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DEAR DR. ROACH: I’m a 77-year-old man who’s prediabetic. I was told by a longtime diabetic that sugar (candy, soda, cake, etc.) doesn’t need to be avoided — just carbohydrates such as pasta, potatoes, etc. I was surprised to hear this. Is it really true? — Anon.
ANSWER: We often use terms like carbohydrates, starches and sugars imprecisely. Strictly speaking, sugars are small molecules (usually 5 or 6 carbon molecules long), like glucose or fructose, which are called monosaccharides. These can also be combined into a single molecule (a disaccharide), which, in the case of glucose plus fructose, is called sucrose (table sugar).
Long strands of sugars that are strung together are called polysaccharides. Starches are very long strands of glucose molecules, with anywhere from hundreds to tens of thousands of them in strings and branches. White rice and processed wheat flour (the primary ingredient in most pastas and many white breads) are largely comprised of starch.
Starch begins turning back into sugar even before you swallow it. An enzyme in saliva (amylase) breaks down the starch structure. White bread made from highly refined flour turns into sugar so rapidly that your blood sugar goes up nearly as fast from bread as it would from eating the same amount of sugar.
Starch and sugars are both carbohydrates, as is fiber, but fiber is indigestible to us. Fiber acts to slow down absorption, reducing the “spike” in blood sugar that occurs with refined starch. However, most foods that we eat are combinations of two or three macronutrients (the sources of calories), which are carbohydrates, proteins and fats.
Protein and fat both slow down the absorption of carbohydrates, similarly to fiber. Whole wheat flour contains fiber and protein, which is why it’s preferred to processed white flour and doesn’t spike the blood sugar as quickly or as high. Most vegetables have lots of fiber, so even though they are mostly carbohydrates (with a little protein), they aren’t treated by your body in the same way that starchy foods are.
Partially because of the slower absorption, people tend to feel more full with fewer calories when protein, fat and fiber are included. Even so, proportions matter. A diet of 80% rice, 10% protein and 10% fat isn’t going to be optimal for any person, especially one with diabetes.
People with diabetes don’t need to completely avoid starches or even simple sugars. They’re an important part of the diet, and as long as they form a modest proportion of a person’s food intake and are consumed as part of a mixed meal, people shouldn’t feel bad about eating them.
Many of my patients with diabetes use a continuous glucose monitor (CGM). By keeping track of what you eat, these monitors can give you immediate feedback on how your food affects your blood sugar. Talk to your doctor about whether a CGM is right for you. People on insulin, people who have low blood sugar (especially if they don’t notice it), and people who are changing their diet and exercise patterns are among those who might benefit from a CGM.
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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.