The FastDay Forum

Drawing on the wise statement that carorees made on the Peter Attia lecture thread “Carbs are best eaten when glycogen stores are low (i.e., at breakfast/after exercise),” along with the accepted view that after eating, carbs are initially used to replenish glycogen stores and once the stores are replete, if there is still excess carbs in the blood then they get stored as fat, the rusty cogs in my grey matter have been turning.

I roughly recalled from my student days the view that to maximize the replenishment time of muscle glycogen post exercise it was best to eat within 2hrs ish of exercise as in this time glycogen synthase(?) was most active and if you didn’t feed within 2 hours it could take up to 48 hrs to replenish muscle glycogen. I think the follow paper may have been what that view was based on: http://www.ncbi.nlm.nih.gov/pubmed/3132449

The following link also talks more on the subject of glycogen replenishment: http://www.thesportjournal.org/article/ ... e-exercise
Am not sure on the details of liver glycogen replenishment , can’t find any useful references

The short of what I’m attempting to say is that it doesn’t look like the view that ingested carbs first go to replenishing glycogen stores post feeding is quite as simple as I first believed and that fat storage may still occur even though glycogen stores are not replete?

Cue: tumbleweed

Cue: tumbleweed

I was concentrating carefully on the biology/chemistry, then I read this. I really made me chuckle

I did used to have a brain, must see if I can find it, perhaps in a box in the loft?

[carorees]

There was a study on 24 hours of fasting in men that showed the insulin response to a CHO containing meal was blunted after fasting suggesting that glucose uptake is slightly impaired, which would mean that fat storage was also reduced, but how that might tie in with glycogen synthesis I don't know. I'd need to investigate the function of the different glucose transporters in the presence/absence of insulin.

However it does suggest that the CHO content of breakfast be low GI. By contrast, I believe that exercise increases GLUT4 activity in the muscle so enhancing glucose uptake. So the post exercise situation is different from post-fast.

(see my signature...Ben Goldacre sums it up nicely)

In my head the priority is that the stuff you just ate is used for fuel first - you burn at least a gram of carb equivalent per 5 minutes and if they're coming from a meal you just ate they aren't going into any sort of storage. Then comes glycogen replenishment, and if there's a surplus finally you get "de novo lipogenesis" (DNL) creating fat for storage.

I've read papers that say DNL is "not significant" in many people, so I'll go read your links properly.

Even if energy from carbs isn't stored as fat it's use as fuel impairs the use of fats from storage so the net effect is similar - no depletion of fat reserves while you're awash with carbs.

I did used to have a brain, must see if I can find it, perhaps in a box in the loft?

You've just given me an idea! Maybe that's where mine is too!

I read these things and start out OK but by the time I get to the end I've forgotten the beginning! If fasting is going to repair my cognitive function I wish it would flamin' well hurry up!