10 March 2009

Why a carb-free diet?

OK, let’s talk about what happens when on a carb-free diet. Normally the diet would not be recommended. Here's why:

Without carbs the liver can do fine using fatty acids converted to triacylglycerols and phospholipids for oxidation in hepatocyte mitochondria (1p167), but the brain and muscles want glucose and, thus, gluconeogenesis occurs using up lactate and alanine and eventually breaking down protein to use amino acids (1p97-98).

Ketones, at a point of starvation of glucose, can end up being an undesired fuel for brain and muscle, but this can be accompanied by a dangerous acid-base balance known as ketosis (1p160). Because of direct oxidation of fatty acids through the TCA cycle, the acetyl CoA can end up in an “overflow” pathway promoting ketone bodies like acetoacetate, beta-hydroxybutyrate and acetone (1p159).

If one wishes to prevent diabetes, however, a carb-free diet may be helpful. According to a German study on mice, a carb-free diet eliminated the hyperglycemia that would normally develop into beta-cell failure, although the mice became morbidly obese (2). Another study confirmed the results: a carb-free diet reduced sensitivity to insulin (3).

In other situations where it is vital to avoid hyperglycemia, a carb-free diet may also be used. An example is after a severe head injury, when possibility of ischemia may occur, because in such cases, carbs may worsen the neurological outcome (4).

Reference List

1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.

2. Mirhashemi F, Kluth O, Scherneck S, et al. High-fat, Carbohydrate-Free Diet Markedly Aggravates Obesity but Prevents β-Cell Loss and Diabetes in the Obese, Diabetes-Susceptible db/db Strain. Obesity Facts 2008;1:292-297. Available at: http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowPDF&ArtikelNr=000176064&Ausgabe=244083&ProduktNr=233731&filename=000176064.pdf.

3. Bainbridge HW. The reduced sensitivity to insulin of rats and mice fed on a carbohydrate-free, excess-fat diet. J Physiol 1925;60:293-300.

4. Ritter AM, Robertson CS, Goodman JC, Contant CF, Grossman RG. Evaluation of a Carbohydrate-Free Diet for Patients with Severe Head Injury Journal of Neurotrauma. August 1996, 13(8): 473-485. doi:10.1089/neu.1996.13.473.

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