The main objective of this project is to study the diurnal variation of the effect of exercise on glycemic metabolism and fat oxidation in humans.

Strong scientific evidence supports the beneficial effects of exercise on cardiovascular health, the regulation of glucose metabolism, and fat oxidation. Physical performance capacity is known to fluctuate throughout the day, however, it is unknown whether there is an optimal time of day to maximize the effects of exercise on health, and specifically on blood glucose metabolism and fat oxidation. Finding the ideal time to perform physical exercise is of clinical and public health interest. Likewise, optimizing the timing of physical exercise to coincide with the greater physiological response of each individual would mean increasing the potential of exercise as a therapeutic tool.

Specific aims of this project are a) to describe possible differences dependent on sex in the diurnal variation of the effect of exercise on glycemic metabolism and fat oxidation, and b) to characterize the molecular mechanisms implicated.

18 men and 17 women with normal weight will be randomized into two conditions (morning and evening) with at least 3 days of separation in between. Each evaluation will conform the following tests:

1. Basal metabolic rate assessment through indirect calorimetry during 30 minutes.
2. Fuel oxidation and energy expenditure assesment during a 60 minutes bout of aerobic steady-state exercise (cycling), through indirect calorimetry.
3. Fuel oxidation and energy expenditure assesment after exercise during 30 minutes, through indirect calorimetry.
4. Blood samples assesment before, inmediately after, and 90 minutes after exercise.
5. Quadriceps biopsies before, and inmediately after exercise. *This will be done only in a sub-cohort.
6. Visual analog scales to assess appetite before, inmediately after, and 90 minutes after exercise.
7. Continous glucose monitoring from the previous 24 hrs to first session day, until 48 hrs after the last exercise session.

Previous to this, participants' body composition and fitness level will be assesed via densitometry and a maximal exercise test, respectively.

Inclusion Criteria:

• Body mass index: 18.5to 27.0 kg/m2.
• Physical activty < 5 days/week.
• To be able to understand instrucctions, objectives and study protocol.

Exclusion Criteria:

• History of a major adverse cardiovascular event, kidney failure, cirrhosis, eating disorder, weight control surgery, or HIV / AIDS.
• Rheumatoid arthritis, Parkinson's disease, active cancer treatment in the past year, type 1 diabetes mellitus, or another medical condition for which fasting is contraindicated.
• Any condition that, in the judgment of the investigator, impairs the ability to participate in the study or poses a personal risk to the participant.
• Use of medications that may affect the results of the study.
• Unstable body weight for 3 months before the start of the study (> 4 kg weight loss or gain)
• Pregnancy and breastfeeding.
• Active tobacco abuse or illicit drug use or a history of alcohol abuse treatment.
• On a special diet or prescribed for other reasons (eg celiac disease).

To blind this study is not possible due to the studied conditions: morning and evening. Only data assesors can be blinded.