Grapes are incredibly good for you, giving you a variety of different vitamins, minerals and even fiber. It is possible, however, to eat too many grapes. Always pre-portion your grapes, instead of nibbling right out of the bag. Otherwise, you might experience negative side effects. If you're allergic to grapes, you might even have problems simply by coming into contact with them.
Sure, grapes are relatively low in calories. One full cup, which is about 30 grapes, has fewer than 105 calories. The issue is, however, that grapes are easy to pop in your mouth. If you sit down with a bag of grapes and turn on the TV — before you know it — you could eat most of the bag. Suddenly, your 105-calorie snack doubles or triples in calories, eventually giving you the same number of calories you'd get from an entire meal. If you eat large portions of grapes on a regular basis without first measuring your portion size, the additional calories could cause you to gain weight.
You need carbohydrates in your diet. They convert to your body's main source of energy — glucose. Carbohydrates should make up 45 to 65 percent of all the calories you consume, according to the Dietary Guidelines for Americans. That's 900 to 1,300 calories from carbs or 225 to 325 grams daily, based on a 2,000-calorie diet. That 1-cup serving of grapes has more than 27 grams of carbs. If you're snacking on grapes freely and not paying attention to your serving size, you could quickly consume more than your daily carb allotment. You'll also throw off your balance of macronutrients, meaning that if your carb intake is high, your protein and fat intake may be lacking.
You'll get a decent dose of fiber from grapes — roughly 1.5 grams from 1 cup. That's probably not enough to cause any disruption in your gut. If you snack on a large serving of grapes, however, you increase your fiber intake. If you don't regularly consume a lot of fiber, you could notice an uncomfortable rumbling in your tummy after devouring a large, fiber-rich portion of grapes. Because your body isn't used to the fiber, it becomes difficult to pass stools, which is a sign of constipation. Sometimes, extra fiber has the opposite effect, however, leaving you with diarrhea, as your system tries to expel the additional fiber.
It's not common to have a grape allergy, although it can happen. If you're allergic to grapes, you might get hives or red patches on your skin by touching grapes or shortly after eating them. In severe cases, you might have difficulty breathing or go into anaphylactic shock. Just because you have an allergic reaction to grapes, however, doesn't necessarily mean that you're allergic to the fruit itself. You may actually be allergic to the pesticide on the grapes, or to the yeast or mold that grows on the grapes. The only way to be certain what you're allergic to is to undergo allergen testing from your physician's office or via a referral to a testing center.