We have extremely limited glucose storage as glycogen, or as circulating glucose.
Glycogen is a large branched polymer of glucose, stored mainly in the liver and the skeletal muscle. Stored glycogen can be mobilized to circulating glucose during fasting periods and to the muscle cells during muscle contraction, a process called glycogenolysis. The total body glycogen storage capacity, in a fasted healthy 70kg human, is 100 g in the liver, and 400 g in the muscle. The total available calorie output from glycogen reserves is approximately 2,000 calories.
Our blood stream normally carries less than 1 1/2 teaspoons of circulating glucose at any given time (less than 5 g of glucose circulates in the blood of a 70kg human). The total available calorie output from circulating glucose is 20 calories.
The exact circulating glucose calculation:
Average blood volume for an adult is 70ml/kg.
Assuming a 70kg male=4,900 ml=49 dl or 4.9l
1 liter is 10dl, therefore we have 50dl.
A nondiabetic glucose concentration is <100mg/dL.
Therefor 100mg/dl multiplied by 49dl equals 4900mg of circulating glucose.
4900mg equals 4.9 g of circulating glucose, where each tsp =4.2gm
Total circulating glucose=1.16tsp
As a comparison, 1 gummy bear contains 1.3g of usable sugar. Consuming 4 gummy bears could double the available circulating glucose, but for the fact insulin rapidly shuttles the glucose out of the circulation and into fat stores.
We have nearly unlimited fat storage capacity, and the primary role of insulin is to remove circulating glucose and convert it to fat storage. Circulating glucose rapidly oxidizes, undergoing a non-enzymatic Maillard reaction, which results in spontaneous glycation, which we measure as HgBA1C. The Maillard reaction was initially used to describe the browning and crisping of bread. This glycation effect changes the cell membranes to a less fluid status, interferes with protein and enzyme function, and often deforms receptor and channels. The end result of excessive glucose is sticky and nonfunctional cells.
Other advanced glycation end-products are measured as fructosamine. Excessive circulating glucose is also rapidly excreted through the kidney above levels of 180 mg/dl, creating excessive urination or polyuria. Glucose metabolism is tightly regulated because it can quickly oxidize and damage cell walls, wreaking havoc on our enzymatic pathways.
What is glycemic index?
The glycemic index (GI) assigns a score to food based on how high your blood sugar rises, compared to pure glucose (sugar). The GI assigns a score to how quickly glucose from a specific food enters the bloodstream. Foods are ranked on a scale of 0 to 100, with glucose having a value of 100. The lower a food’s glycemic index, the slower the blood sugar rises after eating that food. In general, cooking and processing foods makes the GI higher for that food. The higher the GI of a food, the greater the insulin release, and therefore the greater the conversion of circulating glucose to fat storage.
The rate of rise and peak glucose concentration after a meal is often expressed as Glycemic Index (GI).
Glycemic load (GL) measures both how quickly glucose enters the bloodstream (GI) and how much glucose per serving is present. Glycemic Load (GL) is a calculated number and gives a patient a better idea of the effect of a particular food based upon the typical serving size. Watermelon has a high glycemic index of 80, but a serving of watermelon has so little carbohydrate, that its total glycemic load is only 5.
When glucose levels drop, insulin production also drops, resulting in a conversion of stored fat to circulating ketone bodies and circulating triglycerides. Circulating ketone bodies are a fuel source, akin to glucose, that do not require insulin to access the cells.
The point of the discussion is that we have extremely limited circulatory capacity for glucose, about 5g glucose or about 20 calories only. Our constant consumption of processed carbohydrate rich food is inconsistent with our human physiology; which attempts to quickly removes glucose out of circulation to prevent tissue damage. The constant high exposure eventually overwhelms our clearance capacity, leading to metabolic inflammation, and eventual diabetes.
Wasserman D. H. (2008). Four grams of glucose. American journal of physiology. Endocrinology and metabolism, 296(1), E11-21. 10.1152/ajpendo.90563.2008