Glucose, or blood sugar, is a biomolecule very important to cellular survival. Body cells rely on glucose to fill many of their energy needs, and brain cells are almost completely reliant upon glucose. Like many physiological parameters, however, overly high blood glucose can be as detrimental as overly low blood glucose. Further, too much glucose in the system can lead to negative effects associated with overabundance of cellular energy molecules.
Normal levels of glucose in the blood are always a good thing---such normal levels have no negative effects on the body. However, rapidly rising levels of glucose in the blood lead to negative effects. For instance, if individuals eat meals very high in sugar or starch and low in fiber, protein or fat, their blood glucose will increase quickly and dramatically. This, explains Dr. Lauralee Sherwood in her book, "Human Physiology," causes a dramatic insulin release, which signals cells to pull glucose out of the bloodstream quickly. This leads to rapid hypoglycemia, or low blood sugar, and a "sugar crash," which most people find is accompanied by feelings of nausea and fatigue.
Most individuals' bodies don't allow blood sugar levels to rise too high---a normal pancreas secretes insulin when blood sugar levels rise. Diabetics, however, lack either the ability to produce insulin or the ability to respond to insulin, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." As such, in diabetics, blood glucose levels can become very high after a meal. High blood glucose has severe negative effects upon the body. It can lead to thirst, excessive urination, cellular starvation, tissue and organ damage, and even seizures. In cases of uncontrolled diabetes, the high blood sugar that follows a meal can even cause death.
In all humans, regardless of disease process, the body cells need a certain amount of energy to stay alive. If individuals eat less energy than their cells require, they will need to use energy stores to maintain cells. If they eat more energy than cells require, the body will store the extra energy as fat. Body cells can use fat or glucose to provide for energy needs, but fat is much lighter than glucose, so the body stores far more fat than it does glucose. If people eat more glucose than they need, the cells convert the extra glucose to fat, explain Drs. Mary Campbell and Shawn Farrell in their book, "Biochemistry." For individuals looking to avoid fat gain, the ability of the body to turn glucose into fat is a negative effect of glucose.