Carbohydrates are among the three macronutrients along with proteins and lipids. They are our main source of energy. Carbohydrates are made up of carbon, hydrogen and oxygen atoms. Examples of carbohydrates include sugars, starch and fiber. They are classified as monosaccharides, disaccharides, oligosaccharides and polysaccharides based on the number of sugar units in their chemical structure. Carbohydrates with a single sugar unit is called monosaccharides. Examples are glucose, fructose and galactose. Carbohydrates with two sugar units are called disaccharides. Example of disaccharides are lactose, maltose and sucrose. You may recognise lactose as the sugar that is present in milk and sucrose as regular table sugar. Carbohydrates consisting of 3 to 10 sugar units are called oligosaccharides. Oligosaccharides in plant foods such as beans and cabbage cause us to feel gassy after we eat them. Examples are raffinose and stachyose. Raffinose has three sugar units while stachyose has 4 sugar units. Polysaccharides have more than 10 sugar units. Examples are starch, and dietary fibers such as cellulose, pectins, gums, fructans, and mucilages.
Our ability to digest carbohydrates is based on how the sugar units are connected. Sugar units in carbohydrates are connected by a bond known as a glycosidic bond. The glycosidic bond may be an alpha glycosidic bond or a beta glycosidic bond. The bond is alpha if it is pointing down and beta if it is pointing up. Our body’s enzymes digest alpha bonds easily but have more difficulty with beta bonds. For example cellulose consist of glucose connect by beta bonds which cannot be digested by humans since we lack the enzyme cellulase. Some people in the population do not have lactase enzyme which is needed to breakdown lactose. We call these people lactose-intolerant since they cannot digest milk and other dairy products. Lactose is made up of a galactose and a glucose connected by a beta glycosidic bond.
How are Carbohydrates Digested?
You will recall from our previous lesson on digestion that we begin to digest carbohydrates in our mouth with the help of salivary enzymes. These enzyme breaks down carbohydrates to smaller carbohydrate chains and maltose. When food gets to the stomach, alpha amylase is deactivated by hydrochloric acid. Further digestion of carbohydrates continue in the small intestine where it is broken down to its individual monosaccharides by pancreatic enzymes. Monosaccharides are then absorbed into the villa walls of the small intestine and into capillaries which takes monosaccharides to the liver via the hepatic portal vein. In the liver, non-glucose monosaccharides are converted to glucose. If the body needs glucose immediately, glucose is released into the bloodstream. However if there is excess, it is stored in the form of glycogen in the liver and muscles.
Soluble fibers and insoluble fiber cannot be digested in the small intestine and so they pass to the large intestine. Soluble fibers such as pectins, gums and mucilages are partially digested to produce energy (1.5-2.5 Calories/gram) and short chain fatty acids (SCFA) such as acetic, propionic and butyric acid. SCFAs have been found to improve the health of the large intestine and reduce the risk of colon cancer.
Insoluble fibers are not fermented but are eliminated in the feces. They however provide many benefits including:
- Absorption of water to soften the stool
- Trapping and ridding the body of excess cholesterol that the liver deposits in the bile. When there is little fiber in the diet, less of this excess cholesterol is trapped and it is therefore reabsorbed back into the blood
- Trapping cancer-causing substances that may lead to reducing the risk of colon cancer
- Providing bulk to the walls of the large intestine which helps in waste elimination
- Giving us a feeling of fullness. Therefore, high consumption of insoluble fiber can be a part of a good weight loss plan
- Preventing diverticulosis. This condition is where sections within the walls of the large intestine becomes bulged due to straining caused by constipation. Each bulge is called a diverticula. Waste in the diverticula cannot be cleaned properly during bowel movement, so they become inflamed.
How to Stay Healthy with the Right Carbohydrates
To stay healthy, you should select complex carbohydrates instead of simple carbohydrates. That means look for high-fiber options instead of high sugar options. For example, reach for an apple before you reach for a donut. That is because you will be getting not only carbohydrates but also other essential nutrients. Foods that are rich in complex fiber such as pulses and whole grain foods digests slowly and therefore release glucose to the blood at a slower rate. This is associated with a feeling of satiety. The term used to describe how fast glucose is released into the blood is glycemic index. A glycemic index of 55 or less is considered to be low, 56 – 69 is medium and above 70 is considered as high. Another term called glycemic load is also used to describe how certain foods affect blood glucose levels. Glycemic load is the number of grams of carbohydrate a food contains multiplied by the glycemic index. If you are diabetic, give you extra reason and motivation to choose healthy carbohydrates.
What is Diabetes?
Diabetes is a condition where there is too much glucose in the blood. Over time, as blood glucose levels remain high, it can cause serious damage to your large and small blood vessels. These conditions can lead to nerve cells being starved of oxygen and nutrients. When this happens the affected nerve cells die, leading to numbness and necrosis. This is the reason for amputations resulting from diabetes.
The two main types of diabetes are Type 1 and Type 2. Type 1 diabetes is when the pancreas does not produce enough insulin because the immune system attacks and destroy the beta cells in the pancreas that are responsible for making them. Synce it is an autoimmune disease it cannot be prevented. In type 2 diabetes the body becomes insensitive to insulin which results in poor absorption of glucose from the blood. Obesity is a big risk factor for Type 2 diabetes. Type two diabetes can be prevented by adopting healthy choices such as eating complex carbohydrates, avoiding sugar foods, managing your weight, and exercising regularly.
If you do not want to give up certain foods that you enjoy, you may substitute natural sugars such as sucrose and high fructose corn syrup with sweeteners. Some sweeteners provide a few calories. These are called nutritive sweeteners, such as sugar alcohols (2-3 Calories/gram). Many of them, for example xylitol and sorbitol are added to chewing gum with a claim that they will reduce dental caries. Some sweeteners such as aspartame and sucralose (Splenda) are non-nutritive, meaning that they provide no calories. These sweeteners are many times sweeter than regular table sugar (sucrose). For example sucralose is 600 times sweeter than sucrose.
Reference: Thompson, J. L., Manore, M. M. & Vaughan, L. A. (2017). The Science of Nutrition (4th ed.). San Francisco, CA: Pearson Education.