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Articles reviewed:



“When it comes to the carbohydrate periodization, the research mainly focuses on high carbohydrates diets, with specific carbohydrate restrictions around certain training sessions. Personally however, I believe this is fundamentally wrong approach for the majority of athletes. The better approach is to maintain a lower-carb diet, with specific carbohydrate injections around certain training sessions. After all, following polarized or pyramidal training intensity distribution means most of our training is performed at a low “fat-burning” intensity; and maintaining a lower carb diet habitually means dietary fat content will be increased (increasing fat oxidation (9)). Last this is also far superior for overall athlete health, wellbeing and body weight maintenance (5).”


Here are a few key takeaways:

  1. Carbohydrate ingestion should be part of your race day nutrition strategy, as they help spare finite endogenous glycogen stores and improve performance
  2. Glucose-only based sources should be ingested at rates up to ~60 grams per hour, whereas combined glucose-fructose (or sucrose) based sources may work up to ~90 grams per hour – it’s worth trialling your approach in training, at competition intensity, in training first
  3. There isn’t yet any compelling evidence that ingesting carbohydrates at rates of 120 grams per hour will improve long-distance triathlon performance, or that it’s worth the logistical hassle and risk of gastrointestinal upset


Dr. Stacy Sims references for female athletes fueling for higher intensities intervals and strength workouts


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How are females different than males with fat oxidation rates?

Women tend to oxidize more fat than men during endurance exercise and High-Intensity Interval Training (HIIT) due to several physiological factors:

Hormonal Differences:

Women typically have higher levels of estrogen, which enhances lipid oxidation (fat burning) during exercise. Estrogen can increase the activity of enzymes involved in fat metabolism, leading to a greater reliance on fat as a fuel source.

Additionally, women have lower levels of glycogen (carbohydrate stored in muscles) compared to men, which can further promote fat oxidation during endurance exercise and HIIT.

Muscle Fiber Composition:

Women tend to have a higher proportion of Type I muscle fibers, which are more oxidative and rely on fat as a primary fuel source during endurance exercise. These fibers have a greater capacity for sustained activity, making women more efficient at burning fat during prolonged exercise bouts.
Metabolic Rate:

Women generally have a slightly lower metabolic rate compared to men, which can result in a greater reliance on fat as a fuel source during exercise. This is because fat oxidation is favored at lower exercise intensities, which women may be more inclined to maintain due to their lower metabolic rate.

Subcutaneous Fat Distribution:

Women tend to have a higher proportion of subcutaneous fat (fat stored under the skin) compared to men, which serves as a readily available energy source during exercise. This abundance of subcutaneous fat may contribute to the higher rates of fat oxidation observed in women during endurance exercise and HIIT.

Estrogen’s Effect on Blood Glucose Regulation:

Estrogen can also influence blood glucose regulation, promoting the sparing of glycogen during exercise. This allows women to utilize fat as a fuel source more efficiently, particularly during longer duration activities where glycogen stores may become depleted.
Overall, the combination of hormonal differences, muscle fiber composition, metabolic rate, and fat distribution contributes to women’s tendency to oxidize more fat than men during endurance exercise and HIIT. However, it’s important to note that individual variability exists, and factors such as training status, diet, and genetics also play a role in determining fuel utilization during exercise.

The differences in carbohydrate timing for female versus male athletes primarily revolve around physiological variations, including hormonal fluctuations, body composition, and metabolic differences.

Here’s how it may differ:

  1. Hormonal Influences:
    • Female Athletes: Hormonal fluctuations throughout the menstrual cycle can impact carbohydrate metabolism, energy levels, and nutrient utilization. During the luteal phase (second half of the menstrual cycle), women may have increased insulin sensitivity and carbohydrate oxidation rates, potentially affecting carbohydrate needs and timing.
    • Male Athletes: Hormonal fluctuations are not as pronounced as in females, so their carbohydrate timing may be less influenced by menstrual cycle phases.
  2. Body Composition:
    • Female Athletes: Women generally have a higher percentage of body fat and lower muscle mass compared to men. This can affect nutrient partitioning and utilization during exercise and recovery, potentially influencing carbohydrate requirements and timing.
    • Male Athletes: Men typically have a higher proportion of lean muscle mass and lower body fat percentage, which may influence their carbohydrate needs for muscle glycogen replenishment and recovery.
  3. Metabolic Rate:
    • Female Athletes: Women often have a slightly lower metabolic rate compared to men, which may impact calorie and carbohydrate requirements. However, individual variances in metabolic rate exist and should be considered.
    • Male Athletes: Men generally have a higher basal metabolic rate and may require slightly more carbohydrates to support energy needs, particularly during intense training periods.
  4. Nutrient Timing Considerations:
    • Female Athletes: Due to potential fluctuations in insulin sensitivity and carbohydrate metabolism during different phases of the menstrual cycle, women may need to adjust carbohydrate intake and timing accordingly. This may involve slightly higher carbohydrate intake during the luteal phase to support energy needs and optimize performance.
    • Male Athletes: While hormonal fluctuations are less of a concern for male athletes, individual differences in metabolism, training intensity, and goals still warrant personalized carbohydrate timing strategies.

Overall, while the basic principles of carbohydrate timing remain similar for both female and male athletes, individual differences in hormonal profiles, body composition, and metabolism should be considered when designing personalized nutrition plans. Consulting with a sports nutritionist or dietitian can help female and male athletes optimize their carbohydrate timing strategies based on their unique needs and goals.

How do you improve your fat oxidation rates??

Improving fat oxidation can be achieved through various lifestyle and dietary strategies. Here are some ways to enhance fat oxidation and how it can be measured using devices like PNOE:

  1. Increase Aerobic Exercise: Engage in regular aerobic exercise, such as walking, jogging, cycling, or swimming, to improve fat oxidation. Aim for at least 150 minutes of moderate-intensity aerobic activity per week.
  2. Incorporate HIIT Workouts: High-Intensity Interval Training (HIIT) has been shown to increase fat oxidation during and after exercise. Alternating between periods of high-intensity exercise and recovery periods can boost metabolism and enhance fat burning.
  3. Eat a Balanced Diet: Consume a balanced diet that includes healthy fats, lean proteins, and natures carbohydrates. Avoid excessive intake of refined sugars and processed foods, which can impair fat metabolism.
  4. Include Omega-3 Fatty Acids: Omega-3 fatty acids found in wild caught fish (SMASH), flaxseeds, and walnuts can enhance fat oxidation and improve metabolic health.
  5. Stay Hydrated: Adequate hydration is essential for optimal fat metabolism. Drink plenty of water throughout the day to support metabolic processes.
  6. Get Sufficient Sleep: Lack of sleep can disrupt hormone levels and impair fat metabolism. Aim for 7-9 hours of quality sleep per night to support overall metabolic health.
  7. Manage Stress: Chronic stress can increase cortisol levels, which may promote fat storage and impair fat oxidation. Practice stress-reducing techniques such as meditation, deep breathing exercises, or yoga.
  8. Use PNOE or Similar Devices for Measurement: PNOE is a metabolic analyzer that measures respiratory gases to determine an individual’s metabolic rate and substrate utilization during rest and exercise. It can provide insights into fat oxidation rates and metabolic efficiency.
    • How PNOE Works: PNOE measures the concentrations of oxygen (O2) and carbon dioxide (CO2) in the air a person inhales and exhales during a breath-by-breath analysis. By analyzing the ratio of O2 consumption to CO2 production, PNOE can calculate an individual’s resting metabolic rate (RMR), respiratory exchange ratio (RER), and substrate utilization (including fat oxidation rates).
    • Using PNOE for Fat Oxidation Measurement: During a metabolic assessment with PNOE, individuals can undergo various exercise intensities while their respiratory gases are continuously monitored. This allows for the determination of fat oxidation rates at different exercise intensities and provides valuable information for optimizing training programs and nutrition strategies to enhance fat metabolism.

By implementing these strategies and utilizing tools like PNOE for measurement, individuals can effectively improve fat oxidation and optimize their metabolic health.

In Summary…

Fat oxidation rates can vary between men and women due to several physiological differences:

  1. Hormonal Variations:
    • Estrogen: Women typically have higher levels of estrogen, which enhances fat oxidation. Estrogen can increase the activity of enzymes involved in lipid metabolism, promoting the use of fat as a fuel source during exercise.
    • Testosterone: Men generally have higher levels of testosterone, which may favor carbohydrate metabolism over fat oxidation during exercise.
  2. Body Composition:
    • Women tend to have a higher proportion of body fat compared to men. This higher fat mass provides a larger reservoir of fatty acids for oxidation during exercise.
    • Men typically have greater muscle mass and lower body fat percentage than women, which can influence their metabolic rate and substrate utilization during exercise.
  3. Muscle Fiber Composition:
    • Women often have a higher percentage of Type I muscle fibers, which are more oxidative and rely more on fat as a fuel source during endurance exercise. Men, on the other hand, may have a higher proportion of Type II muscle fibers, which are more glycolytic and rely more on carbohydrates for energy.
  4. Metabolic Rate:
    • Men generally have a higher resting metabolic rate compared to women, which may result in greater overall energy expenditure during exercise. However, this higher metabolic rate does not necessarily translate to higher fat oxidation rates during exercise.
  5. Substrate Preference:
    • Women may have a greater preference for fat as a fuel source during exercise compared to men, particularly during low to moderate intensity activities. This may be influenced by hormonal factors and substrate availability.

Overall, while women tend to have higher fat oxidation rates compared to men during endurance exercise, individual variability exists and is influenced by factors such as hormonal fluctuations, body composition, muscle fiber composition, metabolic rate, and substrate availability. These differences highlight the importance of personalized nutrition and training strategies tailored to individual needs and goals.


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