During even a brief period of fasting, your metabolism can rapidly change. For example, let’s look at the macronutrient carbohydrate, like those found in rice, breads, and fruit. The storage form in our bodies, glycogen, generally decreases, which can shift metabolism from burning carbohydrates to burning fat for energy. This occurs in multiple tissues including the one responsible for movement, muscle.
Cleanse Days involve fasting to speed up metabolism and aid in fat burning. Now a recent study confirms fat burning is vastly increased during fasting, but even more interesting is its effects on muscle. What researchers found was certain genes in muscles were actually altered in a very beneficial way (1).
Muscle can use both carbohydrates and fat for energy and can adapt to different nutrient intakes including fasting (2). This occurs through a complex, energy and nutrient-sensing pathway that is involved with many different energy states, like being full or fasting (3). Understanding this pathway under different dietary conditions is very important as it can affect how genes are expressed and regulated (1).
Researcher Marjolein van Wijngaarden and colleagues from the Department of Endocrinology and Metabolic Diseases in Leiden University Medical Center in the Netherlands investigated the effects of fasting on carbohydrate and fat metabolism in relation to gene expression in healthy, young men. It was the first study to actually provide a map of the signaling pathways and changes in human muscle during short-term fasting.
The study used 12 young, lean healthy males who stopped exercise and performed an overnight fast. The subjects started after a breakfast of 300 kilocalories and then followed up with a 24-hour fast where only water and caffeine-free tea were allowed. Blood samples and muscle biopsies were collected at multiple times throughout the well-controlled study period.
Not surprisingly there was a rapid decrease in carbohydrate oxidation (sugar burning) with a large increase in fat oxidation (fat burning) mostly in muscle. Insulin and leptin (a hormone involved in appetite control) levels decreased as free fatty acids and growth hormone levels increased. Most interesting in this study was that genetic expression changed for more than 900 genes in muscle after only a 24-hour fast.
The study represents roughly seven percent of the genes expressed in muscle tissue. However, only 23 genes, mostly involved in the regulation of metabolic processes, were found to be significantly affected by the fasting at both early (10 hours) and late (24 hours) time points. Many of these genes helped move metabolism to fat oxidation (fat burning) at the whole-body level.
In relation to fat burning in muscle tissue, short-term fasting appeared to be quite effective. However, prolonging the daily fast or fasting too often to speed up fat loss is not advisable as fasting for too long can cause a loss in muscle mass and negatively affect metabolism (2;3).
Bottom line: When done correctly, cleansing works!
- Wijngaarden MA, Bakker LE, van der Zon GC et al. Regulation of skeletal muscle energy/nutrient-sensing pathways during metabolic adaptation to fasting in healthy humans. American Journal of Physiology-Endocrinology and Metabolism 2014;307:E885-E895.
- Wijngaarden MA, van der Zon GC, van Dijk KW, Pijl H, Guigas B. Effects of prolonged fasting on AMPK signaling, gene expression, and mitochondrial respiratory chain content in skeletal muscle from lean and obese individuals. American Journal of Physiology-Endocrinology and Metabolism 2013;304:E1012-E1021.
- Laplante M, Sabatini DM. Regulation of mTORC1 and its impact on gene expression at a glance. Journal of cell science 2013;126:1713-9.