The aims of this study are as follows: AIM 1: To examine the weight loss and weight maintenance efficacy of an alternate day fasting-high protein (ADF-HP) diet; AIM 2: To examine the effects of an ADF-HP diet on metabolic disease risk factors (plasma lipids, fasting glucose, fasting insulin, insulin resistance).

Background: Accumulating evidence suggests that alternate day fasting (ADF) is an effective diet strategy to help individuals with obesity lose weight and lower metabolic disease risk. ADF regimens include a "feast day" where food is consumed ad-libitum over 24 h, alternated with a "fast day" where intake is limited to ~600 kcal over 24 h. What has yet to be elucidated is whether consuming a high protein diet (as meal replacements) during ADF is effective for weight loss, weight maintenance, and metabolic disease risk reduction in individuals with obesity.

Objective: The aims of this study are as follows: AIM 1: To examine the weight loss and weight maintenance efficacy of an alternate day fasting-high protein (ADF-HP) diet; AIM 2: To examine the effects of an ADF-HP diet on metabolic disease risk factors (plasma lipids, fasting glucose, fasting insulin, insulin resistance).

Methods: A 24-week, single-center, longitudinal pilot study will be conducted to test the study objectives. The trial will be divided into 2 consecutive intervention periods: (1) 12-week weight loss period, and (2) 12-week weight maintenance period. Subjects with obesity will participate in an ADF-HP: alternate day fasting-high protein diet: 600 kcal "fast day" alternated with ad libitum "feast day", 35% kcal as protein. ADF-HP subjects will consume the Optifast HP Shake Mix (Nestle) on each day of the trial. Body weight, insulin, glucose, and insulin resistance, will be measured at baseline, week 12, and week 24.

Significance: These findings may show that alternate day fasting combined with high protein meal replacements may be implemented as an effective diet therapy to help individuals with obesity lose weight, maintain weight loss, and sustain reductions in metabolic disease risk.

Inclusion Criteria:

• Age between 18 to 65 years old
• BMI between 30.0 and 49.9 kg/m2
• Previously sedentary or lightly active

Exclusion Criteria:

• Diabetic
• History of binge eating disorder
• Taking weight loss-inducing medications
• Not weight stable for 3 months prior to the study (weight gain or loss > 4 kg)
• Perimenopausal or have an irregular menstrual cycle (menses that does not appear every 27-32 days)
• Pregnant or trying to become pregnant
• Night shift worker
• Smokers