This is a cross-over intervention study designed to evaluate how four weeks of time restricted feeding (16 hours fasting and 8 hours feeding), compared to four weeks of a more traditional eating pattern (12 hours fasting and 12 hours feeding), affects resting energy expenditure, subjective and biochemical markers of satiety and hunger, body composition, cardiovascular health, substrate utilization and fitness in male competitive runners.

Weight loss and improvements in body composition (increasing muscle mass and decreasing fat mass) are common goals for both dieters and athletes. Although a traditional method of achieving this has been accomplished through caloric restriction, an alternate method that is becoming increasingly popular is fasting. Fasting is described as the absence of food and/or calorie-containing beverage consumption for a period of time. The majority of people fast for 8-10 hours daily, which occurs during the overnight period when people are asleep Intermittent fasting, specifically time-restricted feeding (TRF), has recently gained popularity because it is a more sustainable means of practicing fasting and it has been shown to enhance the loss of fat mass with or without caloric restriction. It has also been shown to enhance overall health due to reports of reduced fasting glucose, insulin resistance, triglycerides, and reduced total and low-density lipoprotein (LDL) cholesterol levels in obese women. Despite its growing popularity among athletes, there is scarce research on how adhering to this type of diet pattern affects athletic performance and other biochemical markers related to health. The participants in this study will be randomly assigned to either a traditional (12/12) or a time restrictive (16/8) eating pattern in a crossover design. Each pattern will last for four weeks with a minimum two week washout in between, and the participants will consume the same calorie and macronutrient amounts based on recommendations made by the American College of Sports Medicine (ACSM) for both eating patterns. The 12/12 pattern will require subjects to consume 5 meals (breakfast, lunch, dinner, and two snacks) per day within a 12 hour period suggested to be around 8am, 10am, 12pm, 3pm, and 7pm. The 16/8 pattern will require subjects to consume 3 meals in an 8-hour period suggested to be around 12pm, 3pm, and 7pm. Subjects may only consume water, unsweetened coffee, or unsweetened tea (no artificial sweeteners) during their periods of fasting. Subjects will perform all exercise in the morning before 8 am in the fasted state. Subjects will visit the Western Human Nutrition Research Center (WHNRC) for test days at baseline (study day 1), after four weeks of the first dietary intervention (study day 28), and at the start (study day 43) and end of the second dietary intervention (study day 70), for a total of 4 test days over the duration of 10 weeks.

Inclusion Criteria:

• Competitive runners who have been actively training for 3 or more years
• Weigh a minimum of 110 lbs
• Training 4-6 times per week and run a minimum of 20 miles per week
• Competed in a race within past 12 months at a distance of 5 - 26.1 km
• Willing to repeat same monthly training protocol during two 4-week interventions
• VO2max range of 40-70 ml/kg/min

Exclusion Criteria:

• Smoker
• Take medications that have cardiovascular or metabolic effects
• Taking dietary supplements
• Following a restrictive diet including restricting calories or carbohydrates
• Evidence of an eating disorder
• Major injuries in past 3 months
• Chronic disease that affects bone health, metabolism or the cardiorespiratory system
• Present with any contra-indication to exercise testing (cardiovascular abnormalities) as evaluated by study physician