The Effect of Fasting on Peak Fat Oxidation, Mitochondrial Coupling and Extracellular Matrix

Four groups of young normal weight men (n=10) and women (n=10) and young obese men (n=10) and women (10) are recruited. After verbal and written information is provided signed consent is obtained.

Prior to the first day of the trial, the participants must complete a four-day dietary and exercise record based on the weighing of all food and beverages consumed and tracking all exercise. This is used to asses the participant's habitual diet and activity level. For female participants, an additional form must be completed regarding their menstrual cycle and form of contraception.

On the two trial days (before and after fasting), participants arrive at the laboratory in the morning, and on the first day, the participant will have fasted since the previous evening. Furthermore, participants have been asked to minimize strenous physical activity for 24 hours leading up to testing. Prior to day 1, participants have been instructed to consume a habitual carbohydrate-rich diet for the 48 hours preceding the first test day.

Initially, the participants' body composition is determined by DXA, after which the participants rest for 15 minutes. Subsequently, blood pressure is measured in the supine position whereafter resting energy expenditure is measured by indirect calorimetry. A venous blood sample is taken from the antecubital vein, as well as a biopsy from the m. vastus lateralis and from the subcutaneous abdominal adipose tissue using the Bergström technique applied with suction. After 30 minutes of rest, a graded exercise test on a cycle ergometer is initiated to determine peak fat oxidation and maximal oxygen uptake. After the test is completed and following a minimum of 10 minutes of rest, handgrip strength and knee extension strength are measured. This concludes the test day.

Thereafter, the 84-hour fast is initiated. During the fasting period, the subjects will communicate by telephone each morning with one of the researchers regarding their well-being and any questions they may have. On day 2 subjects will do 1 hour of submaximal exercise. During the fasting subjects will be provided with electrolyte tablets. Following the 84 hours, the subjects return to the laboratory. Here, the same protocol as described above is carried out. This concludes the test day and the experiment.

Inclusion criteria

• Healthy normal weight men aged [18,40] and BMI [18,25]
• Healthy normal weight women [18,40] and BMI [18,25]
• Healthy obese men [18,40] and BMI [30,40]
• Healthy obese women [18,40] and BMI [30,40]

Exclusion criteria

• Present or former cardiovascular disease and chronical inflammatory disease
• Consumption of medications and supplements that influence substrate use at rest and/or during exercise
• Pregnancy