Underweight

The body mass index, a ratio of a person's weight to their height, has traditionally been used to assess the health of a person as it pertains to weight: under the cut-off point at a BMI of 18.5, a person is considered underweight.{{cite web|title=Assessing Your Weight and Health Risk|url=http://www.nhlbi.nih.gov/health/educational/lose_wt/risk.htm |publisher=National Heart, Lung and Blood Institute|access-date=23 September 2012}} The calculation is given by the mass per height squared, with results usually given in kg/m^2 (for an approximate conversion from lb/in^2, divide by 703). Another measure of underweight is through comparison to the average weight of a cohort of people of a similar age and height: people who are at least 15% to 20% below the average weight for the group are considered underweight.{{cite book|last=Mahan|first=L. Kathleen|title=Krause's Food, Nutrition & Diet Therapy, 10th Ed.|year=2000|publisher=W.B. Saunders Co.|location=Philadelphia}}

Body fat percentage has been suggested as another way to assess whether a person is underweight. Unlike the body mass index, which is a proxy measurement, the body fat percentage takes into account the difference in composition between adipose tissue (fat cells) and muscle tissue and their different roles in the body.{{cite journal|title=Body mass index and measures of body fat for defining obesity and underweight: a cross-sectional, population-based study|first1=Julie A.|last1=Pasco|first2=Kara L.|last2=Holloway|first3=Amelia G.|last3=Dobbins|first4=Mark A.|last4=Kotowicz|first5=Lana J.|last5=Williams|first6=Sharon L.|last6=Brennan|date=23 June 2014|journal=BMC Obesity|volume=1|page=9|doi=10.1186/2052-9538-1-9|pmid=26217501|pmc=4511447 |doi-access=free }} The American Council on Exercise defines the amount of essential fat, below which a person is underweight, as 10–13% for women and 2–5% for men.{{cite web|title=ACE Fit - Body Fat Percentage Calculator|url=https://www.acefitness.org/education-and-resources/lifestyle/tools-calculators/percent-body-fat-calculator|access-date=28 February 2020}} The greater amount of essential body fat in women supports reproductive function.{{citation needed|date=June 2022}}

Using the body mass index as a measure of weight-related health, with data from 2014, age-standardised global prevalence of underweight in women and men were 9.7% and 8.8%, respectively. These values were lower than what was reported for 1975 as 14.6% and 13.8%, respectively, indicating a worldwide reduction in the extent of undernutrition.{{cite journal |title=Trends in adult body-mass index in 200 countries from 1975 to 2014: a pooled analysis of 1698 population-based measurement studies with 19·2 million participants |journal=Lancet |volume=387 |issue=10026 |pages=1377–96 |date=April 2016 |pmid=27115820 |doi=10.1016/S0140-6736(16)30054-X |doi-access=free |author1=NCD Risk Factor Collaboration (NCD-RisC) |pmc=7615134 }}

A person may be underweight due to genetics,{{cite web|url=http://www.indianexpress.com/news/body-shape-is-down-to-genes/695697/|title=Body Shape 'Is Down to Genes'|publisher=Indian Express|access-date=October 23, 2010}}{{cite web|url=https://www.sciencedaily.com/releases/2007/09/070904122434.htm|title='Skinny Gene' Exists|publisher=Science Daily|date=September 5, 2007|access-date=October 23, 2010}} poor absorption of nutrients, increased metabolic rate or energy expenditure, lack of food (frequently due to poverty), low appetite, drugs that affect appetite, illness (physical or mental) or the eating disorder anorexia nervosa.{{cite web|title=What are Eating Disorders?|url=http://www.nimh.nih.gov/health/topics/eating-disorders/index.shtml|website=NIMH|access-date=24 May 2015|url-status=dead|archive-url=https://web.archive.org/web/20150523184510/http://www.nimh.nih.gov/health/topics/eating-disorders/index.shtml|archive-date=23 May 2015}}{{cite journal | vauthors = Attia E | title = Anorexia nervosa: current status and future directions | journal = Annual Review of Medicine | volume = 61 | issue = 1 | pages = 425–435 | year = 2010 | pmid = 19719398 | doi = 10.1146/annurev.med.050208.200745 }}

Being underweight is associated with certain medical conditions, including type 1 diabetes,{{Cite news|url=http://www.diabetes.co.uk/symptoms/unexplained-weight-loss.html|title=Unexplained Weight Loss - Reasons, Symptoms & Causes|access-date=2017-11-28}} hyperthyroidism,{{cite web|last=Milas|first=Kresimira |title=Hyperthyroidism Symptoms - Signs and symptoms caused by excessive amounts of thyroid hormones|url=http://www.endocrineweb.com/conditions/hyperthyroidism/hyperthyroidism-symptoms|publisher=Endocrine Web|access-date=24 September 2012}} cancer,{{cite web|title=Signs and Symptoms of Cancer|url=http://www.cancer.org/Cancer/CancerBasics/signs-and-symptoms-of-cancer|publisher=American Cancer Society|access-date=24 September 2012}}{{deadlink|date=April 2025}} and tuberculosis.{{cite journal|last=Hira|first=S. K.|author2=H. L. Dupont |author3=D. N. Lanjewar |author4=Y. N. Dholakia |title=Severe weight loss: the predominant clinical presentation of tuberculosis in patients with HIV infection in India.|journal=National Medical Journal of India|year=1998|volume=11|issue=6|pages=256–58|pmid=10083790}} People with gastrointestinal or liver problems may be unable to absorb nutrients adequately. People with certain eating disorders can also be underweight due to one or more nutrient deficiencies or excessive exercise, which exacerbates nutrient deficiencies.{{cite journal|display-authors=6 |vauthors=Fisher MM, Rosen DS, Ornstein RM, Mammel KA, Katzman DK, Rome ES, Callahan ST, Malizio J, Kearney S, Walsh BT |date=July 2014 |title=Characteristics of avoidant/restrictive food intake disorder in children and adolescents: a "new disorder" in DSM-5 |journal=The Journal of Adolescent Health |volume=55 |issue=1 |pages=49–52 |doi=10.1016/j.jadohealth.2013.11.013 |pmid=24506978 |doi-access=free}}{{cite journal |vauthors=Latner JD, Wilson GT |title=Cognitive-behavioral therapy and nutritional counseling in the treatment of bulimia nervosa and binge eating |journal=Eating Behaviors |volume=1 |issue=1 |pages=3–21 |date=September 2000 |pmid=15001063 |doi=10.1016/S1471-0153(00)00008-8 |url=http://www2.hawaii.edu/~jlatner/downloads/pubs/eating_behaviors2000.pdf |citeseerx=10.1.1.578.4563}}

A common belief is that healthy underweight individuals can ‘eat what they want’ and then burn it off either by high levels of activity or elevated metabolism. It has been shown, however, that individuals with BMI < 18.5 eat about 12% less calories than individuals with normal BMI (21.5 to 25) and they are 23% less physically active (by accelerometry).{{cite journal|last=Hu|first=S.|author2= Zhang X.|author3= Stamatiou M.|author4= Hambly C.|author5= Huang Y.|author6= Ma J.|author7= Li Y.|author8= Speakman J.R.|title=Higher than predicted resting energy expenditure and lower physical activity in healthy underweight Chinese adults|journal=Cell Metabolism|year=2022|volume=34 |issue=10 |pages=1413–1415 |doi=10.1016/j.cmet.2022.05.012 |pmid=35839758|s2cid=250576685 |doi-access=free}} Underweight people tend to have low appetites and typically eat little, sporadically or infrequently.

Being underweight can be a symptom of an underlying condition, in which case it is secondary. Unexplained weight loss may require a professional medical diagnosis by a physician.{{Cite web |title=Weight loss - unintentional: MedlinePlus Medical Encyclopedia |url=https://medlineplus.gov/ency/article/003107.htm |access-date=2022-11-18 |website=medlineplus.gov |language=en}}

Being underweight can also cause other conditions, in which case it is primary. Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection. According to Robert E. Black of the Johns Hopkins School of Public Health (JHSPH), "Underweight status ... and micronutrient deficiencies also cause decreases in immune and non-immune host defenses, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes."{{citation |last1=Black |first1=Robert E. |last2=Morris |first2=Saul S. |last3= Bryce |first3= Jennifer |title=Where and Why are 10 Million Children Dying Every Year? |journal=The Lancet |volume=361 |issue=9376 |pages=2226–34 |date=28 June 2003 |pmid=12842379 |doi=10.1016/S0140-6736(03)13779-8|s2cid=14509705 }}

People who are malnourished raise special concerns, as not only gross caloric intake may be inadequate, but also intake and absorption of other vital nutrients, especially essential amino acids and micronutrients such as vitamins and minerals.{{citation needed|date=June 2022}}

In women, being severely underweight, often as a result of an eating disorder or due to excessive strenuous exercise, can result in amenorrhea (absence of menstruation),{{Cite web|url=http://contemporaryobgyn.modernmedicine.com/%5Bnode-source-domain-raw%5D/news/modernmedicine/modern-medicine-now/amenorrhea-caused-extremes-body-mas?page=full|title = MJH Life Sciences™ | Informing Healthcare Professionals •| date=10 August 2022 }} infertility or complications during pregnancy if gestational weight gain is too low.{{citation needed|date=June 2022}}

Malnourishment can also cause anemia and hair loss.

Being underweight is an established{{cite journal |pmid=18221845 |year=2008 |author1=Gjesdal |first2=JI |first3=GE |first4=JG |first5=GS |title=Impact of lean mass and fat mass on bone mineral density: the Hordaland Health Study |volume=59 |issue=2 |pages=191–200 |doi=10.1016/j.maturitas.2007.11.002 |journal=Maturitas |last2=Halse |last3=Eide |last4=Brun |last5=Tell}} risk factor for osteoporosis, even for young people. This is seen in individuals suffering from relative energy deficiency in sport, formerly known as female athlete triad: when disordered eating or excessive exercise cause amenorrhea, hormone changes during ovulation leads to loss of bone mineral density.{{cite journal |pmid=8013041 |year=1994 |author1=Nattiv |first2=R |first3=B |first4=KK |title=The female athlete triad. The inter-relatedness of disordered eating, amenorrhea, and osteoporosis |volume=13 |issue=2 |pages=405–18 |journal=Clinics in Sports Medicine |last2=Agostini |last3=Drinkwater |last4=Yeager|doi=10.1016/S0278-5919(20)30338-0 }}{{cite journal |pmid=8173855 |year=1994 |author1=Wilson |first2=RL |title=Osteoporosis and fracture complications in an amenorrhoeic athlete |volume=33 |issue=5 |pages=480–1 |journal=British Journal of Rheumatology |doi=10.1093/rheumatology/33.5.480 |last2=Wolman}} After this low bone mineral density causes the first spontaneous fractures, the damage is often irreversible.

Although being underweight has been reported to increase mortality at rates comparable to that seen in morbidly obese people,{{cite journal |pmid=6585126 |year=1984 |author1=Waaler HT. |title=Height, weight and mortality. The Norwegian experience |issue=679 |pages=1–56 |doi=10.1111/j.0954-6820.1984.tb12901.x |journal=Acta Med Scand Suppl |volume=215 }} the effect is much less drastic when restricted to non-smokers with no history of disease,[http://www.medicine.ox.ac.uk/bandolier/booth/hliving/BMIwords.html Body Weight and Mortality:What is the optimum weight for a longer life?]. suggesting that smoking and disease-related weight loss are the leading causes of the observed effect.

Underweight individuals may be advised to gain weight by increasing calorie intake. This can be done by eating a sufficient volume of sufficiently calorie-dense foods.{{cite web|url=http://www.mayoclinic.com/health/underweight/AN00597|title=Underweight? See how to add pounds healthfully|author=Zeratsky, Katherine|date=23 August 2011|work=Nutrition and healthy eating|publisher=Mayo Clinic|access-date=19 March 2012}}{{cite web|url=http://nutrition.about.com/od/tryingtogainweight/a/gainehalthyweig.htm|title=Gain Weight and Be Healthy|publisher=About.com|access-date=October 23, 2010}}{{cite web|url=http://www.healthcentral.com/diet-exercise/c/56/57317/achieving-gain|title=Achieving Healthy Weight Gain|publisher=Health Central|access-date=October 23, 2010}} Body weight may also be increased through the consumption of liquid nutritional supplements.{{cite web|url=http://www.youngwomenshealth.org/gain_healthy_parent.html|title=Healthy Weight Gain|publisher=Children's Hospital Boston, Center for Young Women's Health|access-date=October 23, 2010}}

Another way for underweight people to gain weight is by exercising, since muscle hypertrophy increases body mass. Weight lifting exercises are effective in helping to improve muscle tone as well as helping with weight gain.{{cite web|url=http://www.menshealth.com/cda/advicedetail.do?site=mensHealth&channel=best.life&conitem=3d084d5dfcf53010VgnVCM100000cfe793cd____&expertId=1c74f5b65fa53010VgnVCM100000cfe793cd____|title=Men's Health|publisher=Men's Health|access-date=October 23, 2010}} Weight lifting has also been shown to improve bone mineral density,{{cite journal|last=Gleeson|first=Peggy|author2=Elizabeth J. Protas |author3=Adrian D. Leblanc |author4=Victor S. Schneider |author5=Harlan J. Evans |title=Effects of weight lifting on bone mineral density in premenopausal women|journal=Journal of Bone and Mineral Research|date=February 1990|volume=5|issue=2|pages=153–158|doi=10.1002/jbmr.5650050208|pmid=2316403|s2cid=72034011}} which underweight people are more likely to lack.{{cite journal|last=Coin|first=A. |author2=G. Sergi |author3=P. Benincà |author4=L. Lupoli |author5=G. Cinti |author6=L. Ferrara |author7=G. Benedetti |author8=G. Tomasi |author9=C. Pisent |author10=G. Enzi|title=Bone Mineral Density and Body Composition in Underweight and Normal Elderly Subjects|journal=Osteoporosis International|year=2000|volume=11|issue=12|doi=10.1007/s001980070026|pages=1043–1050|pmid=11256896 |s2cid=1298271 }}

Exercise is catabolic, which results in a brief reduction in mass. However, during recovery, anabolic overcompensation causes the muscles to grow, which results in an overall increase in mass. This can happen through an increase in muscle proteins, or through enhanced storage of glycogen in muscles.{{citation needed|date=May 2022}} Exercise can also help stimulate the appetite of a person who is not inclined to eat.

{{main|Orexigenic}}

Certain drugs may increase appetite either as their primary effect or as a side effect. Antidepressants, such as mirtazapine or amitriptyline, and antipsychotics, particularly chlorpromazine and haloperidol, as well as tetrahydrocannabinol (found in cannabis), all present an increase in appetite as a side effect. In states where it is approved, medicinal cannabis may be prescribed for severe appetite loss, such as that caused by cancer, AIDS, or severe levels of persistent anxiety. Other drugs or supplements which may increase appetite include antihistamines (such as diphenhydramine, promethazine or cyproheptadine).{{cite journal|doi=10.1002/ppul.20265 | pmid=16015665 | volume=40 | issue=3 | title=Long-term trial of cyproheptadine as an appetite stimulant in cystic fibrosis | year=2005 | journal=Pediatric Pulmonology | pages=251–256 | last1 = Homnick | first1 = Douglas N.| s2cid=22837100 }}

• Essential nutrient
• List of phytochemicals in food
• Body image
• Emaciation
• Malnutrition
• Stunted growth

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{{Medical resources

| ICD10 = {{ICD10|R|62|8|r|50}}

| ICD9 = {{ICD9|783.22}}

| MeshID = D013851

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{{Nutritional pathology}}

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Category:Human body weight

Category:Symptoms and signs: Endocrinology, nutrition, and metabolism