Although grapes are a nice healthy snack in moderation, too many of them can result in someone's body absorbing too many calories and taking in too many carbohydrates and fiber, all of which can have negative effects on health. Instead, it is best to diversify the type of fruits and vegetables someone eats.
One cup of grapes has approximately 110 calories, 29 grams of carbohydrates and 1.4 grams of fiber. If a person eats an excess of grapes, he or she could end up causing an imbalance of nutrients. This would likely lead to weight gain, because a large percent of the energy in grapes comes from carbohydrates in the form of sugars.