UNLOCK THE FASTING CODE: MUSCLE FIBER TYPES & INSULIN SENSITIVITY

ADAPTING FASTING PROTOCOLS TO YOUR MUSCLE FIBER TYPES 

KEY POINTS

• Researchers found that subjects’ glucose tolerance and insulin sensitivity decreased after  3-days of fasting. However, subjects with greater type muscle fibers experienced a greater decrease in insulin sensitivity and increased fat oxidation.

• The researchers found that when they analyzed type I fibers, those with more type I had a 35% increase in fat oxidation compared to those with high type II fibers.

• The study found that people’s metabolic responses to fasting can differ based on their muscle fiber types.

Fasting is a practice that involves voluntarily abstaining from food for a specific period of time. It can be practiced in various forms, such as intermittent fasting, time-restricted feeding, and alternate-day fasting. (Anton et al., 2018) There are pros and cons of fasting and exercise.

FASTING BENEFITS

Fasting has gained significant attention recently as a popular approach to losing weight, improving overall health, and enhancing longevity. (Mohiuddin, 2019) Some potential health benefits of fasting include accelerated weight loss, reduced inflammation and blood pressure, lower cholesterol and heart disease, longer lifespan, blood sugar stabilization, and prevention of type 2 diabetes. (Washburn et al., 2019)

FASTING AND DIABETES

Fasting can potentially adversely affect those with metabolic disorders such as type II diabetes. Prolonged fasting can lead to dehydration, hypoglycemia, electrolyte imbalance, and patient discomfort. (Falconer et al., 2014)

Fasting can be potentially risky for patients with type II diabetes. For example, in Ramadan, more than 50 million Muslims with type 2 diabetes fast for one lunar month, which involves fasting during daylight hours for 29-30 days. (Almaatouq, 2012)

Interestingly, recent research has highlighted the role of muscle fiber types (i.e., Type I and Type II muscle fibers) and the metabolic responses to fasting. In this article, I will discuss a new study examining fasting, insulin resistance, and muscle fiber types, focusing on how individuals with type I muscle fibers experience better metabolic responses than their type II counterparts. This article will discuss how fasting can elicit different metabolic responses based on muscle type.

WHAT HAPPENS WHEN YOU FAST?

During fasting, the body switches from using glucose as its primary energy source to utilizing stored fat, leading to weight loss and improved body composition. (Arner, 1984). Furthermore, fasting enhances insulin sensitivity, reducing the risk of developing insulin resistance and type 2 diabetes. (Varady, 2011)

While shorter fasting periods may improve insulin sensitivity, fasting over 24 hours may worsen glucose tolerance. (S. Atkinson et al., 2020) In a 1988 study, a 72-hour fast led to a transient decrease in insulin sensitivity in healthy, lean men. This decrease was attributed to increased fat oxidation, which may have interfered with insulin signaling. (Ho et al., 1988)

FASTING, INSULIN RESISTANCE, AND MUSCLE FIBER TYPES

Insulin resistance is when the body’s cells become less responsive to the hormone insulin, leading to elevated blood sugar levels. (Petersen & Shulman, 2018) A Western diet high in saturated fats added sugars, and calories is a key component of an insulin-resistant diet. Prolonged insulin resistance can result in type 2 diabetes, cardiovascular disease, and other health complications. A new study suggests that the type of muscle fibers you have affects your metabolic responses.

The American Journal of Physiology-Endocrinology and Metabolism published the study, “Insulin resistance after a 3-day fast is associated with an increased capacity of skeletal muscle to oxidize lipids.”

The study examined the effect of three days of fasting on insulin sensitivity, glucose tolerance, and fat oxidation in healthy adults (15 men and 4 women). They divided the subjects into categories based on fiber types. Subjects were divided into two groups: those with high slow-twitch muscle fibers and those with fast-twitch fibers.

TYPE I FIBERS AND FASTING

Slow twitch fibers or Type I fibers are called red fibers (i.e., aerobic fibers) and have a high oxidative capacity. They have many mitochondria, increased capillary density, and burn fat as their primary fuel source. Type I fibers are particularly suited for aerobic metabolism during low-intensity activities, such as long-distance running, swimming, and cycling.

Type I fibers have been associated with better metabolic responses and improved insulin sensitivity. Their high amount of mitochondria allows them to utilize fatty acids as a primary fuel source during fasting, preserving glycogen stores and maintaining blood glucose levels. (Simoneau & Bouchard, 1995)

Fast twitch muscle fibers or Type II fibers are recruited during anaerobic metabolism and appear white. Type II fibers have a lower number of mitochondria, reduced capillary density, and less fatigue resistance compared to type I fibers.

Type II fibers are essential for high-intensity strength, power, and speed exercise during physical activities, such as sprinting, jumping, and weightlifting. Individuals with more type II fibers may be at a greater risk of developing insulin resistance and type 2 diabetes. (Tanner et al., 2002)

The authors hypothesized that those with more type I fibers would have a preferential response to fasting compared to those with more type II fibers. They also took muscle biopsies before and after the fast in both groups.

RESULTS

The study found that glucose tolerance and insulin sensitivity decreased in both groups after a 3-day fast. However, those with more type I experienced a greater decrease in insulin sensitivity and increased fat oxidation. (Blackwood et al., 2023) Interestingly, at baseline (after the overnight fast) and after the 3-day fast, the number of circulating lipids and ketone bodies was similar between the groups. But when the individual muscle fibers from each group were incubated with medium-chain triglycerides and the ketone body beta-hydroxybutyrate, the group with high type I fibers had an 35% greater fat oxidation.

The researchers found that when they analyzed type I fibers, they had a 35% fat oxidation compared to those with high type II fibers.

This suggests that muscle fiber type might play a role in how fasting affects insulin resistance.

While the exact cause of the greater reduction in insulin sensitivity in those with type I fibers is unknown, the researchers suggested that it is possible that the higher insulin sensitivity at the beginning of the study with those with higher type I fibers could have contributed to the greater reduction in insulin sensitivity.

Insulin Sensitivity and Fasting

In other words, the more insulin sensitive you were before the fast, the less insulin sensitive you were after. At baseline, those with more type I fibers had a 50% greater insulin sensitivity than those with type II fibers.

So why do we experience insulin resistance during fasting anyway? Researchers have suggested that this is a survival mechanism to divert glucose away from the muscle and to the brain. (Soeters & Soeters, 2012)

The author stated, “In this context, the findings of the current study would suggest that individuals with a high percentage of type I muscle fibers would be at an advantage in surviving a prolonged period of food deprivation as more glucose would be diverted away from skeletal muscle. In contrast, conditions that lead to prolonged lipid loads (e.g., high-fat diets) in such individuals can raise the risk of developing insulin resistance and subsequent diseases.”

FASTING AND HEALTH

Fasting has emerged as a promising strategy for improving insulin sensitivity and reducing the risk of insulin resistance-related health complications. However, it is essential to recognize that fasting is not a one-size-fits-all solution, and individual factors must be considered to tailor a fasting regimen that is both safe and effective.

Fasting has been known to have varying effects on insulin resistance, and this study sheds light on how muscle fiber type might influence these effects. Although more research is needed, understanding your muscle fiber composition can help you make informed decisions about fasting and its potential impact on your insulin sensitivity. As always, consult with a healthcare professional before making significant changes to your diet or exercise routine.

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