Carbohydrates

Hydrates of Carbon - Cn(H2O)n 

Carbohydrates benefit the body by being converted into glucose - a major fuel source for organs, tissue and cells.  *According to the USDA's "Dietary Guidelines for Americans 2005", glucose is the only energy source for red blood cells (which transport oxygen throughout the body) and is the preferred source of energy for the brain and central nervous system as well as the placenta and fetus during pregnancy. Two other main energy sources for our bodies are proteins and fats.

Despite their importance, carbs have been the focus of negative publicity from weight-loss diets to manging glucose levels for diabetics. Yet, product labels only list the number of carbohydrate grams per serving without any mention of their type or affect on the body.

Carbohydrates are composed of carbon, hydrogen and oxygen.  However, all carbs are not created equal and as such, impact the body differently. Essentially, carbs consist of sugar and starch molecules that combine to form either simple or complex carbohydrates. How these carbs affect our bodies can be described in terms of their glycemic index and/or their glycemic load. Understanding their impact will enable consumers to make intelligent decisions on types and quantities of carbohydrates to consume .

USDA Recommended Daily Allowance

The RDA of carbohydrates for a person on a 2,000 calorie per day diet is 300 grams.

When determining carbohydrate intake - subtract the grams of fiber listed on the food label from the grams of carbs. The revised amount is known as 'net carbs' or sometimes referred to as 'available carbs.'

While fiber is a type of carbohydrate - it is considered indigestible thus not counted toward your total carb intake.

Carbohydrates minus fiber = 'net carbs'

     Simple

       Complex

  Glycemic Index

Glycemic Load

Simple Carbohydrates

Simple carbs are comprised of one or two sugar molecules. Examples include fructose in fruit, lactose from dairy and sucrose in table sugar.

Simple carbs found naturally in fruits, dairy and vegetables contain vitamins and minerals thus their calories 'count' toward promoting health.

They can also be found in processed foods and refined sugars such as candy and soda. Lacking nutritional value, their calories are considered 'empty' thus their intake should be limited.

Complex carbs involve longer chains of sugar molecules branched together. These include starchy vegetables (potatoes, corn), whole-grain breads and fiber.

Complex Carbohydrates

Glycemic

Index

Food labeling symbol used in Australia to identify healthy, low GI products.

***Symbol reproduction courtesy of GI News, Human Nutrition Unit, University of Sidney - link below.

While all carbohydrate types are converted into glucose during the digestive process, the rate of this conversion can vary depending their nature. It is theorized that carbs which convert to glucose quickly may have harmful impacts on blood sugar management and appetite control. Furthermore, The United States Food and Drug Administration (FDA) states that a rapid rise in blood sugar triggers an insulin surge that drives fat within the blood into fat cells while also making it more difficult to breakdown and release fat that is stored in cells already.

The glycemic index (GI) was developed in the early 1980's by Dr. David Jenkins at the University of Toronto as a means to determine which foods were best for diabetics. The index has since been incorporated into various diet plans meant for everyone including SouthBeach. The purpose of the GI is to measure the ease by which our bodies convert a particular carbohydrate into glucose which in turn impacts the speed and degree of the associated rise in blood sugar levels. **Surprisingly, the previous concept that simple carbs converted more quickly than complex carbs did not always hold true.

In determining glycemic index levels, blood sugar changes are measured and compared between a test food and a reference food over a set period of time - usually two hours. Both the test food and the reference food use portion sizes that produce 50 grams of carbohydrates .The most common reference food used is glucose dissolved in water due to its rapid absorption rate (although white bread has also been used).  For comparison purposes, the blood glucose levels produced by the reference food (glucose) are given a GI rating of 100.

Determining a product's glycemic index:

*The method below was described in glycemicindex.com, associated with the University of Sydney, School of Molecular and Microbial Biosciences, Human Nutrition Unit.

• A minimum of 10 healthy test subjects fast overnight.
• A portion size is determined by utilizing enough of the product to generate 50 grams of available carbohydrates (carbs minus fiber - aka 'net' carbs).
• The food is consumed within 10 to 12 minutes and changes in blood sugar levels are measured in specific 15-30 minute intervals for the next two hours and used to construct a blood sugar response curve.
  
• The blood sugar levels are then compared to the curve created when the same test subjects consumed 50 grams of glucose - which is given a GI rating of 100.
• The percentage to which the test food's glucose measurements compare to the reference food's (glucose) measurements becomes its glycemic index. If they were the same, then the test food would also have a GI of 100. If it measured half, then its GI would be 50 (50%) and so on. 
  

Foods considered to have a low glycemic index have a rating of 55 or less. The moderate range lies from 56 to 69 while foods with a rating of 70 and above are considered high.

Proponents of the glycemic index recommend eating foods that have a low rating for a variety of reasons:

• High GI foods cause a rapid rise of blood sugar which then triggers a surge of insulin release causing the body to store extra glucose as fat.
• Insulin surges causes rapid decrease in blood sugar which may triggering hunger binges to restore normal levels.
• Low GI foods sustain energy levels longer helping you to feel more satisfied and less likely to binge eat while helping to loose body fat, maintain lean muscle and lower cholesterol.
• Low GI foods produce more gradual fluctuations in blood sugar levels which help prevent TypeII Diabetes and heart disease.
• Gradual blood sugar fluctuations stimulate less insulin release therefore causing less glucose to be stored as fat.
  

Critics, however, will point out deficiencies and limitations of the glycemic index concept including:

• Glycemic index incorporates a standard net carbohydrate intake amount (50 grams) for all items tested although obtaining that amount may utilize serving sizes inconsistent with normal consumption. For instance, 50-grams of carbohydrates  from a baked potato may represent about one-third the amount of potato normally consumed while it may take five slices of bread to obtain 50g of  net carbs. from watermelon would be exorbitant.
  
• The same food's glycemic index also varies based on factors such as length of cooking time and ripeness.
• A carbohydrate's absorption rate (thus its GI) will be affected by other (fat & protein) stomach contents.
  
• A food's GI is not necessarily a reflection of its nutritional value. For instance, a baked russet potato has a higher glycemic index than potato chips as the chip's high fat content slows its absorption rate. Even some popular candy bars score low GI ratings yet have little nutrition while high in fat and calories.
  
• Insulin secretions are not constant but vary between individuals as does a person's response to insulin.
• In a 2007 study at Tufts University using white bread as the carbohydrate source, there was a 30% variability in response in the same volunteers on different occasions, suggesting that the glucose responses to a single food varies within individuals.

Glycemic

Load

Developed by scientists at Harvard University, the glycemic load (GL) utilizes a food's glycemic index but then adjusts its rating to account for the quantity of available carbohydrates (carbs minus fiber or 'net' carbs) actually found within a portion size (rather than using the amount of a product needed to produce 50 grams of net carbs which may be significantly more or less than what a typical portion size would be).

The following formula can be used to determine glycemic load:

Glycemic Index multiplied by grams of available carbohydrates then divided by 100

simply put:

GI x Carbs /100

For example, if a bagel has a GI rating of 72 with 25 grams of net carbs, then

72 x 25 = 1800/100 for a GL of 18

Foods considered to have a low glycemic load have a rating of 10 or less. The moderate range lies from 11 to 19 while foods with a rating of 20 or more are considered high.

Comparing the rating scales of glycemic index versus glycemic load:

• Glycemic index   -   Low = 55 or less     Moderate = 56 to 69     High = 70+
• Glycemic load     -    Low = 10 or less     Moderate = 11  to 19     High = 20+

Essentially, foods that are dense with carbs are likely to have a lower glycemic index rating than glycemic load since the 50 grams of net carbs used to calculate its GI my be less than what is consumed in an actual serving portion used to determine its GL.

i.e. Macaroni - glycemic index = 47 (low)   glycemic load = 23 (high)

Likewise, foods that are sparse in carbs are likely to have lower glycemic load ratings since the amount of carbs consumed in a normal serving is less then the 50 grams consumed to determine the glycemic index.

i.e. Watermelon - glycemic index = 72 (high)   glycemic load = 4 (low)

While the adjustment for portion size does address one of the criticisms levied against the glycemic index, the other concerns regarding GI outcome variations based on cooking times, ripeness and additional stomach contents also apply to glymecic load scores as does individual insulin responses.

References and Referrals

The information regarding glycemic index and glycemic load has been obtained from the web sites listed below. For further information or to view glycemic index and glycemic load ratings of common foods, click on their links.

*http://www.health.gov/dietaryguidelines/dga2005/document/default.htm

**http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/carbohydrates-full-story/index.html

http://www.health.harvard.edu/newsweek/Glycemic_index_and_glycemic_load_for_100_foods.htm

***www.GlycemicIndex.com

www.fda.gov/Food/LabelingNutrition/ReportsResearch/ucm081857.htm

While specific glycemic index and glycemic load numbers may provide us with a better understanding of a product's glucose absorption rate - careful attention still needs to also be paid to the nutritional value in order to avoid excess fat and make the calories count wisely.

Eat more whole-grain breads, cereals and pastas as well as high fiber products as they have a more gradual impact on glucose levels.

Limit highly processed foods as they require less digestive process thus increase glucose levels more rapidly.

Avoid overcooking as starch molecules will swell and soften thus speeding up their rate of digestion (overcooking a pasta can increase its GI by nearly 50%).

AgeDenial.com provides this page to help stimulate interest in nutrition and is not meant to serve as a source of medical advice. As always, consult your personal physician for guidance specific to you as each person's approach to diet and exercise should be guided by a medical professional who will consider individual benefits and risks as they relate to personal medical conditions and needs.