The goal of this study is to examine how regular dairy intake affects body weight, body composition, and metabolic health in overweight and obese adolescents (ages 15-18) and older adults (ages 60-80) over six months. We are inviting eligible individuals to participate in this study at the Nutrition Intervention Center, Department of Nutritional Sciences, University of Toronto. Participants will be randomly divided into two groups. The intervention (dairy) group will consume three servings of dairy per day (milk, yogurt, and cheese) before breakfast, lunch, and dinner. The control (low-dairy) group will continue their usual diet, keeping dairy intake to less than one serving per day. Participants will have biweekly sessions with a registered dietitian to be guided to follow Canada's Food Guide, monitor their dietary intake and ensure compliance with their assigned group. In addition, they will visit the Nutrition Intervention Center at weeks 0, 12, and 24 for anthropometric and body composition assessments (weight, height, waist circumference, blood pressure, lean mass, and fat mass), resting metabolic rate, (gait speed, grip strength, and chair stand test just for older adults) and blood markers (fasting blood glucose, insulin, C-peptide, HbA1c, lipid profiles, and inflammatory markers). Each visit will take approximately 4 hours. Before each visit, the participants will be asked to fast overnight for 12 hours, maintain their usual diet and sleep patterns, and avoid exercise and alcohol the day before. The participants will be compensated for their time and travel expenses. Our secondary objective is to compare the effects of dairy and plant-based alternative products on blood sugar and appetite regulation in adolescents and older adults. Participants will be asked to come to the Nutrition Intervention Center on 3 separate occasions: once for an in-person screening lasting approximately 30 min and 2 times for study visits lasting approximately 2.5 hrs each. The entire study will take at least 2 weeks to complete. You will be asked to fast for 12 hrs (overnight) before each study visit. You will also be instructed to maintain the same dietary and sleep patterns and refrain from exercise and alcohol consumption on the days before the study visits. During these study visits, you will be asked to consume either a dairy or a plant-based alternative product as well as a pizza meal. You will periodically fill out questionnaires rating your feelings and perceptions, and provide blood samples through finger pricks and intravenously through your forearm to measure blood sugar, insulin, hormones, and amino acids. You will be compensated for your time and travel expenses.
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Body Fat Mass (kg)
Timeframe: BFM will be measured at weeks 0, 6, 12, 18, and the end of the study (week 24).
Lean body mass (Kg)
Timeframe: LBM will be measured at weeks 0, 6, 12, 18, and the end of the study (week 24).
Body Mass Index (BMI) z-score
Timeframe: BMI z-score will be measured at the beginning (week 0), midpoint (week 12), and end of the study (week 24).
Blood Glucose (mmol/L) (short-term study)
Timeframe: Over 145 minutes after treatment consumption at each session, for a total of 2 weekly sessions.