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As with "zero trans fat," a product can claim to be "sugar-free" if it has less than 0.5 grams of sugar per serving. But serving sizes can be miniscule, meaning when you ate that bag of sugar-free cookies, you really consumed 4 grams of sugar. Also, sugar-free foods can be loaded with carbohydrates such as maltodextrin, which raise blood sugar just the same. Or they may contain sugar alcohols such as mannitol or sorbitol, which have fewer calories than refined sugar but still add up fast.