And, even fruits and vegetables are limited because they, too, contain carbs. For most people, the keto diet requires making big shifts in how they usually eat. When you eat less than 50 grams of carbs a day, your body eventually runs out of fuel it can use quickly.In fact, 1 cup of raspberries provides only 7 grams of net carbs, as this serving size has around 15 grams of carbs and 8 grams of fiber . One cup of raw tomatoes contains about 7 grams of carbs and 2 grams of fiber .