To quote Lyle McDonald:

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.

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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.

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.

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.

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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.

And that provides the answer:
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.

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 mobilization and blood flow defects. That’s why that specific group found decades ago that fasted morning cardio worked best.

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