I've been trying to understand the answers to these questions since I recorded my blood sugar readings last weekend while running a half marathon distance. You can check out that post for the details, but here's the chart:
I fully expected blood sugar levels to drop throughout the run. In a keto-adapted state from a low carbohydrate diet, I assumed fat would be my primary fuel as muscle glycogen levels should have been relatively low. I don't know the ratio of fuels from fat vs. glucose, but I was surprised that blood glucose levels were consistently higher from the baseline inactive start. If I wasn't getting much glucose from dietary carbohydrates, then where was that glucose coming from that caused the higher readings? The search for an answer resulted in googling a lot of words starting with the letter "g".
It's probably safe to say, or even perhaps obvious to say that the increased blood glucose was a hormonal response. Running is an exercise that quickens the heart rate, causes heavy breathing and sweating. In short, it's stressful. Adrenal glands respond to stress by releasing epinephrine (adrenaline) into the bloodstream. This triggers the secretion of the hormone glucagon from the pancreas.
Glucagon is the opposite hormone of insulin. Insulin acts to lower blood sugar, glucagon acts to raise blood sugar. Glucagon works to raise blood sugar by two mechanisms. First, it converts stored glycogen in the liver into glucose and releases it into the bloodstream for fuel. This process is called glycogenolysis. Second, as glycogen stores are depleted in the liver, glucagon then synthesizes glucose from noncarbohydrates like lactate, glycerol (the backbone of fatty triglycerides) and amino acids (proteins). This process is called gluconeogenesis.
Besides glucagon, another hormone released from adrenal glands in response to stress is cortisol. Cortisol also increases blood sugar by gluconeogenesis. Research has shown that cortisol enhances glucagon stimulation of gluconeogenesis. I found one ultra running site that suggested that cortisol doesn't affect the body until about 15 miles of running. If this is accurate, then my increased blood sugar from 3 miles on would be only due to the effects of glucagon, and most likely from the gluconeogenesis mechanism as my store glycogen levels were relatively low.
I especially like Wikipedia's description of gluconeogenesis, describing it as a highly endergonic process. Dang, there's another new word I had to Google:
By thermodynamic standards, work, a form of energy, is defined as moving from the surroundings (the external region) to the system (the internal region). Thus, an endergonic process, as contrasted with an exergonic process, is one wherein the system absorbs energy from the surroundings. As a result, during an endergonic process, energy is put into the system.
Doesn't that sound like a smart, efficient way to fuel a run?
Leave it to Jimmy Moore to explain gluconeogenesis in his special way:
Getting back to the original question, got sugar? In the diet I don't think sugar is necessary. The liver makes sure our body has glucose. If body tissues are in a glycogen depleted state, then the mechanism is gluconeogenesis: converting fats and proteins to carbohydrates.
Asked a different way, should dietary sugar be avoided? I think so. Carbohydrates put glucose into our bloodstream and elevate hormonal insulin levels. If muscle tissues are insulin resistant, then that glucose is stored as fat. No runner wants to be fat, it slows em down.
But you might say, "Kelly, I eat plenty of carbs and don't get fat." God love ya for being glucose tolerant. However, I would still suggest that reducing the carbohydrates in the diet will adapt the body to fuel with fat instead of the limited storage of glycogen. Speaking for myself, I have plenty of fat storage to fuel a run.
But you might also say, "Kelly, I run a lot further distances than a half marathon, try 100 miles." I don't have a response to this, yet. I plan on conducting more running trials with blood sugar monitoring, and my goal is to discover what happens to my blood sugar at longer distances. But I have never run a distance longer than the marathon, and I'm not sure I plan to. I'll leave that experiment to some other guinea pig.