Question: How does sugar affect metabolism? My trainer told me not to eat fruit after 3 pm because it can have an adverse affect on metabolism.
Answer: While some trainers may have limited knowledge of nutrition, generally it’s best if they stick to what he or she is good at – training -- and leave nutrition to the experts. The advice you got regarding fruit has no basis in fact or physiology, so please disregard it.
Sugar has been vilified by the popular press but moderate amounts will not hurt any healthy person. In fact, the right amounts and kinds of “sugar” can be a positive contributor to the training-induced results of athletes and exercisers (for more info see Xtreme Muscle Stack: Creating the Perfect Anabolic Storm, Use Pre/Post Snacks to Maximize Performance, & Proper Hydration). But remember, too much of anything can be bad for you.
More info
Sugar is only guilty by its association with many people’s uncontrollable addictions to sweetness. Sugar is found in many popular foods that offer little nutritional value and little satiety relative to calorie content (think donuts). These foods are frequently eaten in excessive amounts in today's society. As such, sugary foods contribute an inordinately large number of calories to most people's diets. Here is the important point: sugar is associated with weight gain and other related health issues because people overeat, leading to weight gain or obesity (which causes its own problems that may be exacerbated by sugar), and much of the food commonly overeaten happens to contain sugar. Additionally, when eating a large portion of your calories from any group of less healthy foods, containing sugar or not, you have little room for nutritious items, which will also contribute to health problems. In other words, weight gain and health problems are not simply a result of eating sugar. Sugar is consumed in moderation is absolutely fine--especially in fruit. For more, see Fact or Fiction? Enduring Fitness & Nutrition Myths.