This study aims to determine the requirements for the sulphur amino acids in adults over the age of 60 years:

1. the total sulphur amino acid requirement as methionine only. Methionine is an essential amino acid.
2. the minimum methionine requirement as methionine in the presence of excess dietary cysteine.

It is known that protein and amino acid metabolism may be altered with age and methionine and cysteine, in particular, may be important in the diet of older adults.

Up to 7 different levels of methionine intake as methionine only (no dietary cysteine) and up to 7 different levels of methionine intake in the presence of excess dietary cysteine will be tested in each subject in random order. Each level of intake will involve a 3-day maintenance diet, with measures being collected on the third.

The sulphur amino acids (SAA) are methionine and cysteine. Methionine is essential and must be obtained from the diet whereas cysteine is considered non-essential and is synthesized from methionine. They have many other important roles that are secondary to their role in protein building. For example, they are needed to make the antioxidant glutathione (GSH), which protects the body's cells from damage. However, protein and GSH metabolism have been shown to change with age and hence, the SAA requirements may also be changed. Despite this, current SAA requirements are based on studies conducted in young adults. Thus, there is a need to determine the SAA requirements directly in older adults.

The purpose of the study is to apply the minimally invasive indicator amino acid oxidation (IAAO) protocol to determine the SAA requirements in healthy adults over 60 years of age. The requirements will be determined as (1) the total sulphur amino acid requirement as methionine only and (2) the minimum methionine requirement by providing methionine in the presence of excess dietary cysteine.

A pre-study assessment will be conducted to assess health status using: medical history and a physical exam; and a blood draw (10mL) for fasting blood glucose and hemoglobin A1c to test for diabetes, vitamin B and folate concentrations, and urea and creatinine to test renal function.

In order to determine the requirements, each participant will be studied at up to 7 varying intakes levels of methionine in random order. Each intake level will be studied over 3-days: two adaptation days (day 1 and 2) and one study day (day 3). On the adaptation days, participants will be provided with a milkshake diet providing all the protein and nutrients they need. Participants will consume 4 meals/day in their usual home environment. On the 3rd day participants will come to the Clinical Research Centre at SickKids where they will be given hourly meals and breath and urine samples at specified times.A blood sample will also be collected for analysis of serum insulin, glucose and secondary parameters (i.e. concentrations of amino acids, glutathione, homocysteine and CRP). The mean (estimate average requirement) total sulphur amino acid requirement and minimum methionine requirement will be calculated with breakpoint analysis using a two-phase linear regression crossover analysis.

Inclusion Criteria:

• Consent provided
• Aged 60 to 90 years old
• In good general health as evidenced by medical history, physical health and blood draw
• Fasting blood glucose, hemoglobin A1c (HbA1c), urea, creatinine, vitamin-B6, vitamin- B12 and folate levels within normal ranges for age.
• Willingness to participate in the study.
• BMI <30 kg/m2.

Exclusion Criteria:

• Presence of chronic disease and/or acute illness known to affect protein/amino acid metabolism (e.g. HIV, diabetes, cancer, liver or kidney disease, acute cold or flu)
• Taking medications known to affect protein/AA metabolism (e.g. steroids)
• Inability to tolerate the diet (i.e. allergy)
• Significant weight loss during the past month or consumption of weight reducing diets.
• Significant caffeine consumption (>2 cups per day)
• Significant consumption of alcohol (>1 drink per day i.e. 1 beer or ½ glass of wine).
• Unwilling to have blood drawn from a venous access, or using a ventilated hood indirect calorimeter for the purposes of the study.