Difference between complex and simple carbs

Difference between simple and complex carbohydrates

Pawel Malczewski
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Short summary

In the biochemical context, a simple carbohydrate has 1 or 2 carbohydrate units while a complex carbohydrate is made up of 3 or more. These terms are also used in a nutritional sense but are not consistent among the sources, are ambiguous and have no practical use. For a quick answer click here.

Explanation

The category of simple and complex carbohydrates can refer to a biochemical definition or have a nutritional context.

The biochemical definition refers to a simple difference of the molecular structure described below.

Nutritional context of simple and complex carbohydrates

The nutritional definition was established in 1977 by the U.S. Senate Select Committee on Nutrition and Human Needs. (1)FAO 1998. Carbohydrates in human nutrition. (FAO Food and Nutrition Paper – 66). Chapter 1 – The role of carbohydrates in nutrition. Report of a Joint FAO/WHO Expert Consultation. Rome, 14-18 April 1997. Available here.

It uses the same terminology as in biochemistry and was created to distinguish unhealthy simple sugars (simple carbs) from other, complex carbs, considered at the time as healthy, such as whole grains, fruit and vegetables.

The usage of the term “complex carbohydrates” has been widespread in recent decades and can now, depending on the source, refer to a wide range of carbohydrates, with the exception of refined sugars.

What is now being realized now is that the meaning of this grouping of complex carbs is too vague to use in the nutritional sense for a number of reasons:

  • it doesn’t distinguish between healthy and unhealthy carbohydrates as it was originally intended;
  • there is no agreement as to what is included and what is excluded from the definition, even between official sources.

In other words, the commonly used definitions of complex carbohydrates in the nutritional sense present several points of view, sometimes conflicting, but most of the time with shortcomings, since:

1. Some foods considered as complex carbohydrates are not as healthy as was intended by this definition. For instance, some complex carbs:

  • have a high GL and exert similar metabolic effects on the body as simple carbs (e.g. potatoes vs sugar). (read more..) The most common foods in this category include tubers such as potatoes and refined or unrefined grains such as white or brown rice, wholegrain and white flour breads.
  • are empty calorie, refined foods, such as white bread, white rice or white flour pasta, which have been stripped of most nutrients and contain mostly glucose in the form of starch.

2. Definitions are conflicting depending on the source:

  • Some definitions consider that the prerequisite for naming complex carbs is the presence of starch while others include non-starchy fruit and non-starchy vegetables in this category.
  • Some sources refer to complex carbs only as plant foods that come from whole fruit, whole vegetables, and legumes and other sources consider foods with any fiber or starch contents as complex carbs.

Despite being ambiguous and varying in meaning between official sources, many health professionals and lay-publications still use this classification in a nutritional sense, adding to the confusion.

Separating carbohydrates into simple and complex has no use in nutrition. The first reason is because the biochemical definition doesn’t have any practical use, since it doesn’t consider the health implications of carbohydrates. The second reason is because of the confusing way it has been used, as previously explained.

Difference between simple carbohydrates and complex carbohydrates – Biochemical definition:

1. Simple carbohydrates (mono or di-saccharides) consist of:

  • one molecule of a single sugar called monosaccharide, such as glucose, fructose, galactose or xylose, or;
  • a pair of two single sugars bonded together called disaccharide, such as sucrose, lactose or maltose.

Simple carbohydrates can occur naturally (e.g. in fruit as sucrose, fructose and glucose, or in milk as lactose), or be available as refined sugars (e.g. purified sucrose, agave syrup or high fructose corn syrup).

2. Complex carbohydrate (polysaccharides):

Consists of a chain of 3 or more sugars (called polysaccharide), such as starch, cellulose (insoluble fiber) or pectin (soluble dietary fiber) (2)Zhao J. Turning Waste Into Food: Cellulose Digestion. Available here.

The following image illustrates various types of carbohydrates that exist in food. Carrots contain them all and are a good visual example.

Different types of carbs in food

Conclusion

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In the biochemical sense, the difference between simple and complex carbohydrates is very straight forward and relates to the different number in the carbohydrate units.

Please note that, since the term complex carbohydrates includes a wide variety of polysaccharides, such as starches, dietary fiber and oligosaccharides, it is often more practical in biochemistry to refer to carbohydrates by their proper names rather than a vague group of “complex carbohydrates”. (3)FAO 1998. Carbohydrates in human nutrition. (FAO Food and Nutrition Paper – 66). Chapter 1 – The role of carbohydrates in nutrition. Report of a Joint FAO/WHO Expert Consultation. Rome, 14-18 April 1997. Available here.

In the nutritional sense, the classification of complex vs. simple carbs is too ambiguous since the definition of complex carbs encompasses a large variety of foods that have different effects on our health (positive as well as negative) and the definitions are conflicting between various sources.

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References   [ + ]

1. FAO 1998. Carbohydrates in human nutrition. (FAO Food and Nutrition Paper – 66). Chapter 1 – The role of carbohydrates in nutrition. Report of a Joint FAO/WHO Expert Consultation. Rome, 14-18 April 1997. Available here.
2. Zhao J. Turning Waste Into Food: Cellulose Digestion. Available here.
3. FAO 1998. Carbohydrates in human nutrition. (FAO Food and Nutrition Paper – 66). Chapter 1 – The role of carbohydrates in nutrition. Report of a Joint FAO/WHO Expert Consultation. Rome, 14-18 April 1997. Available here.

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