12 DAYS OF FITNPHYSMAS
Day 12: How Fat Loss Really Works
12 days of exercise science & sports nutrition training tips, curated into your research-based training guide
Since you're here, reading this, it's safe to assume you want to learn about fat loss. You want to know how this stuff works in the body, and what the science says so that you can apply these principles into your fitness and lifestyle routines. If this is you, your first step in understanding metabolism is to enroll in my FREE mini course:
This online course introduces the definition of Calories, anaerobic and aerobic metabolism, and where carbs and fats fit into the whole mix! Go to courses.fitnphys.training to learn more!
Fat loss is a function of:
how adipose, or stored body fat, releases fatty acids
how those fatty acids are transported into active muscle
and then how those fatty acids are metabolized.
So the question next is: What influences numbers 1, 2, and 3? What can you do in the gym to make those functions work to your advantage?
Study this graph for a moment. The X-Axis represents exercise intensity, and the Y-Axis represents the amount (and source) of fuel that generates energy for the increasing amount of work. As you can see here, fatty acids, represented in blue, satisfy a small contribution of total energy production. Furthermore, at a certain intensity (around 50% of VO2max), you can see that fat metabolism starts to drop, and by the time the subject is exercising at around 85% of their VO2max, they are exclusively relying on carbohydrates for energy production.
Allow me to explain some important things about what is happening here:
As exercise intensity increases, so does the circulating concentration of acid in the blood. The drop in pH (i.e., rise in acidity) changes the chemical environment in the body. Things like pH and temperature can catalyze enzyme reactions. In the body we see that fatty acids are no longer released from adipose, nor are they broken down via metabolism within the muscle. Thus, it appears that the increase in acidity due to exercise intensities greater than 60 - 70% of VO2max alters the pH needed for fat metabolism to occur.
Carbohydrates are our primary fuel source. It should be noted that I personally was a scientist involved in the graph above - these are my data that we're looking at. The participants came to the lab in a fasted state, on an empty stomach. This is important because it is well documented in the research that "fat burns in a carbohydrate flame", meaning that you need carbohydrates present to metabolize fats at the fastest and greatest rates possible.
Your ability to rely on fats is "flexible"; we call this, metabolic flexibility. The greater your fitness, the greater you can rely on fats. In other words, the blue segment of the above graph would be greater in magnitude if you we improved your cardiorespiratory fitness.
Referring back to the 3 items functions of fat loss, we see that exercise intensity limits our reliance on fats DURING THE WORKOUT. This is where the dominant bro-science principle of the "fat burn zone" comes from - that you must exercise at a moderate intensity to mobilize and use the greatest amount of fatty acids for energy production. This is true during the workout, but flawed in it's application. In other words, the "fat burn zone" will not be a successful fat loss strategy. The reason is because of the above-mentioned metabolic flexibility. Exercising at very high intensities, while at the moment relying on carbohydrate metabolism, will stress our fat loss for greater adaptation and improved metabolic flexibility. In other words, the harder you work, the more blue will be on your graph the next time you go to the gym.
Studies exploring sprint interval training on a bike or rowing machine have been able to measure rapid changes in metabolism and rapid improvements in fat metabolism adaptations. So exercising at a low and slow intensity uses a good ratio of fats and carbs for fuel during the workout, but it's not intense enough to make the rapid changes in your metabolism. Sprint interval training uses almost exclusively carbohydrates for fuel, yet its high intensity workload makes for the greatest and most rapid changes in how our body fat behaves, transports fatty acids, and how our muscles metabolize fats for fuel.
I recommend sitting on a rowing machine or a stationary bike for your sprint intervals so you don't fall over when your legs get exhausted (which they will). Warm up for a few minutes, and then sprint at an all-out intensity for 30 seconds, followed by a 60 second rest interval. Repeat that process for at least 8 total intervals, or in other words: 4 minutes of total, accumulated work and 8 minutes of total, accumulated rest. Perform more intervals if you can handle it. Your total interval training session doesn't need to exceed 30 minutes (20 intervals of work + rest).