Unlike overdoses of many substances, sugar "overdoses" don't lead to toxicity reactions--though in extreme cases, they can cause dehydration, which has its own set of side effects. Instead, a sugar overdose leads to two separate problems: high blood sugar, and over-consumption of calories. Each of these are associated with side effects.
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One of the first effects most people notice upon consuming too much sugar is the feeling of a sugar "high," which involves a buzzing, caffeinated feeling combined with shaking of the hands or tremors in some individuals. The sugar high is the result of very high levels of blood sugar, notes Dr. Lauralee Sherwood in her book, "Human Physiology." Blood sugar, while it's necessary in certain concentrations to maintain normal cellular function, is a parameter that the body works very hard to maintain within strict ranges. As such, the body responds to very high blood sugar as an emergency situation, which partially contributes to the buzz or high feeling of a sugar overdose.
The second symptom, which generally follows a sugar high within an hour or less, is a sugar "crash." This is a result of the body's compensatory mechanisms working to decrease blood sugar levels to within normal physiological limits, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." The pancreas responds to high levels of blood sugar by releasing the hormone insulin, which signals cells to take up sugar from the blood. Sugar overdoses lead to pancreatic overreactions, which causes cells to take up far too much blood sugar. As such, a sugar overdose results in very high blood sugar, shortly followed by very low blood sugar, which causes fatigue, nausea, weakness and hunger.
Once cells have taken up excess sugar from the bloodstream, they can utilize the sugar to fulfill their own energy needs. A sugar overdose, however, results in cells taking up far more sugar than they actually need for their own purposes. As such, fat cells are free to convert excess sugar into fat, which is the body's primary energy storage medium. The reason the body converts sugar to fat, note Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry," is that fat takes up less room in the body and is lighter in weight than sugar. A sugar overdose generally leads to significant body fat storage.