what happens when we burn fat.

KittensMaster

Love this article. It summarizes everything I have learned and adds more.

EPOC - love this topic and it is often wildly exaggerate or unheard of

Fat as Fuel

The Mitochondria - the organelle that makes it all happen

How fat breaks down

Exercising when fasted, why is actually does work and is not a myth.

The role of insulin in fat being burnt as fuel, and why carb levels matter.

Just an awesome read.

I don't post articles often. This is a paper from the University of New Mexico

https://www.unm.edu/~lkravitz/Article folder/physiologgfatloss.html

Foamroller

Great share and good overview!

Adaptations to Exercise that Improve Fat Usage
Trained people are able to use more fat at both the same absolute (speed or power output) and relative (% of VO2 Max) exercise intensity than untrained people (Achten and Jeukendrup, 2012). Interestingly, lipolysis (breakdown of fats to release fatty acids) and fat release from adipocytes is not different between untrained and trained people (Horowitz and Klein, 2000). This suggests that the improved ability to burn fat in trained people is attributed to differences in the muscle's ability to take up and use fatty acids and not the adipocyte's ability to release fatty acids. The adaptations that enhance fat usage in trained muscle can be divided into two categories: 1) those that improve fatty acid availability to the muscle and mitochondria and 2) those that improve the ability to oxidize fatty acids.

TL;DR Exercise speeds up the mitochondria's ability to burn fat and make more fatty acids "seen" by muscle to utilize.

Fatty acid breakdown
IMTAGs are a readily available substrate for energy during exercise because they are already located in the muscle. Trained athletes have an increased ability to use IMTAG efficiently during exercise (Shaw, Clark and Wagenmakers, 2010). Athletes also tend to have larger IMTAG stores than lean sedentary individuals. Overweight and obese individuals, interestingly, also have high levels of IMTAG but are not able to use IMTAGs during exercise like athletic individuals can (Shaw, Clark and Wagenmakers, 2010).
So what causes the reduced ability to use IMTAGs in obese individuals? A logical guess would be that they have dysfunctional mitochondria that cannot use fatty acid properly. Research has shown however, that the mitochondria from muscles of obese individuals are not dysfunctional (Holloway et al. 2008). Instead, the number of mitochondria per unit of muscle (mitochondrial density) is reduced in an obese population (Holloway et al. 2008). Reduced mitochondrial density is a more likely explanation for reduced ability to use fat for energy in obese individuals. An important adaptation to exercise training is increased mitochondrial density (Horowitz and Klein 2000; Zuhl and Kravitz, 2012). Increasing mitochondrial density would improve the ability to use fat and benefit individuals with fat loss goals.

Endurance exercise training is an effective way to improve the body's fatty acid usage abilities by improving the availability of fatty acids to the muscle and mitochondria and by increasing fatty acid oxidation (Horowitz and Klein, 2000). HIIT training has also been shown to result in similar fat burning adaptations while requiring fewer workouts and less total time commitment (Zuhl and Kravitz, 2012)

TL;DR You don't need to exercise to lose fat, but it will enhance the process. Obese people probably have much less ATP producing mitochondria than trained individuals = burn less. Cardio improves fat oxidation on a cellular level.

Tbh, I think that EPOC is overrated. However, I do think that the hormonal response that training gives the body is the prize. For example: resistance training (RT) "burn" less than cardio, yet it is the "recommended" training form here on MFP and many other places. Although muscle do burn slightly more than fat, the difference is minor looking at crude numbers. My theory is that the hormonal effects RT gives is more noticeable than cardio. And we have yet to figure out exactly how that works in comparison. Plus lookin' good naked and all that. It would explain the gap in "effects". However...the article you linked is a very good argument why all forms of training, including the much hated cardio, is probably a good idea.

This winter and spring I have consciously increased training volume and in "moderate" zone cardio by doing fast, hilly walking. I googled mitochondrial biogenesis sometime in December and found this:

(...) 6-week-old rats subjected to exercise 5 days per week for 3 months showed an increase in skeletal muscle cytochrome c concentration, as well as increased activities of key mitochondrial enzymes and OXPHOS (oxidative phosphorylation) [4]. Further studies established the same observations in humans [5, 6]. Another study compared exercise-trained individuals, elite distance runners and sedentary individuals and found a much greater percentage of oxidative, slow-twitch skeletal muscle fibres and more succinate dehydrogenase activity in runners than in sedentary controls [7]. (...)

In humans, a study examined the effect of caloric restriction on skeletal muscle mitochondrial function in non-obese humans and found that the expression of genes involved in the regulation of mitochondrial biogenesis, including PGC-1α, Tfam, eNOS and SIRT1, was increased in the caloric restriction group.(...) Moreover, fasting by itself induces AMPK in skeletal muscle [69], although caloric restriction seems less efficient.

Ofc, I'm doing numerous experiments very unscientifically at the same time, but I will say that I'm often eating way over "maintentance" for an approaching middle age, short lady with little muscle. I'm very surprised I haven't gained scale weight and I think that since daily fasting with eating window and high volume of exercise are two constants in my lifestyle, they greatly contribute to being able to eat more while maintaining. It works for ME and it doesn't feel like too much strain. It might not suit everyone. YMMV.

ELI5: Aerobic training 5x/week can improve your fat burning.

Edit: some minor changes, not meanings. Ofc, it could also be argued that I could be underestimating my exercise burns, leaving me in maintenance. I don't think so, but it is a possibility.

wabmester

Good article. It hit most of the adaptations that make exercise worthwhile:

• increased enzyme concentration
• increased mitochondrial biogenesis
• increased vasculature

Looking at some of his other articles, somebody should clue him in about how to speed-up the fat-adaptation process by restricting carbs.

GaleHawkins

Typically converting fat to ketones are mentioned when this subject is discussed. This fat burning info was new to me for the most part. Thanks for the link.

KittensMaster

I have been working over the last months just getting my body used to sourcing fat As fuel.

It has been interesting to gradually build up to being able to do a 40+ mile bike ride at 17+ mph on very few calories

I have been approaching getting into a keto state by just burning all my glycogen out.

It seems it works either by not eating carbs or eating a low amount and exercising off 2500 calories.

The body is an interesting and flexible thing.

I'm 54 and my calorie intake needs to be low around 1700 or so. But then when doing a long ride it goes over 4000. And I do find I can eat like mad, or let it come from fat.

The benefit of exercising is immediate and long term.

Sajyana

This is really interesting information. Thanks.

mlinton_mesapark

Great, highly detailed article. Thanks @KittensMaster!

saraphim41

SOLD!!