Lecture4 - The Low-Carb(Ketogenic Diets History and...

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The Low-Carb (Ketogenic) Diets History and Clinical Implications Stephen Phinney, MD, PhD Prof Emeritus, Univ of CA at Davis
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Starvation Adaptation : Insulin falls (but not to zero), allowing net fatty acid release from adipocytes Liver increases ketone production and balances gluconeogenesis to maintain normal blood glucose levels Gluconeogenesis uses amino acids from protein breakdown, plus glycerol from TG lipolysis “prolonged starvation” means more that 5-7 days until ketone production stabilizes, regulated by insulin. Human Ketone Metabolism
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Feeding Your Head Your brain burns 600 kcal/day whether you play tournament chess, sleep, or watch TV re-runs. Because the brain burns only glucose or ketones, but ketones stay low unless dietary carbs are restricted, there’s a potential problem with most “balanced” weight loss diets. Example: A 1200 kcal diet providing 75 grams protein (300 kcal), 300 kcal as fat (25%), and 600 kcal carbs (50%). This looks kind of like “Weight Watchers”.
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Feeding Your Head The 600 kcal as carbs in this 1200 kcal diet is enough to feed the brain, and about half of the 75 grams of protein can be made into carbs by gluconeogenesis (another 100 kcal or so). But what happens if our person decides to go run 5 miles, burning 500 kcal, half of which will come from glucose/glycogen. Suddenly there’s not enough glucose to “go around”. To compensate, either gluconeogenesis needs to go up or carb use down, but ketones stay low. This gluconeogenesis occurs at the expense of tissue protein (ie, loss of lean body mass). What typically happens is the person stops exercising or eats more (or both). To not eat more in this setting risks “bonking”.
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The Physiology of Bonking Also called “hitting the wall”, bonking occurs with the body acutely runs out of food for the brain. Typically after 1-3 hrs of intense endurance exercise without on-going carb intake, the following occurs: Food fantasies (intrusive thoughts of food) Chills and shakes (adrenaline) Dramatic loss of performance Profound depression Even in a very lean athlete (10-15% body fat), the body still has more than 50,000 kcal of fat reserves plus 20- 30,000 kcal protein when a bonk happens.
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