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Tuesday, March 23, 2010

Carbohydrate restriction regulates the adaptive response to fasting

Carbohydrate restriction regulates the adaptive response to fasting

The importance of either carbohydrate or energy restriction in initiating the metabolic response to fasting was studied in five normal volunteers. The subjects participated in two study protocols in a randomized crossover fashion. In one study the subjects fasted for 84 h (control study), and in the other a lipid emulsion was infused daily to meet resting energy requirements during the 84-h oral fast (lipid study). Glycerol and palmitic acid rates of appearance in plasma were determined by infusing [2H5]glycerol and [1-13C]palmitic acid, respectively, after 12 and 84 h of oral fasting. Changes in plasma glucose, free fatty acids, ketone bodies, insulin, and epinephrine concentrations during fasting were the same in both the control and lipid studies. Glycerol and palmitic acid rates of appearance increased by 1.63 +/- 0.42 and 1.41 +/- 0.46, respectively, during fasting in the control study and by 1.35 +/- 0.41 and 1.43 +/- 0.44, respectively, in the lipid study. These results demonstrate that restriction of dietary carbohydrate, not the general absence of energy intake itself, is responsible for initiating the metabolic response to short-term fasting.

I've felt for a long time that LC is equivalent to "starvation mode".  It is interesting that NEFA/FFA were the same in both the fasted group and that getting the lipid infusion.  This would indicate to me that "dietary" fat in and of itself does not stimulate further lipolysis.  It would be interesting to see if the result would differ if the fats were ingested.


Todd said...

Is it fair to compare a LC diet to the lipid study diet here and equate LC to starvation mode when the lipid study diet had zero protein? Seems more appropriate to compare this to the Atkins Fat Fast not normal low carb.

CarbSane said...

Hi Todd, when I say LC puts you in "starvation mode" it is because all of the adaptive mechanisms that kick in during longer fasts are the very same ones that do with severe carb restriction. Increased NEFA (fat mobilization), insulin resistance, ketones enhanced, gluconeogenesis and glyceroneogenesis enhanced, de novo lipogenesis decreased. What the lipid study did was to see if adding nutrition in the form of fats modified the adaptation (e.g. just the body sensing calories). It didn't. An amino acid only infusion would be another interesting study to have done.

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