Foods containing carbohydrates have the greatest impact on blood sugar levels, according to the University of Illinois Extension. Fruit, milk products, cereals, legumes and starchy vegetables are examples of carbohydrate-containing foods, reports MedlinePlus.
Blood sugar levels increase after someone eats a carbohydrate-containing food, prompting the pancreas to produce insulin, a hormone that tells the cells to absorb sugar from the blood. Once cells start to absorb the sugar, blood sugar levels decrease, explains the Harvard School of Public Health.
Three factors affect how quickly blood sugar levels increase and how much of a spike occurs: portion size, food composition and timing. Eating large portions causes blood sugar to rise more than eating small portions does. Food composition refers to the contents of the food. Juice, milk and other carbohydrate-containing liquids cause blood sugar levels to increase more quickly than bread and other carbohydrate-containing solid foods do, reports the University of Illinois Extension. Eating meals and snacks at the right time is also important for preventing dangerous blood sugar spikes.
The glycemic index is a measurement of the effect a carbohydrate-containing food has on blood sugar levels. Foods with high GI values raise blood sugar more than foods with low or medium GI values, states the American Diabetes Association. Instant oatmeal, bagels, pretzels, melon, pumpkin and Russet potatoes all have high GI values.