The Effect of Dairy Intake on Body Weight and Composition and Metabolic Health in Adolescents and Seniors

The goal of this study is to examine how regular dairy intake affects body weight, body composition, and metabolic health in overweight and prediabetic adolescents (ages 14-17) and older adults (ages 60-75) over a six-month period. We are inviting eligible individuals to participate in this study at the Nutrition Intervention Center, Department of Nutritional Sciences, University of Toronto, St. George campus. Participants will be randomly divided into two groups. The intervention (high-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 less than one serving per day.

Participants will have biweekly sessions with a registered dietitian to monitor their dietary intake and ensure compliance with their assigned group. In addition, they will visit the Nutrition Intervention Center at week 0, week 12, and week 24 for anthropometric and body composition assessments (weight, height, waist circumference, blood pressure, lean mass, fat mass), glycemic and metabolic markers (fasting blood glucose, insulin, HbA1c, lipid profiles in seniors, and appetite hormones in adolescents). Each assessment visit will take approximately 4 hours.

Before each visit, you will be asked to fast for 12 hours overnight, maintain your usual diet and sleep patterns, and avoid exercise and alcohol consumption the day before. Your participation in this study will help us better understand the role of dairy in metabolic health and may contribute to improved dietary recommendations for adolescents and older adults. You will be compensated for your time and travel expenses.


Eligibility Criteria:
Inclusion Criteria:

* Age 14-17 years old (adolescents) or 60-75 years old (seniors)
* BMI z-score \>+1SD, \<+2SD (adolescents) or BMI 25-30 kg/m2 (seniors)
* Waist circumference \> 88 cm for women and \> 102 cm for men (seniors)
* FBG 5.6-6.9 mmol/L (prediabetes)
* Consumes ≤1 serving of dairy per day
* Willing to maintain habitual diet, physical activity, and body weight throughout the study.
* Willing to maintain current dietary supplement use throughout the study. On the first day, the participants agree not to take any dietary supplements until the completion of the study.
* Willing to abstain from alcohol consumption for 24 hours prior to all test visits.
* Willing to avoid vigorous physical activity for 24 hours prior to all test visits.
* Understands the study procedures and is willing to provide informed consent by 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 ≥ 7mmol/L
* Blood pressure systolic ≥ 130mmHg or diastolic ≥ 80mmHg (adolescents) or systolic ≥ 160 mmHg or diastolic pressure ≥ 100 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).
* History of childhood obesity
* 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 \>10 lbs. 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