This study aims at assessing whether breath acetone concentration is a good bio-marker of instantaneous energy balance in healthy subjects.

Each participant will be studied on three occasions, separated by 1-4 weeks. Each evaluation will consist in the same set of metabolic measurements, but will be performed fasting or during administration of a 70% fat or 70% carbohydrate liquid diet, according to a randomized order.

On each occasion, subjects will consume a standardized dinner the day before the experiment. They will remain fasted since 10 pm until the test day.

On the day of the experiments, one venous cannula will be inserted into a forearm vein for the collection of blood samples. An indirect calorimetry (measurement of respiratory gas exchanges using a ventilated canopy at rest, or a face mask during exercise) will be performed throughout the 6-hour test to measure energy expenditure and net substrate oxidation rate. Blood samples will be obtained every 60 min for the measurement of plasma glucose, insulin, free fatty acids, and beta-hydroxybutyrate concentrations. Breath samples will be also collected every 60 min (in 500 ml sampling bag) for the measurement of breath acetone concentration using a novel gas analyzer based on infrared laser spectroscopy.

Measurements will be done during 3 consecutive periods:

• 0-120 min: Fast + Rest: subjects will remain lying in bed in fasting conditions (negative energy balance).
• 120-360 min: Fed + Rest: subjects will remain in bed in a fasting state (C) or will receive every hour a liquid meal containing either 70% fat, 15% carbohydrate and 15% protein (Fat), or 70% carbohydrate, 15% fat and 15% protein (Sugar), corresponding to 1.5 X hourly resting energy requirement (positive energy balance).
• 360-480 min: Fed + Exercise: subjects will be transferred to an ergometric bicycle and will be asked to bike at a power output of 25 W [i.e. increasing total energy expenditure to ca. 2 time resting energy expenditure (negative energy balance)].

Inclusion Criteria:

• BMI 19.5-29.9 kg/m2,
• Healthy

Exclusion Criteria:

• any known disease (with the exception of allergies);
• consumption of alcohol > 40 g/day;
• any current drug treatment (with the exception of contraceptive agents, antihistaminergic drugs for allergies, or multivitamin supplementations),
• history of lactose intolerance,
• history of exercise intolerance.