Currently there are insufficient published studies on amino acid requirements in the elderly. Phenylalanine is one of the indispensable amino acids. Low levels of phenylalanine has been observed in in the blood of older adults. It is due the increased uptake by the liver and intestine. This decreased levels of phenylalanine could lead to decreased skeletal muscle protein synthesis. Requirement for elderly individuals have been set based on studies conducted in younger adults hence may be inaccurate. The goal of this study is to directly measure the phenylalanine requirement of male and female elderly subjects over 65 years using the direct amino acid oxidation (DAAO) technique.

1. To determine and compare the phenylalanine requirement of healthy elderly male and female adults over the age of 65 years using the DAAO technique (by measuring the oxidation of L-[1-13 C] phenylalanine to 13 CO 2 [F 13 CO 2] ) in response to graded intakes of phenylalanine.
2. To compare the requirement so derived to that of young healthy adult previously determined in our lab.

Inclusion Criteria:

1. Healthy elderly males and females > 65 years
2. Willingness to participate in the study and completed the screening procedures (height, weight, fasting blood sample and medical history questionnaire).
3. No recent history of weight loss
4. Absence of chronic disease or acute illness that could affect protein and AA metabolism (diabetes, cancer, liver or kidney disease, HIV, acute cold or flu, hypo or hyperthyroidism, rheumatoid arthritis treated with anti-inflammatory medications.
5. Non-smoking
6. Willingness to consume the diet provided

Exclusion Criteria:

1. Presence of disease known to affect protein and AA metabolism (diabetes, cancer, liver or kidney disease, HIV, acute cold or flu, hypo or hyperthyroidism, rheumatoid arthritis treated with anti-inflammatory medications).
2. On medications known to affect protein and amino acid metabolism (steroids).
3. Recent significant weight loss.
4. Individual on weight reducing diets.
5. Inability to tolerate the diet
6. Unwilling to have blood drawn from a venous access, or using a ventilated hood indirect calorimeter for the purposes of the study.
7. Significant coffee consumption of more than 2 cups/day
8. Significant alcohol consumption of more than one drink/day