When a person has diabetes, either their body does not produce enough insulin, or it cannot use the insulin correctly, so glucose accumulates in the blood. High levels of blood glucose can cause a range of symptoms, from exhaustion to heart disease.

One way to control blood sugar is to eat a healthful diet. Generally, foods and drinks that the body absorbs slowly are best because they do not cause spikes and dips in blood sugar.

The glycemic index (GI) measures the effects of specific foods on blood sugar levels. People looking to control their levels should pick foods with low or medium GI scores.

A person can also pair foods with low and high GI scores to ensure that a meal is balanced.

Below are some of the best foods for people looking to maintain healthy blood sugar levels.

1. Whole wheat or pumpernickel bread

Many kinds of bread are high in carbohydrates and quickly raise blood sugar levels. As a result many breads should be avoided.

However, pumpernickel bread and 100% stone-ground whole wheat bread have low GI scores, at 55 or less on the GI scale.

Pumpernickel and stone-ground whole wheat breads have lower GI scores than regular whole wheat bread because the ingredients go through less processing.

Processing removes the fibrous outer shells of grains and cereals. Fiber slows digestion and helps to stabilize blood sugar levels.

In a 2014 study, researchers reported that spelt and rye both caused low initial glycemic responses in rats. They also found that these ancient wheat types, as well as emmer and einkorn, suppressed genes that promote glucose metabolism.

2. Most fruits

Except for pineapples and melons, most fruits have low GI scores of 55 or less.

This is because most fruits contain lots of water and fiber to balance out their naturally occurring sugar, which is called fructose.

However, as fruits ripen, their GI scores increase. Fruit juices also have very high GI scores because juicing removes the fibrous skins and seeds.

A large 2013 study found that people who consumed whole fruits, especially blueberries, grapes, and apples, had significantly lower risks of developing type 2 diabetes.

The researchers also reported that drinking fruit juice increased the risk of developing the condition.

3. Sweet potatoes and yams

Regular potatoes have a high GI score, but sweet potatoes and yams have low scores and are very nutritious.

Some research indicates that the flesh of the sweet potato contains more fiber than the skin, indicating that the whole vegetable could be beneficial for those with diabetes.

Reporting the findings of an animal study, the researchers also noted that sweet potato consumption may lower some markers of diabetes.

While there is still no conclusive evidence that sweet potatoes can help to stabilize or lower blood sugar levels in humans, they are undoubtedly a healthful, nutritious food with a low GI score.

People can substitute sweet potatoes or yams for potatoes in a variety of dishes, from fries to casseroles.

4. Oatmeal and oat bran

Oats have a GI score of 55 or lower, making them less likely to cause spikes and dips in blood sugar levels.

Oats also contain B-glucans, which can do the following:

• reduce glucose and insulin responses after meals
• improve insulin sensitivity
• help maintain glycemic control
• reduce blood lipids (fats)

A 2015 review of 16 studies concluded that oats have a beneficial effect on glucose control and lipid profiles in people with type 2 diabetes. Determining the impact of oat consumption on type 1 diabetes requires more research.

Doctors still recommend that people with diabetes limit their consumption of oatmeal because 1 cup contains roughly 28 grams of carbohydrates.

5. Most nuts

Nuts are very rich in dietary fiber and have GI scores of 55 or less.

Nuts also contain high levels of plant proteins, unsaturated fatty acids, and other nutrients, including:

• antioxidant vitamins
• phytochemicals, such as flavonoids
• minerals, including magnesium and potassium

A 2014 systemic review concluded that eating nuts could benefit people with diabetes.

As with other foods in this article, it is best to eat nuts that are as whole and unprocessed as possible. Nuts with coatings or flavorings have higher GI scores than plain nuts.

6. Legumes

Legumes, such as beans, peas, chickpeas, and lentils, have very low GI scores.

They are also a good source of nutrients that can help maintain healthy blood sugar levels. These nutrients include:

• fiber
• complex carbohydrates
• protein

A 2012 study found that incorporating legumes into the diet improved glycemic control and lowered the risk of coronary heart disease in people with type 2 diabetes.

Avoid legume products that contain added sugars and simple starches, such as those in syrups, sauces, or marinades. These additions can significantly increase a product's GI score.

7. Garlic

Garlic is a popular ingredient in traditional medicines for diabetes and a wide variety of other conditions.

The compounds in garlic may help reduce blood sugar by improving insulin sensitivity and secretion.

In a 2013 study, 60 people with type 2 diabetes and obesity took either metformin alone or a combination of metformin and garlic twice daily after meals for 12 weeks. People who took metformin and garlic saw a more significant reduction in their fasting and post-meal blood sugar levels.

People can eat garlic raw, add it to salads, or use it in cooked meals.

8. Cold-water fish

Fish and other meats do not have GI scores because they do not contain carbohydrates.

However, cold-water fish may help manage or prevent diabetes better than other types of meat.

A 2014 study included data taken from 33,704 Norwegian women over a 5-year period. The researchers found that eating 75–100 milligrams of cod, saithe, haddock, or pollock daily reduced the risk of developing type 2 diabetes.

However, the researchers were uncertain whether the reduction in risk was a direct result of eating the fish or whether other healthful lifestyle factors, such as exercise, could have influenced the findings.

9. Yogurt

Eating plain yogurt daily may reduce the risk of type 2 diabetes.

Authors of a large 2014 meta-analysis concluded that yogurt may be the only dairy product that lowers the risk of developing the condition. They also noted that other dairy products do not seem to increase a person's risk.

Researchers are still unsure why yogurt helps lower the risk of type 2 diabetes.

However, plain yogurt is generally a low-GI food. Most unsweetened yogurts have a GI score of 50 or less.

It is best to avoid sweetened or flavored yogurts, which often contain too much sugar for a person looking to lower their blood sugar levels. Greek-style yogurt can be a healthful alternative.

Other ways to lower blood sugar levels

Eating a healthful, well-balanced diet is key. Additional strategies to help lower or manage blood sugar levels include:

• staying hydrated by drinking plenty of clear liquids
• exercising regularly
• eating small portions more frequently
• not skipping meals
• managing or reducing stress
• maintaining a healthy body weight or losing weight, if necessary

People with diabetes may also need to take medications and measure their blood sugar regularly to reduce the risk of potentially dangerous symptoms and complications.

Speak with a doctor about how to incorporate a healthful diet into a diabetes care plan.