Amy P. Campbell, MS, RD, CDE, and Richard S. Beaser, M.D.
The week’s excerpt answers the following questions
- Does using the "glycemic index" work?
- Is the type of carbohydrate important?
- Carbohydrate counting vs. the glycemic index and glycemic load
- How much carbohydrate do we really need?
- What percentage of consumed carbohydrates are converted to glucose?
- What is the caloric range for weight loss and recommended level for weight maintenance?
- Does insulin may you fat?
Food types or nutrients can be divided into two categories, macronutrients and micronutrients. To understand how to utilize these nutrients in a medical nutritional treatment plan requires a basic understanding of the nutritional roles of each. Macronutrients — carbohydrate, protein and fat — will be covered in detail here; the micronutrients, vitamins and minerals, will be discussed later.
As noted above, in 1994, the ADA revised its nutritional recommendations, stating that there is no standard ADA or diabetic diet! A meal plan must be individualized to each person’s personal eating style and metabolic needs. In fact, for many people with diabetes, the dietary guidelines are in essence the same as those that would be recommended as a healthy nutritional plan for most adults.
Carbohydrate is the major source of energy for the body’s needs. It is the major constituent of the "starchy" foods such as breads, cereals, grains, and pasta. These polysaccharide carbohydrates are referred to as complex carbohydrates, as compared with the refined or simple mono- and disaccharide carbohydrates like sugar. Carbohydrate is also the main component of "sugary" foods, such as cake, cookies, candy, table sugar, milk, fruits and vegetables.
In the past, the recommendation for people with diabetes was to consume primarily complex, or starchy, carbohydrates. The assumption was that these sources of carbohydrate were more slowly absorbed, and thus were present at a time that was more closely coordinated with either the second-phase insulin secretion of a patient with type 2 diabetes or the regular insulin action pattern for someone treated with exogenous insulin therapy.
Today, however, much more is known about carbohydrate. Research shows that eating 50 grams of carbohydrate from a sugar, such as maple syrup, has the same effect on blood glucose as eating an equivalent amount of carbohydrate from a starch, such as bread. In fact, more than twenty research studies show that when individuals choose a variety of foods containing either starches or sugars in meals, if the total amount of carbohydrate is the same, the glucose response will be essentially the same. A key education point for patients first learning about diabetes meal planning is that because foods containing either starches or sugars are digested into glucose at approximately the same rate, it is important to control the total amount (and not the type) of carbohydrate consumed. Of course, good nutrition principles prevail, and the message regarding sweet foods should still be one of moderation, as these foods are often high in fat and calories and provide little nutritional value.
The concept of the "glycemic index" was developed by staff at the University of Toronto and shows how certain food affects blood glucose levels. A more precise definition of glycemic index (GI) is: a system of ranking foods containing equal amounts of carbohydrate according to how much they raise blood glucose in comparison with a reference food (50 grams of glucose or 50 grams of bread).
The glycemic index of a carbohydrate food is determined by assigning that food a number from 0 to 100, where 100 means that 1 gm of carbohydrate from this food raises the blood glucose to the same level as 1 gm of carbohydrates from bread. In other words, foods with a lower GI have less of an effect on blood glucose than do foods with a higher GI. Low GI foods are ranked between 0 and 50; intermediate GI foods are ranked from 56 and 69, and high GI foods are ranked 70 or higher. Some foods are surprisingly fairly low on the glycemic index: the glycemic response of sucrose, for example, resembles more closely that of rice or potatoes.
Fruits and milk (sugars) produce a much lower glycemic response than starches. Even M&Ms, the chocolate-coated candy, have a lower glycemic index than other, more healthful foods, including pasta.
To further complicate matters, many factors can affect the glycemic index of a food, including how it is prepared and in what form it is eaten. Furthermore, the glycemic index can be challenging for patients to apply to their daily food choices because foods are compared with one another not in usual portions but in equivalent amounts of carbohydrate. For example, a pound and a half of carrots and one cup of pasta each contain 50 grams of carbohydrate, and this amount is used to determine their GI even though it is very unlikely that anyone would consume one and a half pounds of carrots at one time.
Because of the difficulty of relating GI with portions size, some researchers suggest using another approach, called the glycemic load (GL). The GL combines the GI value and the carbohydrate content of an average serving of a food or meal, and is calculated by multiplying the GI number of a food by the number of grams of carbohydrate in a serving and then dividing by 100. A GL of 10 or less is low; 11–19 is medium; and 20 or more is high.
The American Diabetes Association concludes in its evidence-based nutrition recommendations that research does not support the glycemic index as a primary method of meal planning for people with diabetes. Recent research also casts doubt on the effectiveness of this approach as an effective meal-planning tool; instead, the primary determinant of the postprandial glucose response is the amount of carbohydrate consumed. Nevertheless, several randomized clinical trials have shown that low GI diets do reduce glycemia in people with diabetes. For people consuming a high GI diet, changing to a low GI diet can improve postprandial hyperglycemia. Not all carbohydrates have the same effects on blood glucose levels. Thus, while it is not necessary to eliminate potatoes, it is important for people with diabetes to choose a variety of carbohydrate foods each day and to understand the differences among the various choices.
The glycemic index and glycemic load may be beneficial for people with diabetes, and this view is in accord with Joslin’s Nutrition Guideline, but these tools should be used only as adjuncts to other meal planning methods, such as carbohydrate counting. People adjusting their rapid-acting insulin based on carbohydrate intake can actually develop their own glycemic index by carefully counting carbohydrate grams and monitoring blood glucose levels before and after meals. The use of self-monitoring of blood glucose (SMBG) or continuous glucose monitoring (CGM), are key for individuals with diabetes to determine their own postprandial response to foods.
How much carbohydrate do we really need? Before insulin was discovered in 1921, the diets recommended by Joslin physicians and others treating people with diabetes were high in fat, high in protein, and low in carbohydrate. This made sense to those physicians — diabetes is a condition in which patients cannot metabolize carbohydrate, so remove carbohydrate from the diet! These diets were not unlike those advocated today in some commercial diet plans for weight reduction like "South Beach" or the "Atkins" program.
Our understanding of the nutritional needs of people with diabetes has come a long way since those early days. We know that about 100% of consumed carbohydrates are converted to glucose and serve as the main source of energy in our diet. Carbohydrate sources are bread, pasta, rice, cereals, fruit, milk, table sugar and sweets.
An analysis of the current low-carbohydrate/high-protein diets advocated by some reveals that the caloric range for weight loss is from 1000 to 1600 kcals. There is also a recommended calorie level for weight maintenance of 1800 kcals. Of course these diets work, they contain fewer calories! What is not discussed in these diet plans is their ability to fit it into a healthy lifestyle. Are they practical? Are there food limitations? Are they providing enough vitamins and minerals that are known to aid in keeping good health? What impact do these diets have on increasing the risk of coronary artery disease?
The fact is that low-carbohydrate/high-protein diets cause ketosis, electrolyte loss and dehydration. They may exacerbate kidney disease and gout, and may cause calcium depletion. Because some of these commercial plans promote the eating of highly saturated fat foods, they also may contribute to coronary heart disease. While these diets may be a short-term fix, they are not ideal for long-term health. In addition, people with diabetes who also have kidney, liver or heart disease, or who are pregnant or lactating should not follow a very low-carbohydrate/high-protein diet.
The ADA no longer recommends that a specific percentage of calories come from carbohydrate; however, it does recognize in its nutrition recommendations that, while the RDA for carbohydrate is 130 grams per day and is an average minimum requirement, 1-year follow-up data from a weight-loss trial showed that fasting glucose was lower in those following a low carbohydrate diet compared with those following a low fat diet. The source and distribution of carbohydrate calories among foods with differing glycemic indices is secondary in concern to the total carbohydrate content. Nevertheless, unrefined, unprocessed carbohydrate foods should be used whenever possible. Joslin Diabetes Center’s Clinical Nutrition Guideline for Overweight and Obese Adults with Type 2 Diabetes, Prediabetes or at High Risk for Developing Type 2 Diabetes recommends approximately 40% of calories from carbohydrate, the total not to be less than 130 grams per day, in accordance with the Recommended Dietary Allowance. This modest decrease in carbohydrate may improve postprandial blood glucose levels and enhance weight loss by utilizing stored fat for energy without causing ketosis or dehydration.
After digestion and absorption into the bloodstream, carbohydrate has three key destinations, and insulin is important for all three to be reached.
- used to provide for immediate energy needs
- stored as glycogen, primarily in liver and muscle, to serve as a rapidly accessible energy supply (e.g., source of glucose for rebound hyperglycemia or fuel for muscle undertaking sudden activity)
- converted to fat, an almost unlimited potential storage space that can be used when glycogen stores are filled
Insulin must be present for glucose to take any of the three pathways described above, including the storage of fat in adipose cells. As a result, many patients think that insulin makes you fat. Of course, this is not true! Insulin is non-caloric! However, insulin, when given to a person with previously uncontrolled glucose levels, reduces calorie loss through glycosuria, can temporarily promote edema, and, when not balanced properly with food intake, can cause hypoglycemia, necessitating excess food consumption. Proper insulin use, balanced in a physiologic manner with carbohydrate intake, should not lead to excessive weight gain, although in the short run some increase in weight may occur.
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