Thu, Mar-25-04, 21:59
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Butter Tastes Better
Posts: 5,201
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Plan: Atkins OWL / IF-23/1 /BFL
Stats: 100/100/100
BF:
Progress: 34%
Location: SF Bay Area
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Free Fatty Acids (FFA) and Ketones
Most tissues of the body can use FFA for fuel if it is available. This includes skeletal muscle, the heart, and most organs. However, there are other tissues such as the brain, red blood cells, the renal medulla, bone marrow and Type II muscle fibers which cannot use FFA and require glucose. The fact that the brain is incapable of using FFA for fuel has led to one of the biggest misconceptions about human physiology: that the brain can only use glucose for fuel. While it is true that the brain normally runs on glucose, the brain will readily use ketones for fuel if they are available.
Arguably the most important tissue in terms of ketone utilization is the brain which can derive up to 75% of its total energy requirements from ketones after adaptation. In all likelihood, ketones exist primarily to provide a fat-derived fuel for the brain during periods when carbohydrates are unavailable.
As with glucose and FFA, the utilization of ketones is related to their availability. Under normal dietary conditions, ketone concentrations are so low that ketones provide a negligible amount of energy to the tissues of the body. If ketone concentrations increase, most tissues in the body will begin to derive some portion of their energy requirements from ketones. Some research also suggests that ketones are the preferred fuel of many tissues. One exception is the liver which does not use ketones for fuel, relying instead on FFA.
By the third day of ketosis, all of the non-protein fuel is derived from the oxidation of FFA and ketones. As ketosis develops, most tissues which can use ketones for fuel will stop using them to a significant degree by the third week. This decrease in ketone utilization occurs due to a down regulation of the enzymes responsible for ketone use and occurs in all tissues except the brain. After three weeks, most tissues will meet their energy requirements almost exclusively through the breakdown of FFA. This is thought to be an adaptation to ensure adequate ketone levels for the brain. Except in the case of Type I diabetes, ketones will only be present in the bloodstream under conditions where FFA use by the body has increased. For all practical purposes we can assume that a large increase in FFA use is accompanied by an increase in ketone utilization and these two fuels can be considered together.
http://www.allyourstrength.com/nutrition_1002Fuel.html
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