What the CDC Calls "Underweight"
The CDC classifies adults age 20 and older as underweight when Body Mass Index falls below 18.5 kg/m². BMI is calculated as weight in kilograms divided by height in meters squared; in US units the formula is (pounds ÷ inches²) × 703. The 18.5 cutoff matches the World Health Organization threshold and is the same for men and women of all ages in the adult chart.
About 1.5% of US adults land in this band, according to NHANES data. The number is small, but the health implications can be serious, particularly when low weight results from illness, restrictive eating, or unintentional weight loss rather than genetics.
CDC BMI Classification Table
| BMI Range | CDC Category | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | Possible nutrient deficiency and immune compromise |
| 18.5 - 24.9 | Normal weight | Lowest risk of chronic disease |
| 25.0 - 29.9 | Overweight | Increased risk of obesity-related conditions |
| 30.0 - 34.9 | Obese (Class I) | High risk of chronic disease |
| 35.0 - 39.9 | Obese (Class II) | Very high risk of complications |
| 40.0+ | Obese (Class III) | Severe, clinically significant risk |
Health Risks of Being Underweight
The CDC and clinical literature link BMI below 18.5 to a different disease profile than overweight or obesity. Risk rises sharply when low weight is accompanied by recent unintentional loss, frailty in older adults, or restrictive eating patterns.
- Protein-energy malnutrition and micronutrient deficiencies
- Impaired immune response and slower wound healing
- Anemia from low iron, B12, or folate
- Reduced bone mineral density and higher fracture risk
- Hormone disruption: amenorrhea, low testosterone, fertility issues
- Hypothermia and cold intolerance from low body fat
- Cardiac changes including bradycardia and arrhythmias in severe cases
- Higher surgical complication rates and longer hospital stays
Older adults are especially vulnerable. The CDC reports that underweight adults over 65 have higher all-cause mortality than those in the normal range, partly because low weight often signals sarcopenia, frailty, or undiagnosed chronic illness.
Common Causes of Low BMI
Treating underweight starts with finding the cause. The CDC and NIH list several categories clinicians work through:
- Inadequate calorie intake from restrictive dieting or food insecurity
- Hyperthyroidism and other endocrine disorders
- Gastrointestinal disease: celiac, IBD, malabsorption
- Chronic infection or inflammatory disease
- Cancer and the cachexia it can produce
- Disordered eating, including anorexia nervosa
- Psychiatric conditions such as depression
- Medication side effects that suppress appetite
- High metabolic demand from intense endurance training
How the CDC Defines Underweight in Children and Teens
CDC growth charts for ages 2-19 use BMI-for-age percentiles rather than fixed cutoffs. A child is classified as underweight when BMI falls below the 5th percentile for their age and sex. Pediatricians watch the trajectory, because a single low value can reflect a normal growth pattern or signal a problem needing workup.
CDC and USDA Guidance for Healthy Weight Gain
The CDC's lifestyle guidance, aligned with the USDA Dietary Guidelines for Americans, recommends a measured approach for adults trying to leave the underweight band:
- Add 250-500 calories per day above maintenance to gain 0.25-0.5 lb weekly
- Prioritize nutrient-dense foods: nuts, nut butters, eggs, dairy, lean protein, whole grains
- Eat 5-6 smaller meals if large meals feel uncomfortable
- Include resistance training 2-3 days per week to build muscle, not just fat
- Drink calories between meals: smoothies, milk, fortified drinks
- Track weight weekly and adjust intake as the trend changes
- Work with a registered dietitian for a tailored plan
A clinician should evaluate anyone with a BMI below 16, unintentional loss of more than 5% body weight in 6-12 months, or signs of restrictive eating. The workup typically includes thyroid panel, complete blood count, comprehensive metabolic panel, and a dietary review.
What to Do Next
Check your BMI and track it over weeks. If you sit below 18.5, pair the number with how you feel, your energy, and a conversation with a healthcare provider. The CDC categories above give context for where your result stands.