How does cardio decrease fat metabolism /efficiency?

[Martin Brown]

Out of curiosity? Is there a direct impact that long duration sub-maximal "cardio" has upon our ability to utilse fat?

[NU_nutrition_TS]

Not so much on our ability to utilise fat - we do that all the time at rest! But on our propensity to store more fat - probably.

If you do long-duration cardio at an intensity that preferentially allows the energy to come from mainly - or exclusively - fat (the so-called 'fat-burning' zone) then you are sending a signal to your body that it should store more fat for such future exertions.

It sounds counter-intuitive because the opposite - short but intense bursts of activity (like HIIT) - uses much more glucose than fat but the by-product of anaerobic glycolysis builds up in the cytosol and continues to be cleared long after the exercise session has ended (the so-called 'after-burn' effect). Added to which, if you limit dietary carbohydrate (especially sugars), you have to replace any lost glycogen via gluconeogenesis, often using the lactic acid built up during the intense exercise as a substrate and fatty acids as a fuel for the gluconeogenic process. These are the reasons given for this type of short, intense burst/rest exercise being more effective for overall fat loss and conditioning than regular LISS cardio.

So why does the latter result in more fat loss while the former encourages more fat storage? Assuming the observation is correct, I'm not sure but I can take an educated guess - in the latter situation fatty acid oxidation follows glucose/glycogen usage and continues/ticks along 'in the background' for an extended period (some sources claim 48-72 hours), in much the same way as it would at rest anyway, while in the former case we never work up to an intensity that would greatly use or exhaust glucose/glycogen so immediately calls upon fatty acid oxidation as a direct fuel source for physical activity - thus creating a greater need for its replacement above and beyond what we may already carry.

This is my understanding of the research out there - either may be in error - but I hope it made some semblance of sense!

[James]

Interesting topic/thoughts.

I feel that carb cycling/resistance training somewhat softens the blow of LISS and possible fat storage, the old 'shock the body' theory seems to run true in my experience using these protocols, especially LISS. I have had my best results using a near zero carb approach on rest days where LISS is preformed (fasted state, of course), as this is forcing the body to use fat for both basal functioning as well as expenditure from the exercise. Come training days, I hit the carbs quite a bit, exercise preformed on these days is obviously anaerobic so the body is somewhat having a bit of a 'recovery' from solely using the fat burning pathways in terms of heavy activity.

After having used LISS most days for months, and becoming very adapted to it, progress has seemed to slow (with of course can be attributed to less bodyfat too also lose) as the body is more efficient at performing the exercise and utilizing fuel sources for it - thus as NU has said above, possibly 'sending signal to your body that it should store more fat for such future exertions'; although, I believe from my experiences from what I lay out above has a very positive impact.

I feel like this post is very dis-jointed/doesn't make much sense, I hope there is a good point in there, somewhere!

[NU_nutrition_TS]

^Yes, I think we are both struggling to make our points clearly and succinctly but intuitively know what it is we want to say!

I agree that LISS (here I am thinking more in terms of brisk 30 minute walks rather than 45-60 minutes spent on a treadmill or stepper!) when allied with some form of high intensity activity, such as the classic HI resistance workout followed by a brisk 30 minute walk, does give you the best of both worlds while limiting the negative adaptations associated with LISS alone. Like you, I think prevailing diet and fasting/meal timing have an input here too.

Even though HIIT-type activities of short duration are supposed to burn around 9 times the calories of LISS done over a longer duration (most of it in the hours to days following the exercise) there is not such a strong signal to store greater amounts of fat because a greater proportion of those calories are coming from glucose (either directly or via liver/muscle glycogen stores) than fat (while exercising). The fat stores are utilised during the recovery period (when glycogen stores are topped up*) so it's usage is not directly tied (in time) to the physical activity itself.

* This is where prevailing diet/meal timing/fasting comes into the equation. If we are limiting dietary carbohydrate or fasting, it is more likely the glucose/glycogen will come from gluconeogenesis, which requires the oxidation of fats to fuel it.

Hopefully that is a slightly clearer, less wordy version of what I have already said!

[James]

Yes! That is a lot clearer, and what I was trying to lean towards, thank you

[Elvis]

Fat Burning: using body fat instead of carbohydrates as fuel

[Martin Brown]

Thanks guys for the responses. I agree with you Nu about the encouragement to store bodyfat because of the longer duration mid-intensity aerobic work.

I can recall reading somewhere about the metabolic changes due to longer duration aerobic work that impair fat oxidation but can't think where? Maybe I'm imagining it.

I'm giving another seminar on Weds on the right exercise to get in shape - obviously I'm not a fan of aerobics/run clubs/core classes, so the more info I have against it the better I have some fairly good examples of real life people being in worse shape due countless extra hours of training each week, and have most of the science to back it up.

Thinking of it, can you recall the studies that showed cardio had no impact upon weight loss?


Elvis - isn't that link a good example of what got us in a mess in the first place? The whole notion of training within a "fat burnign zone"?

[Elvis]

Quote

Elvis - isn't that link a good example of what got us in a mess in the first place? The whole notion of training within a "fat burnign zone"?

I think the information as a whole is useful (edit: and it indicates that cardio actually increases ability to metabolise fat). I think the picture people are missing is that cardio for leaness (rather than fat burning) is a poor deal.

The reason being it does little to retain lean body mass (as it provides a low stimulus for muscle growth), if we maintain a consistent weight, without increasing our lean body mass (which cardio is not very effective at doing), we will never get any leaner.

Alternatively if we reduce our bodyweight whilst doing cardio alone, we will lose fat, but we will also start to lose muscle (as we are not replacing it as fast as we are losing it, because the body has insufficient stimulus to do so). Long distance runners don't approach lean until they are at pretty minimal BMI.