Carbohydrates

ACRONYMS AND DEFINITIONS

CC- Carbohydrate counting

Carb- carbohydrate

OS - Oligosaccharides

USDA - United States Dept of Agriculture

CARBOHYDRATE OVERVIEW

Macronutrients

There are 3 types of macronutrients:

Carbohydrates
Fats
Protein

Carbohydrates can be subdivided into 3 main categories:

Sugars
Starches
Fibers

Energy

Macronutrients supply the following amounts of energy:

Carbohydrates - 4 calories/gram
Fats - 9 calories/gram
Protein - 4 calories/gram

Carbohydrate utilization

Glucose is the fundamental energy source in humans
Certain forms of carbohydrates (sugars and some starches) are easily absorbed and broken down into glucose by the body
Because of this, carbohydrates have the most direct effect on blood sugar levels

RECOMMENDED DIETARY INTAKE

USDA recommended intake

The USDA recommends that 45 - 65% of total calories come from carbohydrates
The USDA recommends that added sugars be limited to < 10% of total calories [14]
See calories for a guide on calculating daily caloric requirements

SUGARS

Sugar categories

Sugars can be divided into 3 main groups:

Monosaccharides
Disaccharides
Oligosaccharides

Monosaccharides (1 sugar unit)

Monosaccharides are easily absorbed and turned into glucose, therefore they have a direct effect on blood sugar

Glucose (also called dextrose)

The main energy source of most living organisms
Found naturally in fruits and vegetables - grapes, peaches, pears, bananas, sweet onions, corn, etc.
Pure corn syrup is pure glucose
Sweeteners used in food are often a combination of glucose and fructose - honey, molasses, high-fructose corn syrup, invert sugars

Fructose

Fructose has the highest sweetness (sensation of tasting sweet) of all the sugars
Found naturally in fruits and vegetables - apples, pears, tree fruits, grapes, carrots, sweet pepper, corn, etc.
Found in sweeteners - high fructose corn syrup, honey, invert sugars

Galactose

Galactose is found mainly in dairy products
A galactose molecule is joined with a glucose molecule to make lactose

Disaccharides (2 sugar units joined together)

Disaccharides are easily absorbed and turned into glucose, therefore they have a direct effect on blood sugar

Sucrose

Sucrose is a glucose and a fructose molecule joined together
Found naturally in fruits and vegetables - sugar cane, sugar beets, pineapples, peaches, green peas, etc.
Table sugar is sucrose derived from sugar cane or sugar beets
Main component of food sweeteners - table sugar, brown sugar, maple syrup, powdered sugar

Lactose

Lactose is a galactose and a glucose molecule joined together
Main carbohydrate found in dairy products
Lactose must be metabolized by the lactase enzyme to be absorbed from the intestine
People who are lactase deficient cannot metabolize lactose and may have gastrointestinal symptoms

Maltose

Maltose is two glucose molecules joined together
Maltose is fermented by yeast to ethanol so it is often used to brew beer
Maltose corn syrup can be added to foods as a sweetener

Oligosaccharides (3 - 10 sugar units joined together)

Oligosaccharides (OS) are short chains (typically 3-10 units) of monosaccharides joined together
Naturally occurring OS include raffinose, stachyose, low molecular weight fructans, and verbascose
OS are not digestible by the upper intestinal tract, and they pass unchanged into the colon where they are fermented by bacteria. Because of this process, OS only yield around 1.5 - 2 calories/gram.
They also produce methane gas which can lead to flatulence
Oligosaccharides have less of an effect on blood glucose levels than other sugars

Foods containing OS:

Raffinose - beans, cabbage, Brussels sprouts, broccoli, asparagus
Stachyose - green beans, soybeans
Fructans (fructo-oligosaccharides) - Jerusalem artichoke, onions, wheat, rye [1,2]

STARCHES

Starches

Starches are chains of many glucose molecules linked together (also called polysaccharides)
Starches are found naturally in many foods - grains (corn, barley, wheat, rye, rice), potatoes, squash, green peas, beans, banana

Starch bioavailability

Food processing is a large determinant in whether the starch in a food will be available for absorption and utilization by humans (bioavailability)

Examples of processes that affect starch bioavailability include:

Heating and cooking - a baked potato will yield more starch than a raw potato
Milling and grinding - grinding grains into flour will increase their starch bioavailability compared to consuming them as whole grain
Food storage - the longer food is stored, the more starch becomes bioavailable
Ripeness - The riper a food, the more starch is bioavailable
Other foods consumed with starches - fiber consumed with starches can decrease the bioavailability of starch

Other starches

Resistant starches

Resistant starches are starches that resist digestion in the small intestine and pass into the large intestine undigested
Because of this property, some resistant starches are considered fibers
Resistant starches can be naturally occurring or synthetic. Naturally occurring resistant starches include many unprocessed starches (e.g. starches in whole grains and beans, starches in uncooked vegetables). Other resistant starches are produced through food processing (pasta, Hi-maize®) [2]

Modified starches

Modified starches are treated chemically or physically to create a functionally desirable characteristic (e.g. viscosity, binding, texture)
Modified starches are added to foods and other materials (e.g. adhesives, paper, pharmaceuticals) to impart their characteristic on that substance. [1,2]

Complex carbohydrates

Term used to refer to oligosaccharides and starches

FIBERS

Overview

Fibers are nondigestible carbohydrates that pass relatively unchanged through the upper digestive tract
Because fibers are not digested in the upper digestive tract, they do not have a direct effect on blood sugars
The USDA recommends adults consume 14 grams of fiber per 1000 calories consumed

Food labeling requirements for fiber are as follows:

Good source of Fiber - defined as foods that provide 2.5 grams of fiber/serving
Excellent source of fiber - defined as foods that provide 5 grams of fiber/serving [3,4]

Types of fiber

Fibers can be divided into three main groups:

Soluble (viscous) fiber
Insoluble (nonviscous) fiber
Mixed fibers

Soluble (viscous) fiber

Soluble fiber passes undigested through the small intestine into the colon
Soluble fiber is fermented in the colon by bacteria to short chain fatty acids which can then be used as energy. After this process, soluble fiber only yields 1.5 - 2.5 calories/gram compared to 4 calories/gram for other carbohydrates [1]
Soluble fibers include psyllium, guar gum, pectin, glucomannan, xanthan

Insoluble (nonviscous) fiber

Insoluble fiber is not fermented in the colon and therefore does not yield any bioavailable energy
Insoluble fibers include cellulose, hemicellulose, corn bran, rice bran, wheat bran, lignin

Mixed fiber (soluble and insoluble)

Mixed fibers share properties of both soluble fibers and insoluble fibers
Mixed fibers include soybean and oat bran [5]

Sources of fiber

Vegetables, fruits, whole grains, dry beans, peas, nuts, seeds, supplements (e.g. Metamucil), cereals (e.g. All Bran)

SUGAR ALCOHOLS

Sugar alcohols are monosaccharides and disaccharides that have been altered slightly
Sugar alcohols are added to foods as sweeteners
They have limited absorption in the intestine and may produce a laxative effect if consumed in large quantities
Sugar alcohols only yield about 2 calories/gram as opposed to 4 calories/gram for other carbs
Examples of sugar alcohols include sorbitol, mannitol, xylitol, lactitol, maltitol [2]

CARBOHYDRATES ON FOOD LABELS

FDA requirements

The FDA requires manufacturers to list certain nutrient values on food labels
Some nutrients are required and some are voluntary

For carbohydrates, the following is true:

Required

Total Carbohydrates
Dietary Fiber
Total sugars
Added sugars

Voluntary

Soluble fiber
Insoluble fiber
Sugar alcohol
Other carbohydrate

Exceptions

Certain foods that contain an insignificant amount of nutrients (e.g. Diet Coke®) are exempt from the requirements
Exempt foods are only required to list total calories, total fat, total carbs, protein and sodium

Carbohydrate categories

On food labels, there are 8 different categories of carbohydrates that may be listed
An example of a food label is given below along with the definition for each carbohydrate category

Example food label:

Nutrition Facts
Serving size 30g

Total
Carbohydrate 25g

Dietary Fiber 8g

Soluble Fiber 1g

Total sugars 6g
includes 5g Added Sugars

Other Carbohydrate 5g

Sugar Alcohol 6g

Total carbohydrates (required)

The sum total of all the different types of carbohydrates in a food (sugars, starches, and fibers)
It is important to note that when the grams of carbs from the subcategories of carbohydrates (ex. Fiber, Total Sugars) are added up, they may be less than the total carbohydrates. This occurs because the category "Other Carbohydrate" is voluntary and not always listed.

Dietary fiber (required)

Listed as "Dietary fiber" or "Fiber"

Total Sugars (required)

Total sugars is the sum of added sugars and sugars found naturally in the food

Added Sugars (required)

Added sugars are sugars that are added during the processing of foods
Added sugars typically include monosaccharides and disaccharides that are found in syrups, honey, and concentrated fruit and vegetable juices

Soluble fiber (voluntary)

Soluble fiber is typically only listed on high fiber foods

Insoluble fiber (voluntary)

Insoluble fiber is typically only listed on high fiber foods

Sugar alcohols (voluntary)

Sugar alcohols consist of lower calorie sweeteners like sorbitol and mannitol
Because sugar alcohols are not a required listing, they may be present in the food, but not listed separately on the label
Sugar alcohols may also be counted under "Other Carbohydrate"

Other carbohydrates (voluntary)

Defined as the "difference between total carbohydrate and the sum of dietary fiber, sugars, and sugar alcohol"

Other Carbohydrates = Total Carbohydrate - Dietary Fiber - Sugars - Sugar Alcohol

Other carbohydrates typically include starches and oligosaccharides
If sugar alcohols are not listed separately, they may be a part of other carbohydrates

CARBOHYDRATES AND SATIETY

Satiety is defined as the sense of fullness or satisfaction a person feels after eating food
Because of the obesity epidemic, there has been interest in the satiety of macronutrients (protein, carbohydrates, and fat) as a means to control weight gain
A number of studies of varying designs have been performed to measure the effect of different macronutrients on satiety
In short, there is no conclusive evidence that one macronutrient is superior to another in providing satiety that leads to reduced calorie intake [1,7]

GLYCEMIC INDEX

Sugars, starches, and fibers are absorbed differently, and therefore, they have varying effects on blood sugar levels
The glycemic index was developed to quantify the direct effect of different foods on blood sugar levels
See glycemic index for a review

CARBOHYDRATE COUNTING

Carbohydrate counting is a method used by diabetics to help dose insulin
See carbohydrate counting for a review

RESOURCES FOR FOOD CONTENT

USDA food composition database - extensive nutritional information about numerous foods including restaurant foods
myfitnesspal - commercial website that has extensive database of food calorie content
Lose it! - commercial website that has extensive database of food calorie content

BIBLIOGRAPHY

1 - Dietary Reference Intakes, Institute of Medicine, 2005
2 - Carbohydrates in human nutrition, FAO food and nutrition paper - 66, 1997
3 - Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010
4 - USDA Dietary Guidelines for Americans 2005
5 - PMID 8283297
6 - PMID 16935851
7 - PMID 12499328
8 - PMID 8862476
9 - PMID 10435117
10 - PMID 7900695
11 - PMID 7661908
12 - PMID 1728818
12 - PMID 20167238
13 - Code of Federal Regulations, Title 21, Sec 101.9
14 - Scientific Report of the 2015 Dietary Guidelines Advisory Committee

CARBOHYDRATES