Carbohydrates: Sugars, Starches, and Fibers

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen. They are classified based on the number of sugar units present.

• Monosaccharides: The simplest form of carbohydrates, consisting of a single sugar unit. Examples include glucose, fructose, and galactose.

• Disaccharides: Composed of two monosaccharide units linked together. Examples include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose).

• Polysaccharides: Long chains of monosaccharide units. Examples include starch (plant storage), glycogen (animal storage), and fiber (structural component in plants).

Example: Table sugar is sucrose, a disaccharide composed of glucose and fructose.

Glycogen

Glycogen is the storage form of glucose in animals and humans, primarily found in the liver and muscles. It is a highly branched polysaccharide that can be rapidly mobilized to meet energy needs.

• Function: Serves as a short-term energy reserve, especially during periods of fasting or intense physical activity.

Fiber: Types, Importance, Recommendations, and Food Sources

Dietary fiber refers to plant-based carbohydrates that are not digestible by human enzymes.

• Types:

  • Soluble fiber: Dissolves in water to form a gel-like material (e.g., oats, apples, beans).

  • Insoluble fiber: Does not dissolve in water and adds bulk to stool (e.g., whole wheat, vegetables).

• Importance: Promotes digestive health, helps regulate blood glucose, lowers cholesterol, and aids in weight management.

• Recommendations: The recommended intake for adults is 25–38 grams per day.

• Food Sources: Whole grains, fruits, vegetables, legumes, nuts, and seeds.

Dietary Recommendations for Carbohydrates

• RDA (Recommended Dietary Allowance): 130 grams per day for adults and children, based on the minimum amount of glucose required by the brain.

• AMDR (Acceptable Macronutrient Distribution Range): 45–65% of total daily calories should come from carbohydrates.

• Added Sugar Recommendations: The Dietary Guidelines for Americans recommend limiting added sugars to less than 10% of total daily calories.

• Fiber Recommendations: 25 grams per day for women and 38 grams per day for men.

Whole Grains vs. Refined Grains

Grains are a major source of carbohydrates in the diet and can be classified based on their processing.

• Whole grains: Contain all parts of the grain kernel (bran, germ, and endosperm). Examples: brown rice, whole wheat bread, oatmeal.

• Refined grains: Have the bran and germ removed, resulting in loss of fiber, vitamins, and minerals. Examples: white bread, white rice.

Natural vs. Added Sugars

• Natural sugars: Occur naturally in foods such as fruits (fructose) and milk (lactose).

• Added sugars: Sugars and syrups added to foods during processing or preparation. Examples: table sugar, high-fructose corn syrup.

Functions of Carbohydrates

• Primary source of energy for the body, especially the brain and red blood cells.

• Protein sparing: Adequate carbohydrate intake prevents the use of protein for energy.

• Precursor for biosynthesis of certain amino acids and fatty acids.

Carbohydrate Metabolism: Glycogenesis, Glycogenolysis, Gluconeogenesis

• Glycogenesis: The process of converting glucose to glycogen for storage in the liver and muscles.

• Glycogenolysis: The breakdown of glycogen to release glucose when energy is needed.

• Gluconeogenesis: The synthesis of glucose from non-carbohydrate sources (e.g., amino acids, glycerol).

Hormonal Regulation: Insulin and Glucagon

• Insulin: A hormone produced by the pancreas that lowers blood glucose by facilitating cellular uptake of glucose.

• Glucagon: A hormone produced by the pancreas that raises blood glucose by stimulating glycogenolysis and gluconeogenesis.

Ketosis and Ketone Bodies

When carbohydrate intake is very low, the body produces ketone bodies from fatty acids as an alternative energy source. This metabolic state is called ketosis.

• Ketone bodies can be used by the brain and muscles for energy during prolonged fasting or low-carbohydrate diets.

Enzymes in Carbohydrate Digestion

• Salivary amylase: Begins starch digestion in the mouth.

• Pancreatic amylase: Continues starch digestion in the small intestine.

• Disaccharidases: Enzymes (e.g., maltase, sucrase, lactase) in the small intestine that break down disaccharides into monosaccharides.

Digestion and Absorption of Carbohydrates

• Carbohydrate digestion begins in the mouth and continues in the small intestine.

• Monosaccharides are absorbed into the bloodstream and transported to the liver.

Lactose Intolerance

Lactose intolerance is the inability to digest lactose due to low levels of the enzyme lactase. Symptoms include bloating, gas, and diarrhea after consuming dairy products.

Artificial Sweeteners and Sugar Alcohols

• Artificial sweeteners: Non-nutritive sweeteners that provide little or no calories (e.g., aspartame, sucralose, saccharin).

• Sugar alcohols: Nutritive sweeteners that provide fewer calories than sugar and are less likely to cause dental caries (e.g., xylitol, sorbitol).

Regulation of Blood Glucose Levels

• Maintained by the coordinated action of insulin and glucagon.

• Normal fasting blood glucose: 70–99 mg/dL.

Hypoglycemia and Hyperglycemia

• Hypoglycemia: Abnormally low blood glucose levels, which can cause shakiness, confusion, and fainting.

• Hyperglycemia: Abnormally high blood glucose levels, often associated with diabetes.

Diabetes Mellitus: Types, Causes, Risk Factors, Treatment, and Prevention

• Type 1 Diabetes: Autoimmune destruction of pancreatic beta cells; requires insulin therapy.

• Type 2 Diabetes: Insulin resistance and eventual beta cell dysfunction; managed with lifestyle changes, oral medications, and sometimes insulin.

• Gestational Diabetes: Occurs during pregnancy; increases risk of type 2 diabetes later in life.

• Risk Factors: Family history, obesity, physical inactivity, ethnicity, age.

• Treatment: Diet, exercise, medication, and blood glucose monitoring.

• Prevention: Healthy diet, regular physical activity, maintaining a healthy weight.

Glycemic Index

The glycemic index (GI) is a measure of how quickly a carbohydrate-containing food raises blood glucose levels compared to pure glucose.

• Foods with a high GI cause rapid spikes in blood glucose; low GI foods result in slower, more gradual increases.