When you eat two foods from the same macronutrient group, such as carbohydrates, they don't necessarily digest in your body at the same speed. This is because carbohydrates' structures vary widely, from simple to complex. The more complex the structure of the carbohydrate, the more slowly it's digested, and vice versa. The rate of digestion determines whether a carb is "fast" or "slow."
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Grains and Grain Products
Grain products vary widely in composition and in rate of digestion, but a simple way to differentiate in this group is between refined grains and whole grains. Refined grains have been highly processed, which removes many of their nutrients and much of their fiber. The amount of dietary fiber in a food affects its rate of digestion because it slows the movement of food through the digestive tract. Foods with more fiber digest more slowly. These include whole grains like brown rice and whole-grain breads and pastas. White breads, rice and pastas, on the other hand, are fast-digesting carbs.
Fruits, Vegetables and Beans
Fruits, veggies and beans are rich sources of dietary fiber, which means they are slow carbs. Although fruit contains simple carbohydrates like fructose, which your body digests and absorbs very quickly, the amount of fiber fruit contains offsets that component. Beans are also a rich source of protein, which digests more slowly than carbohydrates. Their carbohydrate and protein contents combined make them a very slow-digesting source of carbs.
Cakes, Cookies and Snack Foods
Cakes, cookies and snack foods are usually made from refined grains, which are low in fiber and therefore digest quickly. They often also have added sugar, which the body digests quickly and delivers to the bloodstream. Although snack foods such as potato chips are made from a vegetable, they are highly processed, which makes them easily digestible by the body. Potatoes are also an exception to the vegetable rule, because even in their whole form they are digested rather quickly.
Milk, Cheese and Yogurt
Although dairy products are often included in the protein food group, and also contain fats, they are a source of carbohydrate. However, it's not as easy to label this group as "fast" or "slow." Dairy products, like fruit, contain a simple sugar called lactose that the body digests and absorbs quickly. But they don't contain fiber like fruit does. The sugars in dairy products are likely to have more of an effect on blood sugar than those from fruit. However, dairy products are a good source of protein, which helps slow their digestion. When choosing dairy products, avoid those with added sugars, which makes them a less nutritious source of carbs.