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*If you suspect a heat-related illness, stop exercising and get out of the heat. Drink water, and wet and fan your skin. If you don't feel better within 60 minutes, contact your doctor. If you develop a fever higher than 102 F (38.9 C) or become faint or confused, seek immediate medical help.
the brain can switch to using ketone bodies. made from fatty acid breakdown instead of glucose and will maintain function like that, the rest of your body will use fatty acids for energy.
In brain, glucose utilization is obligatoryThe brain normally derives almost all of its energy from the aerobic oxidation of glucose, but this does not distinguish between preferential and obligatory utilization of glucose. Most tissues are largely facultative in their choice of substrates and can use them interchangeably more or less in proportion to their availability. This does not appear to be so in the brain. Except in some unusual and very special circumstances, only the aerobic utilization of glucose is capable of providing the brain with sufficient energy to maintain normal function and structure. The brain appears to have almost no flexibility in its choice of substrates in vivo. This conclusion is derived from the following evidence.Glucose deprivation is followed rapidly by aberrations of cerebral function. Hypoglycemia, produced by excessive insulin or occurring spontaneously in hepatic insufficiency, is associated with changes in mental state ranging from mild, subjective sensory disturbances to coma, the severity depending on both the degree and the duration of the hypoglycemia.
The brain utilizes ketones in states of ketosisIn special circumstances, the brain may fulfill its nutritional needs partly, although not completely, with substrates other than glucose. Normally, there are no significant cerebral arteriovenous differences for d-β-hydroxybutyrate and acetoacetate, which are “ketone bodies” formed in the course of the catabolism of fatty acids by liver. Owen and coworkers  observed, however, that when human patients were treated for severe obesity by complete fasting for several weeks, there was considerable uptake of both substances by the brain. If one assumed that the substances were oxidized completely, their rates of utilization would have accounted for more than 50% of the total cerebral oxygen consumption, more than that accounted for by the glucose uptake. [...]Under normal circumstances, that is, ample glucose and few ketone bodies in the blood, the brain apparently does not oxidize ketones in any significant amounts. In prolonged starvation, the carbohydrate stores of the body are exhausted and the rate of gluconeogenesis is insufficient to provide glucose fast enough to meet the requirements of the brain; blood ketone concentrations rise as a result of the rapid fat catabolism. The brain then apparently turns to the ketone bodies as the source of its energy supply.Cerebral utilization of ketone bodies appears to follow passively their concentrations in arterial blood . In normal adults, ketone concentrations are very low in blood and cerebral utilization of ketones is negligible. In ketotic states resulting from starvation; fat-feeding or ketogenic diets; diabetes; or any other condition that accelerates the mobilization and catabolism of fat, cerebral utilization of ketones is increased more or less in direct proportion to the degree of ketosis
I have a friend who is diabetic who informed me that ketosis over longer periods of time can be dangerous. I'm not quite sure how different body chemistry between diabetics and non-diabetics is though. Incidentally I just read an article about the ketogenic diet, not advised for anyone because it seems to increase risk of kidney stones, the patient consumes massive fat and almost no sugars.
The September 2002 issue Bicycling magazine talked about bonk training, where an individual purposefully bonks for a short period of time in order to loose weight -fast.