Published in The Journal of Clinical Endocrinology and Metabolism, the study, titled “Lipid metabolism links nutrient-exercise timing to insulin sensitivity in men classified as overweight or obese” investigated a concept known as “nutrient-exercise timing”.
Essentially, this concept relates to the time frame in which nutrients are consumed in relation to exercise. In particular, whether you exercise while in the fasted or the fed state, and how this influences your response to physical activity and your adaptation to it.
12 men completed two conditions in a randomized crossover order. In one condition, they completed a 60-minute session of moderate intensity (65% V02 max) cycling followed by a high-carb breakfast.
In the other condition, the order was revered — breakfast then exercise (90 minutes later).
Measures: Expired gas samples during exercise to determine what amount of fat and carbohydrates they were utilizing, blood samples to test for glucose, glycerol, free-fatty acids, and insulin, muscle biopsies for skeletal muscle triglyceride content, glycogen, and protein expression of genes related to metabolism.
Main outcome: comparison of lipid (fat) utilization during exercise when fasted vs. fed
30 men were randomized into a control (no exercise) group, fed-exercise training group, or fasted-exercise training group. The fed group received a carbohydrate-containing beverage 2 hours before exercise, and the fasted group consumed a placebo (no carbs or calories) before exercise and had the drink after.
Measures: Pre- and post-training oral glucose tolerance test (OGTT) to test insulin sensitivity, muscle biopsy for muscle lipid composition, metabolic enzyme activity, and proteins involved in glucose transport/insulin signaling.
First, the acute study.
Compared to fed exercise, fasted exercise resulted in an increase in whole body lipid utilization. During the exercise bout, the group who consumed breakfast first had an increased respiratory exchange ratio (RER), indicating they were utilizing more carbohydrates for energy. On the other hand, the fasted exercise group was burning a greater amount of fat throughout the exercise bout.
In the chronic study, similar findings were observed.
The group who completed the exercise training in a fasted state experienced a 2-fold increase in their lipid utilization during exercise, which was maintained throughout the entire 6-week trial. They reduced their “reliance” on carbohydrates and burned more fat during exercise.
The fasted exercise training group also experienced an increase in their insulin sensitivity after 6 weeks — their insulin response to the OGTT was lower — meaning they needed less insulin to take up the same amount of glucose (basically, insulin “worked better”).
There were also some interesting changes at the cellular level. Specifically, a protein known as AMP kinase (AMPK) was increased 3-fold in the fasted-exercise group. AMPK is known as a cellular “nutrient sensor” which is able to upregulate gene expression by detecting whether our body is “high energy” (i.e. fed) or “low energy” (i.e. fasted).
AMPK plays a big role in the regulation of fatty acid oxidation, the building of new mitochondria, and glucose uptake (all things that contribute to insulin sensitivity). Thus, the response of AMPK to fasted exercise likely explains many of the beneficial metabolic effects observed in this study and others.
Study Key Takeaways
- A single bout of fasted exercise increased lipid utilization
- Fasted exercise training increased fat utilization 2-fold
- Fasted training improved insulin sensitivity
- Fasted training increases energy sensing (AMPK) and glucose transport (GLUT4) proteins