BLOG: Clarification on the carbohydrate controversy

Read more blogs from Jeffrey Anshel, OD.

These days, all we hear is the bad news about carbs: how they are in all of the tasty foods but cause us to get fat. But do we really know the details of what a carbohydrate is and why it acts the way it does? What about the “good” carbs?

First, a bit of biochemistry. A carbohydrate is an organic compound that consists only of carbon, hydrogen and oxygen, usually with a hydrogen:oxygen atom ratio of 2:1, such as in water.

Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as storage for energy (eg, starch and glycogen) and as structural components (eg, cellulose in plants and chitin in arthropods). The five-carbon monosaccharide, ribose, is an important component of coenzymes (eg, ATP, FAD and NAD) and is the backbone of RNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting and development.

Carbohydrate, the term, is most common in biochemistry, where it is a synonym of “saccharide.” The carbohydrates (saccharides) are divided into four chemical groupings: monosaccharides, disaccharides, oligosaccharides and polysaccharides. In general, the monosaccharides and disaccharides, which have a lower molecular weight than carbohydrates, are commonly referred to as sugars. In food science and in many informal contexts, the term carbohydrate often means any food that is particularly rich in the complex carbohydrate starch (such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found in candy, jams and desserts). Carbohydrates are a common source of energy in living organisms; however, no carbohydrate is an essential nutrient in humans.

Carbohydrates are not necessarily building blocks of other molecules; the body can obtain all its energy from protein and fats. The brain and neurons generally cannot burn fat for energy, but use glucose or ketones. Humans can synthesize some glucose from specific amino acids, from the glycerol backbone in triglycerides and in some cases from fatty acids. Carbohydrate and protein contain 4 kcal per gram, while fats contain 9 kcal per gram. In the case of protein, only some amino acids are usable for fuel.

Organisms typically cannot metabolize all types of carbohydrates to yield energy. Glucose is a nearly universal and accessible source of calories. Many organisms also have the ability to metabolize other monosaccharides and disaccharides, though glucose is preferred.

Polysaccharides are also common sources of energy. Many organisms can easily break down starches into glucose; however, most organisms cannot metabolize cellulose or other polysaccharides. These carbohydrate types can be metabolized by some bacteria. Cows and termites, for example, use microorganisms to process cellulose. Even though these complex carbohydrates are not very digestible, they represent an important dietary element for humans, called dietary fiber. Fiber enhances digestion, among other benefits.

Based on the effects on risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 45% and 65% of dietary energy from carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55% to 75% of total energy from carbohydrates, but only 10% directly from sugars — their term for simple carbohydrates.

Nutritionists often refer to carbohydrates as either simple or complex; however, the exact delineation of these categories can be ambiguous. The term “complex carbohydrate” was first used in the U.S. Senate Select Committee on Nutrition and Human Needs publication DietaryGoalsfortheUnitedStates (1977), where it was intended to distinguish sugars from other carbohydrates that were perceived to be nutritionally superior. However, the report put “fruit, vegetables and whole-grains” in the complex carbohydrate column, despite the fact that these may contain sugars as well as polysaccharides. This confusion persists as today some nutritionists use the term complex carbohydrate to refer to any sort of digestible saccharide present in a whole food where fiber, vitamins and minerals are also found, as opposed to processed carbohydrates, which provide calories but few other nutrients. The standard usage, however, is to classify carbohydrates chemically: simple if they are sugars (monosaccharides and disaccharides) and complex if they are polysaccharides (or oligosaccharides).

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In any case, the simple vs. complex chemical distinction has little value for determining the nutritional quality of carbohydrates. Some simple carbohydrates (eg, fructose) raise blood glucose slowly, while some complex carbohydrates (eg, starches), especially if processed, raise blood sugar rapidly. The speed of digestion is determined by a variety of factors, including which other nutrients are consumed with the carbohydrate, how the food is prepared, individual differences in metabolism and the chemistry of the carbohydrate.

The U.S. Department of Agriculture’s DietaryGuidelinesforAmericans2010 call for moderate to high carbohydrate consumption from a balanced diet that includes six 1-oz servings of grain foods each day, at least half from whole grain sources and the rest from enriched sources. That flies in the face of the Paleo Diet concept, which argues that our bodies were not designed to eat grains from our early Paleolithic days. This is a topic that I’ll cover in a future blog.

The glycemic index and glycemic load concepts have been developed to characterize food behavior during human digestion. They rank carbohydrate-rich foods based on the rapidity and magnitude of their effect on blood glucose levels. Glycemic index is a measure of how quickly food glucose is absorbed, while glycemic load is a measure of the total absorbable glucose in foods. The insulin index is a similar, more recent classification method that ranks foods based on their effects on blood insulin levels, which are caused by glucose, or starch, and some amino acids in food.

So, the bottom line on carbs is more related to the way they interact with glucose and insulin levels, as well as how they are processed in the body. If not “used” properly, they will be stored as fat. Thus, eating fat does not make you fat, but eating simple-sugar carbs can!