Carbohydrates are the sugars and starches that provide the fuel for the human body. These substances are found in many foods and are broken down during digestion into a simple sugar called glucose (blood sugar). Once glucose is absorbed into the bloodstream, a hormone in the body called insulin moves it into the cells, where it is converted to energy.

Carbohydrates also are the source of a substance called glycogen. This fuel reserve is stored in the liver, fat and muscles and can be broken down into glucose when the body does not receive enough carbohydrates from outside food sources.

People with diabetes either do not produce insulin (type 1 diabetes) or cannot properly use the insulin they do produce (type 2 diabetes). As a result, the level of glucose in the blood is prone to quickly rising too high (hyperglycemia). Conditions including insufficient consumption of carbohydrates can also cause patients’ blood glucose to fall too low (hypoglycemia).

Severe episodes of acute hyperglycemia or hypoglycemia can result in a diabetic coma, and chronic hyperglycemia can cause many complications ranging from heart conditions to diabetic retinopathy. Diabetic individuals can help prevent glucose irregularities by controlling the amount and type of carbohydrates they consume.

The body gains energy by breaking down three major classes of foods. These groups, known as macronutrients, are:

• Carbohydrates.
  
  
• Fats. Fats are a major source of energy for the body, with each gram providing nine calories. Types include saturated, monounsaturated and polyunsaturated. Saturated fats are the so-called “bad” fats found in animal and dairy products. Harmful, artery-clogging artificial trans fats are unsaturated fats processed to act like saturated fats.
  

• Proteins. Proteins provide energy as a last resort when other energy sources are depleted. The main job of proteins is to build and repair cells.  

In addition, many alcoholic beverages including beer, wine and cocktails contain carbohydrates.  If a physician approves the use of alcohol, these carbs must be included in meal planning.

Carbohydrates are broken down and converted to glucose, which is the fuel for the body’s activities. Such activities include autonomic functions such as breathing, digestion and heartbeat, and forms of exertion such as running and lifting. In addition, glucose is normally the sole fuel source for the brain.

Carbohydrates are divided into two classes:

• Complex carbohydrates (starches). These include whole-grain breads and cereals, legumes and starchy vegetables that provide nutrition in the form of vitamins, minerals and fiber. Fiber, in carbohydrates such as oatmeal, protects starchy carbohydrates from digestive enzymes. This slows the absorption of glucose from the small intestine into the bloodstream, which can be beneficial to diabetic individuals trying to control their blood glucose levels. Fiber can also help control cholesterol. However, people who have gastroparesis, a type of autonomic neuropathy that delays digestion, may need to limit fiber.
  
  
• Simple carbohydrates (sugar). These are found in milk products, fruit, candy, soda and other sweets. Fruit and milk products are important sources of valuable nutrients, vitamins and minerals. Candy, soda and sweets provide few or no nutrients. Though these carbohydrates help provide fuel for energy, they are “empty calories” with no nutritional value. An excess intake of simple carbohydrates can lead to undesired weight gain and obesity, a major risk factor for several forms of diabetes.

Each type of carbohydrate contains four calories per gram. Gram for gram, simple and complex carbohydrates eventually add the same amount of glucose to the bloodstream. However, complex carbohydrates are broken down into glucose more slowly than simple carbohydrates. Complex carbohydrates provide important sources of essential nutrients for a good diet. People with diabetes are usually advised to favor the slower-acting complex carbohydrates, as they lessen the likelihood of rapid rises in blood glucose.

Some carbohydrates have been highly refined (for example, white bread, cake and cookies), and may be considered neither simple carbohydrates nor complex carbohydrates. For example, grains can be classified as whole grains or refined grains.

A grain is made up of:

• Bran (outer shell with fiber, B vitamins and minerals)
• Germ (next layer, with nutrients such as fiber, essential fatty acids and vitamins)
• Endosperm (center of the grain, containing the starch)

Whole grains are complex carbohydrates that include all three elements, but refined grains contain only the endosperm, or starch. Whole grains are therefore far more nutritious than refined grains, which provide energy but few nutrients.

The further a food is away from its natural state, the less nutritious it is. Healthy carbohydrate choices in appropriate amounts are vital for nutrition.

Studies have shown that controlled-carbohydrate diets can improve control of glucose and blood pressure and may decrease insulin resistance. However, diets very low in carbs can cause problems including headaches, constipation, muscle cramps and fatigue.

Eating fewer than 130 grams of carbohydrates a day can cause ketosis, according to the National Institutes of Health (NIH). Ketosis is a state in which the body, deprived of carbohydrates to use for fuel, burns fats instead. Though ketosis promotes weight loss, it can also lead to gout and kidney stones and may be especially problematic for diabetic patients, people with kidney disease (e.g., diabetic nephropathy) and pregnant women, according to the NIH.

A dangerous condition called diabetic ketoacidosis (DKA) involves severe hyperglycemia and ketosis. Dietary ketosis may contribute to DKA, but DKA is typically triggered by causes such as a missed dose of insulin or a severe illness.

Some proponents assert that low-carb diets can help prevent type 2 diabetes, but all individuals interested in trying such plans, including those with prediabetes, a family history of diabetes or other risk factors for diabetes, are advised to consult their physician first.

Diabetic patients are advised to establish a meal plan with the help of their physician and a registered dietitian. They should also ask their physician how often to perform glucose monitoring and ketone tests. During illness, people with diabetes typically need to eat their usual amount of carbohydrates. A factor that can affect consumption of carbs is exercise.