If you're familiar with Bodyopus or any of the other cyclical diets of this sort, you're probably wondering about the importance (or not) of ketosis, which I haven't mentioned at all. Ketosis is a metabolic state that occurs when fatty acid oxidation is ramped up to a very high degree, such that the liver is unable to use them all for energy. This occurs under conditions of total starvation, low-carbohydrate diets, and even extensive endurance exercise. Under those circumstances, fatty acids are only partially oxidized. This results in excess acetyl-CoA which is then converted to ketones which are released into the bloodstream. When ketones accumulate beyond a certain concentration, the body is said to be in ketosis. When ketosis occurs and blood glucose is low (as in the case of a low-carbohydrate/ketogenic diet), ketones become the preferred fuel in many tissues.
Now, ketones have a very important role in human physiology: to provide the brain with fuel when glucose is available in only low amounts. Recall from previous chapters that the brain can't use fatty acids for fuel; its primary fuel is glucose. When glucose becomes unavailable, the body needs an alternate energy source; that source is ketones. In fact, after 3 weeks of ketosis, the brain will derive nearly 75% of its total energy requirements from ketones; the remaining comes from glucose which is made in the body from amino acids, pyruvate, lactate and glycerol (from fat metabolism).
Depending on who you talk to, ketosis is either thought to be wonderfully beneficial or deadly. As with most extremist stances, the truth is somewhere in the middle. It's true that diabetic ketoacidosis, which only occurs in Type I (insulin dependent diabetics) can be lethal but this level of ketosis will never develop in non-diabetics. There are various feedback loops that prevent it. And, under certain conditions, ketosis has potential benefits.
One is protein sparing. Arguably the main reason the brain shifts to using ketones during starvation is to reduce its reliance on glucose; this means less body protein needs to be broken down to make glucose. In studies where protein intake is too low, ketosis may also be protein sparing. For the most part, if protein intake is adequate to begin with, I haven't seen any convincing data that ketosis has much of an additional benefit.
One problem may be that lean individuals can't make enough ketones to exert a protein sparing effect; this is a consequence of the difficulties in mobilizing fatty acids in the first place. Even during total starvation, when you'd expect ketosis to have the greatest impact, ketones aren't protein sparing in lean individuals (<15% bodyfat or so). Perhaps this is the shining moment for MCTs, by producing ketones in larger amounts, we can exert a protein sparing effect beyond simply providing quick fat energy. Assuming protein intake is sufficient in the first place, I still tend to doubt ketosis has any huge advantages in this regards. If it does, it simply hasn't shown up in real world experience. Bodyopus did generate better fat loss and less muscle loss for some people, but I suspect this had more to do with the carb-load.
A second argued advantage is that ketones are inefficient, that you'll lose more fat for a given calorie deficit in ketosis than without. The mechanism given is that one pound of fat converted to ketones doesn't provide the same energy as one pound of fat burned directly. This may be somewhat true but the difference is minor, amounting to a few percentage points at most. As well, except in very obese individuals, most tissue of the body aren't using ketones past the first couple of weeks; they are using fatty acids. There is also a small loss of ketones in the urine but this also tends to amount to very little. As with the protein sparing effect, increased fat loss from being in ketosis just hasn't shown up in the real world with lean athletes and bodybuilders.
For the most part, I simply see ketosis as a "side-effect" of fat loss (burning to be more accurate), more than something to be explicitly sought out. That is, when you accelerate fat oxidation with the methods above, you tend to enter ketosis. Ketosis in and of itself isn't any big deal. For that reason, I won't talk about monitoring ketone levels with Ketostix or anything like that. Frankly, using a low-carbohydrate/ketogenic diet for the fat loss phase of the UD2 has more to do with lowering insulin, raising catecholamines, and ramping up fat oxidation; ketosis is simply a tangential effect. A low-carbohydrate diet is also the only way to reduce calories as low as I'm going to suggest; there simply isn't caloric room for many carbohydrates in the fat loss phase of the diet.
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