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When is Fasted Cardio Useful for Fat Loss?

Question: I’ve long seen it claimed that cardio has to be done first thing in the morning on an empty stomach (i.e. fasted cardio) for optimal fat loss, is this true?

Answer: This is probably one of the most commonly asked questions which is why it’s worth addressing. It’s worth keeping in mind that this idea usually comes out of the bodybuilding subculture, usually contest bodybuilders who, assuming their diet is working properly, are getting towards the low-end of body fat levels.  And the short-answer to your question is that the body fat of the person is going to be the main determinant of whether doing cardio fasted in the morning is important or not.

How is Bodyfat “Burned”?

To understand that, I need to cover a bit of background physiology, I’d mention that this is discussed in much more detail in both my Ultimate Diet 2.0 and The Stubborn Fat Solution for anybody who is truly interested in the topic.  But simply, there are three primary steps involved in ‘losing’ fat, they are:

  1. Mobilization
  2. Transport
  3. Oxidation (burning)

Mobilization refers to actually getting stored fat (specifically fatty acids) out of the fat cell; this process is under the primary control of insulin and the catecholamines although hormones such as growth hormone, cortisol and others play secondary or tertiary roles.

Transport refers to the actual transport of fatty acids (bound to albumin) within the bloodstream; this step can be an issue when folks are dealing with stubborn body fat (such as lower ab/low back fat in men and hip/thigh fat in women); blood flow is impaired in those areas.

Finally is oxidation which is the actual burning of fatty acids within tissues such as skeletal muscle, liver and heart.

The Impact of Bodyfat Percentage

Now, in lean individuals (where lean is around 12-15% body fat for men and about 19-22% for women), fat mobilization becomes a problem; blood flow is often an issue as well.  As folks get leaner, the body undergoes a series of adaptations that occur to make getting fat out of the fat cells more difficult.

For the most part, oxidation isn’t so much of a problem although there are strategies (such as skeletal muscle glycogen depletion) that can enhance the process; read my Ultimate Diet 2.0 for more details.

At the other extreme, that is in the very obese (here I’m talking about perhaps 35%+ body fat for men and 40%+ for women), the reverse problem is present.  There are tons of fatty acids floating around in the bloodstream, but for a variety of reasons, oxidation has become impaired.  To fully discuss this issue (along with approaches of fixing it) would require a full article an I won’t say much more about this group here.

And between those two extremes (so from about 15-35% body fat in men and ~20-40% body fat in women), there are really no issues.  Mobilization is usually not a problem since the body hasn’t started to fight back, transport isn’t an issue since stubborn fat isn’t being targeted, and oxidation is rarely a problem since the defects which show up at the extremes of obesity generally aren’t present.

When Can Fasted Cardio be Useful?

For the lean trying to get very lean (15% body fat or less for men, 22% or less for women), various strategies, including fasted cardio are probably going to be required to offset the mobization and blood flow defects.

That’s why that specific group found decades ago that fasted morning cardio worked best.  And why I wrote The Stubborn Fat Solution since it deals with how to overcome all of the problems.

But for folks who aren’t that lean yet, the folks in the middle range of body fat levels, it really doesn’t matter.  The best time to do cardio will be whenever it will most consistently get done.  If that’s first thing in the morning, fantastic.  If not, also fantastic.  It’s more important in this situation that it gets done than when it gets done.

Again, for the extremely obese, different strategies entirely are required but, again that would take a full article to address so I won’t talk about it here.

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