Your own response to sugar is the best test of how much you can handle. In some people, sugar triggers mood swings - it brings on a rush of energy followed later by a "crash" into lethargy and depression. Others don't get the rush; they just feel logy and sleepy after consuming sugar. And, of course, some people don't notice any physical or mental effects at all.
In general, sugar is bad for the teeth (because it contributes to cavities). More specifically, diets high in sugar may predispose some people, especially women, to yeast infections, may aggravate some kinds of arthritis and asthma and may raise triglyceride levels. In people genetically programmed to develop insulin resistance, high-sugar diets may drive obesity and high blood pressure and increase risks of developing type 2 diabetes. Although conventional medical studies haven't shown that sugar causes hyperactivity in children, many parents are convinced that sugar does have that effect and that limiting sugar intake improves kids' behavior and attention. Recent research also indicates that sugar, rather than saturated fat, is the real culprit in America's high rates of cardiovascular disease.
I recommend cutting down or eliminating sugar if you experience mood swings, fluctuating energy levels, suffer from rheumatoid arthritis, are overweight or obese, have a family history of heart disease, or have frequent vaginal yeast infections. You may notice an improvement in your moods, a lessening of your arthritis symptoms and the frequency of yeast infections when you reduce or eliminate the sugar in your diet.
Even if you have no particular health or mood problem related to your sugar intake, realize that sugar's negative impact on health can slowly, insidiously accumulate over the years, and virtually all Americans consume too much. The best way to satisfy a sweet tooth is via foods in which the sugar is part of a whole food, such as in fresh or dried fruit, because the sugars are bound in a matrix of fiber that slows digestion and limits blood sugar spikes. No one needs foods made with copious amounts of added sugar.
Andrew Weil, M.D.