I want to talk about long runs, but find I have to talk about sugars first.
There are onlt two sugars you need to know about, glucose and fructose. Table sugar (cane sugar, sucrose) gets converted in your body into one of each. Honey is slightly more than half fructose, the rest glucose. Most fruits contain just fructose (hence the name), the exceptions being dates, figs, canteloupe and watermelon. Starches are long chains of glucose; if the starches aren't digestible they're called fiber. Maltodextrin, found in many preparations for runners, is the simplest starch, made of three glucose units.
Glucose gets absorbed into the bloodstream very quickly. The glucose from starches takes a little longer (potato and corn starch almost immediately). What isn't needed by the body at once is converted to the storage starch glycogen and, when the body decides it has enough glycogen, what's left over is made into fat.
Fructose gets absorbed much more slowly and only 5% gets converted into glycogen under the best of conditions, so fat cells are adapted to using it when available.
Red blood cells use only glucose for fuel and the brain, under non-starvation or diabetic situations, uses only glucose. This is why the body needs to store glycogen in the liver (and to an extent, the kidneys). Muscles can use a number of different fuels, but store glycogen for anaerobic conditions. If there is no sugar available for the brain, the body starts breaking down proteins, which can be converted to a small degree into fuel.
What can go wrong
Eating a lot of carbohydrates quickly causes the blood sugar level to rise quickly. This leads to a large amount of insulin to be released to get the sugar level back to normal. This can then lead to too much sugar being removed from the blood for a while. This is an insulin reaction, or "sugar crash," which is very serious in diabetics and a major nuisance to runners, causing overwhelming tiredness. Visual disturbances, drunken behaviour and mood changes are common.
Muscles can be depleted of their glycogen. This is "hitting the wall." Once this happens in a run, there's no turning back the clock (though, if it happens to me in a 100 miler or 24 hour run, I'd be tempted to down every carb I can find and wait two hours, to see if it helps).
The liver can be depleted of it's glycogen, which is called "bonking." The physical characteristics are like the insulin reaction, but can be reversed by consuming sugars. Bicyclists are prone to this and not hitting the wall; apparently, it's a matter of bicycling using mostly large muscle groups whereas running uses a lot of little muscles for balancing and those run out of glycogen first.
The reason this is important is that one can manipulate the amount of glycogen stored in the body and hence, run longer. This is done through carboloading (which I won't go innto here) and by intentionally running until one's muscles are depleted. Muscles that are regularly depleted of glycogen adapt by storing more.
Fisher's Big Wheel
1 week ago