The Athlete's Secret: Why Exercise Makes Fat Your Friend

According to a new study published in the journal Diabetologia exercise fuels our bodies with fatty acids and glucose, with their relative contributions depending on factors like training and health. Trained athletes excel at burning fat, while insulin-resistant individuals struggle. These fatty acids come from various sources, like circulating blood fats and muscle stores. Interestingly, muscle fat storage in athletes doesn't hinder insulin sensitivity, unlike in others. This "athlete's paradox" is linked to the size, location, and even protein coating of these fat droplets. Exercise can remodel these droplets, improving fat burning and potentially insulin sensitivity, especially in those with metabolic issues like diabetes. This review dives into the complex interplay between exercise, fat metabolism, and insulin sensitivity, highlighting the potential of exercise to reshape fat storage and improve overall health.

Key Points

1. Exercise Fueling:

• Fuel Sources: During exercise, the body uses both glucose and fatty acids (FAs) for energy, with the relative contribution dependent on various factors like:

  • Prandial state: Whether your stomach is full or empty affects fuel availability.

  • Exercise intensity: lower intensity uses more fat, while higher intensity uses more glucose.

  • Training status: endurance-trained athletes have a higher capacity to burn fat.

2. Fat Sources for Exercise:

• Circulation: FAs can come from circulating triglycerides released from white adipose tissue during lipolysis.

• Muscle Stores: Muscles also store FAs in small droplets called intramyocellular lipids (IMCLs).

• Liver Fat: In some cases, hepatic fat stored in the liver may also contribute to fuel during exercise.

3. The "Athlete's Paradox":

• High IMCL and Insulin Sensitivity: Non-athletes with high IMCL tend to have lower insulin sensitivity, while trained athletes can have high IMCL content and remain highly insulin-sensitive. This is the "athlete's paradox."

• Droplet Characteristics Matter: Research suggests the key lies not just in the total IMCL amount but also in the size, number, location, mitochondrial tethering, and even the protein coating of these fat droplets.

4. Exercise and Lipid Droplets:

• IMCL and IHL Content: In metabolically compromised individuals like obese or type 2 diabetes patients, both IMCL and intrahepatic lipid (IHL) content are increased, with lower fat oxidation capacity.

• Training Effects: Endurance training can:

  • Reduce the IHL content in the liver.

  • Remodel IMCL in muscle, potentially increasing the proportion of "athlete-like" droplets associated with good insulin sensitivity. However, total IMCL may not decrease and might even increase.

  • Improve fat oxidation capacity in both healthy and insulin-resistant individuals.

5. Future Research Needs:

• Acute Exercise Effects: While training studies exist, understanding the immediate effects of single exercise bouts on fat metabolism, especially in insulin-resistant individuals, is crucial.

• Sex Differences and Intensity: More research is needed to clarify the impact of sex and exercise intensity on IMCL utilization and droplet remodeling.

• IHL Regulation: The mechanisms underlying exercise-induced changes in IHL are much less understood compared to muscle fat droplets.

• Timing of Exercise: Diurnal rhythms in lipid metabolism suggest exploring the optimal timing of exercise for improving insulin sensitivity in specific populations, like type 2 diabetes patients.

The Basics: Fatty Acids as an Energy Source

Before we delve into the intricacies of fat oxidation, it's imperative to grasp the fundamentals. When you engage in physical exercise, your body's energy demand escalates. This heightened demand is chiefly met through the oxidation of two primary sources: glucose and fatty acids. The relative and absolute contributions of these sources are contingent upon several factors, including your prandial state (whether you've recently eaten), exercise intensity, and your overall training status.

Endurance Athletes and Fat Oxidation

Endurance-trained athletes, often revered as the epitome of aerobic capacity, possess a remarkable ability to harness fatty acids for energy during exercise. Their bodies are finely tuned for fat oxidation, making them exceptionally efficient at utilizing stored fat as a source of energy. This high oxidative capacity is a result of years of unwavering training and dedication, setting them apart from the average individual.

The Challenge of Insulin Resistance

On the flip side, we encounter insulin-resistant individuals. In these cases, the body's ability to effectively oxidize fat is compromised. Insulin resistance is frequently associated with conditions like obesity and type 2 diabetes. These individuals grapple with using fatty acids for energy, and glucose often takes precedence in their energy expenditure.

The Impact of Training

Now, let's venture into a fascinating twist. Endurance training, even in individuals with insulin resistance, has demonstrated its capability to enhance fat oxidative capacity. This implies that with the right training regimen, even those with compromised fat oxidation abilities can elevate their performance and endurance.

The Sources of Fatty Acids

Fatty acids utilized during exercise stem from multiple sources. They can be obtained from the circulation, where they are encapsulated in triacylglycerol-rich particles originating from the liver. Alternatively, they may be in the form of NEFAs (non-esterified fatty acids), predominantly originating from adipose tissue lipolysis.

Exercise Intensity and Fat Oxidation

The intensity of your exercise plays an instrumental role in determining the source of fatty acids utilized for energy. Research indicates that fat oxidation experiences an upswing between 40% and 55% of your maximal power output (Wmax) and subsequently begins to decline toward 75% Wmax. This pattern is attributed to various factors, including alterations in the oxidation of NEFAs and triacylglycerol fat sources.

Uncovering the Role of IMCLs

Intramyocellular lipids (IMCLs) constitute another wellspring of fatty acids for exercise. These lipids are chiefly stored in triacylglycerol-rich lipid droplets dispersed throughout the muscle. Their involvement in exercise-induced fat oxidation is nothing short of intriguing.

The Athlete's Paradox

An intriguing phenomenon in the fitness world is the 'athlete's paradox.' It is characterized by trained athletes storing IMCLs to a similar extent as insulin-resistant individuals, yet they maintain a high level of insulin sensitivity. This suggests that the mere presence of IMCLs is not the sole factor influencing insulin sensitivity.

The Role of Lipid Droplets in Understanding Lipid Metabolism in Human Skeletal Muscle

In recent years, there has been a growing interest in understanding the effects of acute exercise on lipid metabolism in human skeletal muscle. This topic not only holds significance for athletes and fitness enthusiasts but also plays a crucial role in addressing conditions such as type 2 diabetes and obesity. Through stable isotope measurements and muscle biopsies, researchers have made significant strides in shedding light on how our muscles utilize lipids during exercise. In this comprehensive article, we delve into the intricacies of lipid metabolism, exploring the differences between healthy, endurance-trained individuals and those with metabolic complications.

The Role of Lipid Droplets

Lipid droplets are dynamic organelles found in our muscles, serving as storage units for fatty acids. These droplets play a pivotal role in releasing fatty acids when the body demands energy. The characteristics of these lipid droplets, including their number, size, location, and protein decoration, have been closely linked to insulin resistance. Importantly, these characteristics vary significantly between individuals, particularly between athletes and those with type 2 diabetes.

Distinct Differences in Lipid Droplet Characteristics

Athletes typically store lipid droplets in smaller quantities within their muscle fibres. In contrast, individuals with type 2 diabetes often store more lipid droplets in the subsarcolemmal region, which is a glycolytic type of muscle fibre. This distinction in lipid droplet storage highlights the differences between these two groups and how their muscles manage fatty acids during exercise.

The Role of Perilipin Proteins

Perilipin (PLIN) proteins, a family of lipid droplet-coating proteins, are integral to the process of lipid-droplet turnover. They interact with lipases, such as adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), to facilitate the release of fatty acids from lipid droplets. Among the PLIN family, PLIN2, PLIN3, and PLIN5 play critical roles in human skeletal muscle.

• PLIN2 negatively regulates ATGL-mediated lipid droplet lipolysis by hindering access to the lipid droplet surface.

• PLIN3 coats nascent lipid droplets and influences fat oxidation rates.

• PLIN5 regulates the rate of lipolysis to match fatty acid release with mitochondrial fatty acid oxidation.

The Impact of Exercise on Perilipin Proteins

While the acute exercise itself does not significantly affect the total content of PLIN5 or ATGL, the redistribution of these proteins to match the changes in energy demand is a fascinating phenomenon. This redistribution occurs during exercise, ensuring that the release of fatty acids from lipid droplets aligns with the body's immediate energy requirements. This phenomenon has been observed in advanced imaging, allowing us to study individual lipid droplets.

Comparison Between Healthy Individuals and Those with Type 2 Diabetes

When we compare the lipid droplet characteristics of healthy, lean participants to those with type 2 diabetes, we notice distinctive differences. Endurance-trained athletes preferentially use lipid droplets coated with PLIN2 and PLIN5 during exercise. Notably, they have a higher number of PLIN5-coated lipid droplets compared to individuals with type 2 diabetes.

Additionally, individuals with type 2 diabetes tend to have a higher myocellular PLIN2 protein content than endurance-trained athletes. This suggests that the muscles of athletes are better equipped for higher exercise-mediated lipid-droplet turnover than those of individuals with type 2 diabetes.

Gender Differences

One interesting observation is that the increase in lipid droplet-mitochondria interaction during exercise may vary between genders. In male elite cross-country skiers, this interaction intensifies despite no noticeable changes in intramyocellular lipid (IMCL) content. However, in endurance-trained women, this interaction increases alongside a reduction in IMCL content. The contrast between these findings may stem from inherent gender differences.

Implications for Individuals with Type 2 Diabetes

The potential impact of lipid droplet-mitochondria interaction extends beyond athletes. Research indicates that trained individuals possess higher levels of PLIN5 and more PLIN5-coated lipid droplets, which may promote greater interaction between lipid droplets and mitochondria. This suggests that individuals with type 2 diabetes, who often exhibit impaired lipid metabolism, may benefit from investigating the role of PLIN5 and lipid droplet-mitochondrial tethering in their condition.

Endurance Training and Mitochondrial Respiratory Capacity

Mitochondrial respiratory capacity, the ability of our cells to produce energy, is significantly compromised in individuals with type 2 diabetes and obesity. However, a powerful solution emerges in the form of endurance training. Numerous studies have demonstrated that mitochondrial respiratory capacity and fat oxidation can be significantly enhanced through endurance exercise, even in individuals with type 2 diabetes and obesity. This revelation offers hope and practical solutions for those struggling with these conditions.

IMCL Storage, Lipid Droplet Morphology, and Interaction

Endurance training not only boosts mitochondrial capacity but also plays a pivotal role in modulating intramyocellular lipid (IMCL) storage and lipid droplet morphology. Research indicates that endurance training can affect the characteristics of lipid droplets within muscle cells without drastically altering the total IMCL content. This is particularly significant for type 2 diabetic, obese, and sedentary individuals, as it not only improves insulin sensitivity but also enhances the way the body stores and utilizes IMCL.

A Closer Look at Lipid Droplet Phenotypes

Upon endurance training, several remarkable changes occur at the lipid droplet level. Lipid droplet size decreases, intramyofibrillar lipid droplet content increases and these smaller lipid droplets tether themselves to mitochondria. This "athlete-like lipid droplet phenotype" is characterized by multiple small lipid droplets in type I muscle fibers, enhancing fat metabolism. In contrast, individuals with type 2 diabetes have larger lipid droplets, predominantly located in type II muscle fibers, hindering efficient fat oxidation. The tethering of lipid droplets to mitochondria increases endurance training in obese individuals, showcasing the profound impact of exercise on lipid droplet dynamics.

Training in the Fasted State

Training in the fasted state has gained popularity for its potential to enhance fat oxidative capacity. Fasting increases adipose tissue lipolysis and plasma NEFA levels, promoting IMCL storage and fatty acid oxidation. The increase in NEFA levels during fasting may stimulate peroxisome proliferator-activated receptor (PPAR)-mediated gene expression, benefiting fat metabolism. Training in the fasted state has also been shown to improve glucose tolerance, though the effects on fat oxidative capacity remain inconsistent.

Exercise and Liver Lipid Metabolism

Intrahepatic lipid (IHL) storage is closely linked to type 2 diabetes and cardiovascular diseases. Studies indicate that both diet and exercise influence IHL content in individuals with non-alcoholic fatty liver disease or type 2 diabetes. Endurance training for a few months can lead to a reduction in IHL content and an improvement in whole-body/muscle insulin sensitivity. This suggests that exercise may reduce de novo lipogenesis in the liver, ultimately contributing to a lower IHL content.

To Summarize

1. Exercise as a Dual Fuel Source: During physical activity, the body utilizes both glucose and fatty acids for energy. The relative use of these fuels depends on factors like whether the individual has eaten recently (prandial state), the intensity of the exercise, and the person's training status, with endurance-trained athletes showing a higher capacity for fat oxidation.

2. Fatty Acids from Various Sources: Fatty acids used during exercise come from different sources, including circulating blood fats released from white adipose tissue and intramuscular fat stores. These sources are tapped differently based on the intensity of the exercise and the individual's metabolic health.

3. The Athlete's Paradox: Trained athletes can have high levels of muscle fat (intramyocellular lipids) without suffering from reduced insulin sensitivity, a phenomenon known as the "athlete's paradox." This contrasts with non-athletes, where high muscle fat often correlates with poor insulin sensitivity.

4. Exercise's Impact on Lipid Droplets: Regular endurance exercise can remodel the fat droplets in muscles, potentially improving fat burning and insulin sensitivity. This effect is particularly beneficial for individuals with metabolic issues, such as those with type 2 diabetes or obesity.

5. Differences in Fat Metabolism: The ability to burn fat efficiently varies greatly between individuals, depending on factors like training status, metabolic health, and potentially gender. Endurance training enhances fat oxidation capacity in both healthy and insulin-resistant individuals.

6. Lipid Droplets and Insulin Sensitivity: The characteristics of lipid droplets in muscles, including their size, location, and protein coating, play a significant role in determining insulin sensitivity. Athletes tend to have smaller, better-distributed lipid droplets with a composition that supports efficient fat metabolism.

7. Future Research Opportunities: There's a need for more research to understand the immediate effects of exercise on fat metabolism, especially in insulin-resistant individuals. Additionally, exploring the impact of exercise intensity, timing, and sex differences on fat metabolism can provide deeper insights into optimizing exercise for metabolic health.

Concluding Remarks

In summary, the effects of exercise training on lipid metabolism in human skeletal muscle are profound and offer hope for individuals with type 2 diabetes and obesity. Endurance training not only improves mitochondrial respiratory capacity but also remodels IMCL content, leading to an athlete-like lipid droplet phenotype. Furthermore, exercise has a positive impact on IHL content and may reduce the risk of metabolic complications. While there is still much to explore in this field, it is clear that exercise is a powerful tool for improving lipid metabolism and overall health.

Reference

Gemmink, A., Schrauwen, P., & Hesselink, M. K. C. (2020, June 12). Exercising your fat (metabolism) into shape: a muscle-centred view. Diabetologia. https://doi.org/10.1007/s00125-020-05170-z

Image credit: :https://www.frontiersin.org/files/Articles/509410/fcell-08-00009-HTML/image_m/fcell-08-00009-g001.jpg

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