A person that weigh’s 70kg has 4 grams of glucose that circulates in his or her blood. When a serious drop in blood glucose occurs it can be characterised as metabolic dysfunction that could lead to a seizure or death in extreme cases. Likewise, a persistent elevation in blood glucose can cause glucose toxicity that can lead to complications of diabetes.
Blood glucose levels are maintained through glycogen reservoirs that are stored in the liver (100g) and in muscle (400g). To recap, a 70kg person has 4 grams of glycogen in his or her blood and 500 grams of reserves on average. As blood glucose is so important, the body is protected from marked deviations by a sophisticated control system that is in place.
When a high carbohydrate meal is consumed it causes increased insulin secretion. It is very important to note that insulin curb the entry of glucose from the liver to the blood and stimulates glucose removal from blood into muscle, liver, and fat. Blood glucose deviations are dampened in this way. Notice here that instead of glucose flowing from the liver to the blood and then to the muscles, glucose is flowing from blood to the muscles, liver and fat stores. So, high-carb meals could be OK if it was just enough to replenish the depleted stores in the blood, liver and muscles. However, anything more than what was used / burned in the form of exercise or daily activities would be stored as fat.
Muscle glucose uptake is increased during exercise. When this happens, one’s liver produces glucose, again keeping deviations in blood glucose to a minimum. So when you are exercising and consuming carbs, the muscles will get its glucose from the blood and glucose will not enter the blood from the liver, but when you are exercising and not eating, the muscles will still receive its glucose from the blood, but the blood will receive its supply of glucose from the liver.
Liver glucose reserves protect the levels of glucose in the blood. However the need for blood glucose during exercise is diminished as muscle glycogen breakdown is used as a source of fuel and impedes the removal of glucose from the blood. Remember the 400g of glucose in the muscles?
We know that meals that contain high amounts of carbohydrates will start a reaction that will ultimately stop the liver from releasing glucose into the blood. This seems bad for weight loss as we want to burn our stored body fat and the liver’s role in supplying ketones is key in burning fat. See http://blogsa.co.za/blog/2019/02/08/what-is-keto/
We also know that insulin stimulates the removal of glucose from blood into muscle, the liver and fat. Well if muscle reserves are already topped up and if the liver is topped up and the release from the liver is stopped by the increase of insulin, then we only have our fat stores left for the excess glucose to go. So if we eat meals that are high in carbohydrates and we do not perform exercise to create “space” for glycogen within our muscle reserves through energy expenditure (burning calories), then our bodies must gain more fat as the only place left for the glucose to go to, is to the fat stores.
After reading this article I am certain that you would agree that high-carb meals must be accompanied by exercise that uses / burns at least the same amount of glycogen as what is consumed so that all depleted stores can be replenished, but if we were to consume anything more, then we will most likely gain weight.
If you are interested in the actual science, you can follow this link from the American Journal of Physiology https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2636990/#r5