Carbohydrates and Blood Sugar

When people eat a food containing carbohydrates, the digestive system breaks down the digestible ones into sugar, which enters the blood.

  • As blood sugar levels rise, the pancreas produces insulin, a hormone that prompts cells to absorb blood sugar for energy or storage.

  • As cells absorb blood sugar, levels in the bloodstream begin to fall.

  • When this happens, the pancreas start making glucagon, a hormone that signals the liver to start releasing stored sugar.

  • This interplay of insulin and glucagon ensure that cells throughout the body, and especially in the brain, have a steady supply of blood sugar.

Carbohydrate metabolism is important in the development of type 2 diabetes, which occurs when the body can’t make enough insulin or can’t properly use the insulin it makes.

  • Type 2 diabetes usually develops gradually over a number of years, beginning when muscle and other cells stop responding to insulin. This condition, known as insulin resistance, causes blood sugar and insulin levels to stay high long after eating. Over time, the heavy demands made on the insulin-making cells wears them out, and insulin production eventually stops.

Glycemic index

In the past, carbohydrates were commonly classified as being either “simple” or “complex,” and described as follows:

Simple carbohydrates:

These carbohydrates are composed of sugars (such as fructose and glucose) which have simple chemical structures composed of only one sugar (monosaccharides) or two sugars (disaccharides). Simple carbohydrates are easily and quickly utilized for energy by the body because of their simple chemical structure, often leading to a faster rise in blood sugar and insulin secretion from the pancreas – which can have negative health effects.

Complex carbohydrates:

These carbohydrates have more complex chemical structures, with three or more sugars linked together (known as oligosaccharides and polysaccharides).  Many complex carbohydrate foods contain fiber, vitamins and minerals, and they take longer to digest – which means they have less of an immediate impact on blood sugar, causing it to rise more slowly. But other so called complex carbohydrate foods such as white bread and white potatoes contain mostly starch but little fiber or other beneficial nutrients.

Dividing carbohydrates into simple and complex, however, does not account for the effect of carbohydrates on blood sugar and chronic diseases. To explain how different kinds of carbohydrate-rich foods directly affect blood sugar, the glycemic index was developed and is considered a better way of categorizing carbohydrates, especially starchy foods.

The glycemic index ranks carbohydrates on a scale from 0 to 100 based on how quickly and how much they raise blood sugar levels after eating. Foods with a high glycemic index, like white bread, are rapidly digested and cause substantial fluctuations in blood sugar. Foods with a low glycemic index, like whole oats, are digested more slowly, prompting a more gradual rise in blood sugar.

  • Low-glycemic foods have a rating of 55 or less, and foods rated 70-100 are considered high-glycemic foods. Medium-level foods have a glycemic index of 56-69.

  • Eating many high-glycemic-index foods – which cause powerful spikes in blood sugar – can lead to an increased risk for type 2 diabetes, (2) heart disease, and overweight

  • Foods with a low glycemic index have been shown to help control type 2 diabetes and improve weight loss.

Glycemic load

The food’s glycemic index does not tell us how much digestible carbohydrate – the total amount of carbohydrates excluding  fiber – it delivers. Hence researchers developed a related way to classify foods that takes into account both the amount of carbohydrate in the food in relation to its impact on blood sugar levels. This measure is called the glycemic load. In general, a glycemic load of 20 or more is high, 11 to 19 is medium, and 10 or under is low.

Here is a listing of low, medium, and high glycemic load foods. For good health, choose foods that have a low or medium glycemic load, and limit foods that have a high glycemic load.

Low glycemic load (10 or under)

  • Bran cereals

  • Apple

  • Orange

  • Kidney beans

  • Black beans

  • Lentils

  • Wheat tortilla

  • Skim milk

  • Cashews

  • Peanuts

  • Carrots

Medium glycemic load (11-19)

  • Pearled barley: 1 cup cooked

  • Brown rice: 3/4 cup cooked

  • Oatmeal: 1 cup cooked

  • Bulgur: 3/4 cup cooked

  • Rice cakes: 3 cakes

  • Whole grain breads: 1 slice

  • Whole-grain pasta: 1 1/4 cup cooked

High glycemic load (20+)

  • Baked potato

  • French fries

  • Refined breakfast cereal: 1 oz

  • Sugar-sweetened beverages: 12 oz

  • Candy bars: 1 2-oz bar or 3 mini bars

  • Couscous: 1 cup cooked

  • White basmati rice: 1 cup cooked

  • White-flour pasta: 1 1/4 cup cooked (15)

Here’s a list of the glycemic index and glycemic load for the most common foods.

References:
Beulens JW, de Bruijne LM, Stolk RP, et al. High dietary glycemic load and glycemic index increase risk of cardiovascular disease among middle-aged women: a population-based follow-up study. J Am Coll Cardiol. 2007;50:14-21.
Halton TL, Willett WC, Liu S, et al. Low-carbohydrate-diet score and the risk of coronary heart disease in women. N Engl J Med. 2006;355:1991-2002.
Anderson JW, Randles KM, Kendall CW, Jenkins DJ. Carbohydrate and fiber recommendations for individuals with diabetes: a quantitative assessment and meta-analysis of the evidence. J Am Coll Nutr. 2004;23:5-17.
Ebbeling CB, Leidig MM, Feldman HA, Lovesky MM, Ludwig DS. Effects of a low-glycemic load vs low-fat diet in obese young adults: a randomized trial. JAMA. 2007;297:2092-102.

https://www.hsph.harvard.edu

Afsi Felsher