Regarding adults, they report a significant decrease in underweight from an estimated 4% in the early 1960s to 1.7% in 2007-2010. The decrease was significant among all age groups. While the prevalence of underweight is greater among women than men, a significant decline was observed in both genders. Women age 20-39 saw a decrease in prevalence of underweight went from 3% in 1988-1994 to 1.9% in 2007-2008.
Regarding children and adolescents aged 2-19, an estimated 3.5% are underweight, down from 5.1% in 1971-1974. Among children aged 2.5, prevalence dropped from 5.8% to 3.4% from 1971-1974 to 2007-2010; among children 6-11, the rate dropped from 5.3% to 3.6%. No significant change was observed among adolescents aged 12-19.
An interesting observation on underweight has come from a study by Koh and colleagues. They looked at demographic, BMI and related data from the Boston Health Care for the Homeless Program and compared the data on the homeless to the NHANES data. They found that only 1.6% of the homeless adults were underweight. They found that the homeless population weight distribution was basically the same as the general population. They state, “Although underweight has been traditionally associated with homelessness, this study suggests that obesity may be the new malnutrition of the homeless in the United States. PubMed: Koh: The Hunger_Obesity Paradox: Obesity in the Homeless