity: Data and statistics. Retrieved August 8, 2012, from http://www.cdc.gov/obesity/data/childhood.html
  4. Ogden, C. L., Carroll, M. D., Kit, B. K., & Flegal, K. M. (2012). Prevalence of obesity and trends in body mass index among U.S. children and adolescents, 1999-2010. Journal of the American Medical Association, 307, 483–490.

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.


  1. National Heart, Lung, and Blood Institute. (2012). How are overweight and obesity diagnosed? Retrieved August 8, 2012, from https://www.nhlbi.nih.gov/health/educational/lose_wt/risk.htm
  2. Barlow, S. E., & the Expert Committee. (2007). Expert committee recommendations regarding the prevention, assessment, and treatment of child and adolescent overweight and obesity: Summary report. Pediatrics, 120, S164–S192.

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.

Learn more about weight-loss techniques, medications, and surgery options.


  1. National Heart, Lung, and Blood Institute. (2012). How are overweight and obesity treated? Retrieved August 8, 2012, from https://www.nhlbi.nih.gov/health-topics/overweight-and-obesity

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. The National Heart, Lung, and Blood Institute offers more information on the causes of overweight and obesity.

Food and Activity

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


  1. National Heart, Lung, and Blood Institute. (2012). What causes overweight and obesity? Retrieved August 8, 2012, from https://www.nhlbi.nih.gov/health-topics/overweight-and-obesity
  2. Centers for Disease Control and Prevention. (2012). Overweight and obesity: Causes and consequences. Retrieved August 8, 2012, from https://www.cdc.gov/obesity/adult/causes.html

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.


  1. Centers for Disease Control and Prevention. (2012). Overweight and obesity: Causes and consequences. Retrieved August 8, 2012, from https://www.cdc.gov/obesity/adult/causes.html
  2. National Heart, Lung, and Blood Institute. (2012). What are the health risks of overweight and obesity? Retrieved August 8, 2012, from
  3. Hannon, T. S., Rao, G., & Arslanian, S. A. (2005). Childhood obesity and type 2 diabetes mellitus. Pediatrics, 116(2), 473–480.
  4. Olshansky, S. J., Passaro, D. J., Hershow, R. C., Layden, J., Carnes, B. A., Brody, J., et al. (2005). A potential decline in life expectancy in the United States in the 21st century. New England Journal of Medicine, 352, 1138–1145.
  5. Han, J. C., Lawlor, D. A., Kimm, S. Y. (2010). Childhood obesity. Lancet, 375, 1737–1748.

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


  1. Zain, M. M., & Norman, R. J. (2008). Impact of obesity on female fertility and fertility treatment. Women's Health, 4, 183–194.
  2. Legro, R. S. (2007). Pregnancy considerations in women with polycystic ovary syndrome. Clinical Obstetrics and Gynecology, 50, 295–304.
  3. Kiddy, D. S., Hamilton-Fairley, D., Bush, A., Short, F., Anyaoku, V., Reed, M. J., et al. (1992). Improvement in endocrine and ovarian function during dietary treatment of obese women with polycystic ovary syndrome. Clinical Endocrinology (Oxford), 36, 105–111.
  4. Du Plessis, S. S., Cabler, S., McAlister, D. A., Sabanegh, E., & Agarwal, A. (2010). The effect of obesity on sperm disorders and male infertility. Nature Reviews: Urology, 7, 153–161.

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.


  1. Institute of Medicine. (2011). Weight gain during pregnancy: Reexamining the guidelines. Retrieved August 8, 2012, from http://nationalacademies.org/hmd/Reports/2009/Weight-Gain-During-Pregnancy-Reexamining-the-Guidelines.aspx External Web Site Policy
  2. NICHD. (2012). NIH obesity research featured in HBO's The Weight of the Nation. Retrieved August 8, 2012, from http://www.nichd.nih.gov/news/resources/spotlight/Pages/051112-HBO-obesity.aspx
  3. Committee on Obstetric Practice. (2013). Obesity in pregnancy. Committee Opinion No. 549. American College of Obstetricians and Gynecologists. Obstetrics and Gynecology, 121, 213–217. Retrieved July 26, 2013, from https://www.acog.org/Patients/FAQs/Obesity-and-Pregnancy External Web Site Policy

Obesity and Overweight: Research Activities and Scientific Advances

Institute Activities and Advances

Obesity is a growing epidemic, but prevention efforts have not been successful. To a great extent, this is due to the challenging task of changing the way people eat, move, and live. Obesity is both a biological and a social problem and must be considered as a function of these larger contexts. Several NICHD organizational units conduct and support research on a broad range of areas related to excess weight gain, including the causes, effects, prevention, and treatment of obesity and related conditions.

Because environment and genetics play important roles in childhood obesity, the NICHD’s Section on Growth and Obesity works to increase understanding of the metabolic and behavioral factors involved in determining body weight regulation and body composition during childhood. The section also studies prevention and treatment interventions, and places a special emphasis on minority populations that are at an increased risk for obesity. For example, in one study on African American and Caucasian children and adolescents, researchers found that the gastrointestinal lipase inhibitor orlistat improved weight loss. Another study found that the medication metformin added to a behavioral program significantly improved weight loss and lowered insulin resistance in severely obese, insulin-resistant children.

The Pediatric Growth and Nutrition Branch (PGNB) also supports a wide range of childhood obesity research, including studies on psychosocial risks of obesity, the natural history and clinical pathophysiology of body composition, metabolic syndrome and diabetes, environmental and policy research in relation to obesity, and preventive and therapeutic interventions for childhood obesity. Funding from the PGNB has led to several noteworthy advances, including one of the first large genome-wide association studies (GWAS) on weight among U.S. children.

Obesity affects pregnancy in several significant ways. The Pregnancy and Perinatology Branch (PPB) conducts obesity-related research in many areas, including the short- and long-term effects of maternal obesity and weight gain during pregnancy on women’s and children’s health. In addition, the Branch supports studies of lifestyle interventions during pregnancy that aim to improve maternal and child health through healthy changes in diet and physical activity levels. The PPB also has explored the possible connection between obesity and postpartum depression.

Obesity also can affect fertility. The NICHD’s Fertility and Infertility (FI) Branch explores ways to prevent and treat infertility related to obesity. One recent scientific advance provided new insights into the relationship between obesity, androgen excess, and polycystic ovary syndrome in adolescents.

Other NICHD units, such as the Intellectual and Developmental Disabilities Branch (IDDB), explore issues related to obesity in specific populations, such as people with Down syndrome. The Child Development and Behavior Branch (CDBB) supports research on the behavioral factors that can lead to obesity and behavior-based prevention and treatment strategies for obesity. In addition, the Division of Intramural Population Health (DIPHR) works to fill critical data gaps to advance understanding of such factors as obesity that affect health. For example, DIPHR launched a 7-year assessment of U.S. adolescents and young adults to identify several health-related factors, including genetic, personal, family, school, and social factors that promote or sustain positive health behaviors.

To ensure that NICHD organizational units advance the most effective obesity-related science, the NICHD’s Obesity Research Strategic Core (ORSC) brings together research and translational activities from across the Institute. Some of the ORSC’s focus areas include:

  • Interventions, such as pediatric obesity prevention or treatment studies in the primary care setting, at home, in schools, and in camps
  • Behavioral and psychosocial observational research, including observational studies that examine social and psychological antecedents, consequences, or correlates of diet, physical activity, sedentary behavior, and/or obesity in children and adolescents
  • Basic sciences research, including human in vitro or animal studies that focus primarily on the genetic, molecular, cellular, or organ-system factors related to diet, physical activity, development of adiposity, or other aspects of energy metabolism
  • Clinical physiology, such as human studies that focus on clinical cohorts of either normal weight or overweight children and adolescents

The ORSC also is working to promote a systems-oriented approach to childhood obesity that includes the following key features:

  • Framing obesity as a complex systems problem
  • Emphasizing cross-level and cross-disciplinary hypotheses at the outset of research
  • Increasing efforts in structural or upstream interventions
  • Building capacity for multilevel research, in terms of training and collaborating with partner organizations
  • Investing in complex systems research methodologies
  • Maintaining a global perspective

Other Activities and Advances

The NICHD works to promote collaborative efforts to understand overweight and obesity and to promote healthy weight. For example:

  • In 2003, the NIH director began addressing the problem of the obesity epidemic by forming the Obesity Research Task Force to promote obesity research efforts across the NIH.
  • The NICHD is a member of the National Collaborative on Childhood Obesity Research (NCCOR) External Web Site Policy, which brings together leading research funders (including the NIH, the Centers for Disease Control and Prevention, the Robert Wood Johnson Foundation, and the U.S. Department of Agriculture) in a public-private collaboration to accelerate progress on reversing the epidemic of overweight and obesity among U.S. youth. The mission of NCCOR is to improve the efficiency, effectiveness, and application of childhood obesity research, and to halt—and reverse—childhood obesity through enhanced coordination and collaboration.
  • The NICHD created the Maternal-Fetal Medicine Units Network to focus on clinical questions in maternal-fetal medicine and obstetrics. The Network is composed of 14 university-based clinical centers and a data coordinating center that have explored a number of issues, including weight gain and nutrition.
  • The NICHD also works to encourage a healthy weight among young people. It joins several other NIH Institutes to support We Can! (Ways to Enhance Children’s Activity and Nutrition), which focuses its evidence-based education activities on parents and caregivers—the primary group for influencing youth. The NICHD’s Media-Smart Youth: Eat, Think, and Be Active!(PDF - 30.0 MB) curriculum is one of four youth curricula offered through We Can! Media-Smart Youth is an afterschool program that helps participants become aware of, and think critically about, the media’s influence on their nutrition and physical activity choices.
  • Research supported by NICHD and other NIH Institutes was featured in the HBO documentary The Weight of the Nation External Web Site Policy, which explored many topics related to the obesity epidemic in the United States, from causes to treatments. Leaders of the NIH Obesity Research Task Force provided guidance and materials to the filmmakers throughout the creation of the piece.
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