Impaired functional status with aging is a major concern of the older population, limiting their daily activities, predisposing them to falls and increasing their mortality and morbidity risk. This condition also increases the admissions to nursing homes and caregiver burden. The purpose of this dissertation is to provide a better understanding of the relationship between nutritional risk factors, physical performance and disability among older Americans. This dissertation consists of three studies which used data from the National Health and Nutrition Examination Surveys (NHANES). The first study examined the association between overall diet quality, as measured by the United States Department of Agriculture’s (USDA) Healthy Eating Index-2005 (HEI-2005), and physical performance, as measured by gait speed and knee extensor power, among adults ≥ 50 y. Older adults with higher HEI-2005 scores had a faster gait speed compared to those with the lowest HEI-2005 score (1.03 and 1.04 m/s in quartiles three and four, respectively, vs. 1.00 m/s in quartile one; both p < 0.05). For knee extensor power, those with the highest HEI-2005 scores had greater knee extensor power compared to those with the lowest HEI-2005 scores (1.60 W/kg in quartile four vs. 1.50 W/kg in quartile one; p = 0.03). The second study examined the association between overall diet quality, measured by HEI-2005, and self-reported disability among adults aged 60 and older. Self-reported measures of disability included activities of daily living (ADLs), instrumental activities of daily living (IADLs), leisure and social activities (LSAs), lower extremity mobility (LEM), and general physical activities (GPAs). Older adults with higher HEI-2005 scores were less likely to experience LEM disability (p for trend < 0.001) and GPAs disability (p for trend < 0.001). Compared to older adults whose HEI-2005 scores were in the lowest quartile, the likelihood of both LEM and GPA disability were significantly lower in individuals whose scores were in quartiles two, three, and four. Compared to those who had HEI-2005 scores in the lowest quartile, the odds of IADLs disability were significantly lower in older adults whose scores were in quartile two and three. The third study examined the association between food insecurity and physical performance including gait speed and knee extensor power, among adults aged 50 and older. Responses to the US Household Food Security Survey Module (HFSSM) were used to assign participants to study food security categories. The association between food insecurity and gait speed varied by smoking status (p = 0.005). For nonsmokers, those who were marginally food secure (0.91 m/s, p = 0.016) and food insecure (0.94 m/s, p = 0.004) had significantly slower gait speeds than food secure participants (1.04 m/s). No statistically significant associations were observed for both current and former smokers. Similar findings were observed for knee extensor power. These results suggest the ensuing proper nutritional status and being nonsmokers are vital to maintain the functional status among older adults. More studies are needed to understand the complex factors that influence the nutritional and functional status of older adults to reduce healthcare costs and enhance quality of life.