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.

Inclusion Criteria (long-term Study):

• Age 15-18 years old (adolescents) or 60-80 years old (seniors)
• BMI z-score >+1SD, <+2SD (adolescents) or BMI 25-34.9 kg/m2 (seniors)
• Waist circumference > 80 cm for women and > 94 cm for men (seniors)
• FBG 5.6-6.9 mmol/L (prediabetes)
• Willing to follow Canada's Food Guide
• Willing to maintain current dietary supplement use throughout the study.
• Willing to abstain from alcohol consumption for 24 hours before all test visits.
• Willing to avoid vigorous physical activity for 24 hours before all test visits.
• Understands the study procedures and is willing to provide informed consent by the parent/guardian and assent by participant to participate in the study and authorization to release relevant protected health information to the study investigator.

Exclusion Criteria:

• Fasting BG ≥ 7 mmol/L
• Blood pressure systolic ≥ 130mmHg or diastolic ≥ 80mmHg (adolescents) or systolic ≥ 140 mmHg or diastolic pressure ≥ 90 mmHg
• Smoking (tobacco and/or cannabis product use in the last 6 months)
• Thyroid problems
• History of CVDs, diabetes, liver or kidney disease, inflammatory bowel disease, celiac disease, short bowel syndrome, any malabsorption syndrome, pancreatitis, gallbladder, or biliary disease.
• Presence of gastrointestinal disorder or surgeries within the past year.
• Use of the prescription or non-prescription drugs, herbal or nutritional supplements known to affect the outcome of the study as per the investigator's judgment (including prebiotics and probiotics).
• Known to be pregnant, lactating, or not postmenopausal for at least a year and/or is taking hormonal treatments
• Unwillingness or inability to comply with the experimental procedures
• Known intolerance, sensitivity, or allergy to dairy, gluten, or any other study treatments.
• Consumption of protein powders/supplements
• Extreme dietary habits (e.g., Atkins diet, very high-protein diets, etc.)
• Weight gain or loss of > 5% in the previous three months.
• Excessive alcohol intake (more than 2 drinks per day or 9 per week)
• Restrained eating, identified by a score of ≥11 on the Eating Habits Questionnaire

Inclusion Criteria (short-term study):

• Age 14-18 years old (adolescents) or 60-75 years old (older adults)
• BMI z-score >+1SD, <+2SD (adolescents) or BMI 25-30 kg/m2 (older adults)
• Willing to maintain habitual diet, physical activity pattern, and body weight throughout the study.
• Willing to maintain current dietary supplement use throughout the study. On study visit days, subject agrees not to take any supplements until dismissal from the Nutrition Intervention Center.
• Willing to abstain from alcohol consumption for 24hrs prior to all study visits.
• Willing to avoid vigorous physical activity for 24hrs prior to all study visits.
• Willing to refrain from cannabis use throughout the entire duration of the study.
• Willing to refrain from protein supplement use (e.g. protein powder) throughout the entire duration of the study.
• Understands the study procedures, provides informed consent to participate in the study, and authorizes release of relevant protected health information to the study investigator.

Exclusion Criteria (short-term study):

• Smoking
• Thyroid problems
• Lactose intolerance and/or allergies, intolerances, or sensitivities to study treatments
• Previous history of cardiovascular disease, diabetes, liver or kidney disease, inflammatory bowel disease, celiac disease, short bowel syndrome, malabsorptive syndrome, pancreatitis, gallbladder or biliary disease
• Presence of gastrointestinal disorder or surgeries within the past year.
• Inability to comply with the experimental procedures and follow our safety guidelines
• Regular breakfast skipping (>3 days a week)
• On a special diet (e.g. ketogenic, Atkins, high protein) or restrained eaters as identified by a score of >20 on the Eating Attitudes Questionnaire
• Difficulties with eating or swallowing
• Fasting blood glucose >5.6mmol/L measured at screening
• Uncontrolled hypertension (systolic blood pressure >120mmHg, diastolic blood pressure >80mmHg) as defined by the average blood pressure measured at screening
• Weight gain or loss of >10lbs in previous three months
• Consuming prescription or non-prescription drug, herbal or nutritional supplements known to affect outcome of the study as per investigator's judgement

IPD will not be shared, as all data will be de-identified and pooled during the analysis. Only researchers directly involved in the study will have access to the data.