Inactivity Duration on Lipid Metabolism

Physical inactivity impairs health, including contributing to chronic diseases and increasing the risk of mortality, especially related to cardiovascular diseases. Regular exercise can help reduce the risk of cardiovascular diseases, but it is not enough on its own. Prolonged sitting and having too little of light intensity of physical activity (like stepping) means lacking physical activity and is harmful for health, even for those who meet the exercise guidelines. Although an acute bout of exercise is able to improve the postprandial lipid metabolism (i.e., attenuate postprandial lipemia), it has been found that this benefit is abolished after a period of physical inactivity. Specifically, taking 8,500 steps per day or more (i.e., being physical active) would assure the benefits in postprandial lipid metabolism from exercise, while taking 5,000 steps or less (i.e., being physical inactive) would abolish this benefit from exercise. Investigators termed this phenomenon 'exercise resistance'. However, it is still unknown of how long of the of a period of physical inactive will cause 'exercise resistance'. Therefore, investigators propose this study to focuses on the effect of the length of physical inactivity on postprandial lipid metabolism the following morning after exercise. Three intervention trials differentiate with different periods of physical inactivity (12 hours, 24 hours, and 36 hours) will be involved for this purpose. The study aims to determine how long a period of physical inactivity is required for 'exercise resistance' to occur.

Twelve male and female participants will be recruited from Austin community to participate this study. All participants will self-report that they have been medically cleared for exercise by submitting a health history questionnaire. Researchers will screen all forms filled out by participants and confirm that participants are eligible for this study. Participants will receive both written and verbal descriptions of the study procedures and measurements, and provide written informed consent.

The study requires at least 7 visits besides the day for questionnaire filling and informed consent signing. One for preliminary test, and 6 for the three trials (one cycling session and one high fat tolerance tests for each trial). All visits will take place in the Human Performance Lab (BEL 820) at the University of Texas at Austin. All participants will complete one preliminary test (incremental exercise test) on a cycle ergometer before three intervention trials in a randomized order.

During the incremental exercise test, participant will cycling on a cycle ergometer with a mask. Participant's breathing air will go through the tubes connecting with the mask. By analyzing the exhaled gas during the test, the maximal oxygen consumption (VO2max) will be measured for determining the cycling intensity for the cycling in the three trials. Between trials, there will be a washout period of at least five days. The three trials varies in length of physical inactive (12 hours, 24 hours, and 36 hours). Thus, the three trials are marked as 12-hr trial, 24-hr trial, and 36-hr trial. During the cycling sessions, participant will cycle for 1 hour at an intensity of 60-65% of VO2max (moderate intensity) on a cycling ergometer. The high fat tolerance test (HFTT) day will be the day for collecting primary data.

To highlight the effect of physical inactivity duration on causing exercise resistance, all HFTT will start at the same time on the testing day of each trial, which is 9 am on the final day of each trial. For easier understanding, the events sequence in each of the trails is listed below in a backward order,

• A HFTT from 9 am to 2 pm on the last day of each trial.
• A physical inactivity phase (<1,000 - 1,500 steps/12 hr) that lasts until the beginning of the HFTT (i.e., 9 am on the last day of the trial). The lengths of this phase will be 12 hours, 24 hours, or 36 hours depending on the trial.
• A 1-hr cycling session finishing 12 hours before the HFTT (i.e., 8pm - 9pm on the second last day).
• A 48 hours of physical activity control phase (> 10,000 steps/24hr) that lasts until the beginning of the physical inactivity phase.

During the whole trials, participants will be given a pedometer for self-monitoring the step counts. A step recording form, a sleep recording form, and a diet form will be provided to participants in all trials and will be returned after each trial. A standard frozen meal will be provided to the participants to consume 2 hours before the cycling sessions to avoid the effect from fat consumption from the meal before the HFTT. Other than the sport drink that will be provided right after the cycling, participants will need to stay fasted (no food or beverage, only water) since the frozen meal to the HFTT. During the HFTT, a high fat meal (a mixture of ice cream and heavy cream) will be provided to participants. Participants will consume the milkshake in 10 min. The amount of ice cream and heavy cream will be determined by participant's body weight. During the test, five measurements (fingertip blood collection and exhaled air collection) will be done including one baseline measurement (before high fat meal), and four measurements each hour after consuming the meal.