Fat Loss, Exercise, and Fasting

Exercising For Fat Loss: What Is Best? High Intensity, Sprinting, Moderate Intensity?  

Exercise is a key component of fat loss, but different types of exercise can have vastly different effects on hormones and metabolism. Huberman suggests that the best way to think about exercise for fat loss is in terms of three general types of training: high intensity interval training (HIIT), sprint interval training (SIT), and moderate intensity continuous training (MICT).

HIIT is defined as sub-maximal, 80 to 100% of VO2 max bursts of activity that last 60 to 240 seconds, interspersed with less intense recovery periods. 

An example of this would be running around a track for 60 seconds, then walking for a minute or two and repeating.

SIT, on the other hand, is defined as all-out, greater than 100% of VO2 max bursts of activity that last 8 to 30 seconds, interspersed with less intense recovery periods. 

This would be sprinting down field for 8 to 30 seconds and then walking back for a minute or two before sprinting again.

MICT, is steady state cardio, performed continuously for 20 to 60 minutes at moderate intensity of 40 to 60% of VO2 max, or if you prefer heart rate, 55 to 70% of max heart rate. This type of exercise is often called zone two cardio on the internet.

It is important to note that the intensity of exercise is important, and low intensity usually means that one could carry on a conversation or maybe gasp every few steps or so while trying to talk and run. 

By understanding the different types of exercise and their corresponding intensities, one can make an informed decision on which type of exercise is best for their specific goals.

Exercising Fasted: Does It Truly Accelerate Fat Loss/Oxidation  

Exercising fasted is a topic that many people are interested in, as it pertains to fat loss and oxidation. 

The question at hand is whether or not burning more fat is possible by doing exercise in a fasted state, meaning not having eaten anything for several hours before exercising. 

There is a lot of controversy surrounding this topic, as some studies have shown that fasting before exercise can lead to increased fat burning, while others have shown no difference.

One of the classic studies on this topic is Alborg et al. from 1976, which found that consuming glucose before exercise reduces fat burning during exercise. 

Other studies, such as Horowitz 1999 and Lee et al. have also shown similar results, with people consuming sweet, sugary milk before exercise leading to less fat being oxidized during the exercise.

However, many other studies have shown that eating before exercise does not reduce the amount of fat that is oxidized during the exercise. 

Fasting and Exercise: The 90 Minute Rule

If you’re looking to maximize your fat-burning potential during exercise, the 90 minute rule is a key protocol to understand. According to the research, after 90 minutes of moderate intensity exercise, there’s a switchover point where the amount of fat burned is affected by whether or not you ate before the workout.

If you ate within 1-3 hours prior to exercising, you’ll burn less fat from the 90 minute mark onward. 

On the other hand, if you fasted (didn’t eat for 3 hours or more) before exercising, you’ll start burning more fat after 90 minutes of moderate intensity exercise. This applies to activities like running, swimming, walking, and hiking.

It’s worth noting that 90 minutes of moderate intensity exercise is a significant amount of time, but for those who participate in all-day activities like hiking or running, it’s a good idea to do so in a fasted state to maximize fat oxidation.

Benefits of Fasting During High-Intensity Training

According to recent studies, high-intensity training done first can bring the benefits of fasting before 90 minutes. If you engage in high-intensity training for 20 to 60 minutes, such as weightlifting, powerlifting, kettlebell swings, or sprints, the switch from burning glycogen to burning body fat will come earlier. This is because insulin levels have to go down far enough for the body to start tapping into fat stores.

It’s important to note that this isn’t about how long you exercise, but about how intensely you exercise. 

If you engage in high-intensity training for a shorter time, the switch from glycogen to body fat will occur earlier. However, if you can’t train unless you eat something, then eating something before the workout is the better option.

It’s also worth mentioning that the type of food you eat before exercising can affect insulin levels and body fat reduction. 

If your goal is to oxidize body fat, it’s best to have low insulin levels, which can be achieved by fasting, ingesting fats and proteins, or eating low carbohydrates.

Post-Exercise Metabolic Increases: How To Bias This Toward Fat Oxidation 

In this part in the pod, Dr. Huberman explains that the conventional understanding of counting calories burned during a workout is only half the equation. 

The more important factor is the increase in metabolism after the workout, also known as post-exercise oxygen consumption.

Studies show that high-intensity exercise, such as weight training, sprints, and push-ups, burns more glycogen during the activity. After the exercise, the post-exercise oxygen consumption goes up, sometimes for up to 24 hours, and it is during this period that more fat is oxidized, not glycogen. 

Long bouts of low to moderate-intensity exercise, such as running or biking, burn more body fat per unit time during the exercise. However, after the exercise, more glycogen is burned.

Dr. Huberman concludes that the best way to continue burning body fat is to pick an exercise that you enjoy and can do regularly, but it seems that high-intensity exercise followed by moderate-intensity exercise is the best way to burn fat overall. 

This combination increases the percentage of body fat burned and the overall increase in basal metabolic rate.

Wrapping Up

Starting with high-intensity exercises before moving on to low-intensity activities can increase your chances of burning fat, especially while fasting. 

Fasting before high-intensity training can help increase the percentage of body fat burned during exercise. However, if you’re unable to train without eating, then it’s best to choose a pre-workout meal that helps keep insulin levels low.

Lastly, while the type of diet you follow may not be the deciding factor with successful weight loss, your ability to stick to a diet is critical.


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The content above is a summary from the Huberman Lab podcast. I’m not affiliated with him.

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