Carbohydrates in Human Diet

Carbohydrates in the Human Diet

• Two major types of carbohydrates in the human diet:
  • Starch: Storage form of glucose in plants, mainly in seeds, roots, and tubers
  • Glycogen: Storage form of glucose in humans and animals, primarily stored in the liver and muscles

Dietary Fiber

• Complex carbohydrate that cannot be digested by humans
• Two main types:
  • Insoluble fiber: Includes cellulose, hemicellulose, and lignin
  • Soluble fiber: Includes pectins, gums, mucilages, and some hemicelluloses

Whole Grains

• Intact, ground, cracked, or flaked seeds of cereal grains
• To be labeled "whole grain", the food must contain:
  • Fiber-rich bran
  • Starchy endosperm
  • Oily germ

Carbohydrate Digestion and Absorption

• Mouth: Salivary amylase digests some starch
• Stomach: Acid inactivates salivary amylase
• Small intestine: Main site for carbohydrate digestion and absorption
  • Pancreatic amylase
  • Maltase
  • Sucrase
  • Lactase
• Liver: Receives absorbed glucose, fructose, and galactose from the small intestine
• Large intestine: Some soluble fiber and undigested carbohydrates are fermented
• Rectum: Very little dietary carbohydrate eliminated in feces

Blood Glucose Regulation

• After meals, blood glucose increases, the pancreas releases insulin:
  • Helps glucose enter most cells
  • Enhances production and storage of fat, glycogen, and protein
  • Decreases hunger
• When blood glucose decreases, the pancreas releases glucagon:
  • Stimulates glycogenolysis (glycogen breakdown) releasing glucose into the blood
  • Stimulates lipolysis (breakdown of triglycerides) for energy

Glucose for Energy

• Cells use oxygen to release energy stored in glucose's chemical bonds.
• Carbon dioxide and water are formed in the process.
• Energy is released.

Ketone Bodies and Ketoacidosis

• Ketone bodies form as a result of incomplete fat breakdown:
  • Fasting or starving
  • Low-carbohydrate, high-protein diet (e.g., Atkins diet)
• Ketone bodies can be used by certain cells for energy.
• Ketoacidosis is a condition that occurs with very high blood ketone bodies, making the blood pH acidic:
  • Poorly controlled type-1 diabetes
  • Unconsciousness and death may occur

Carbohydrate Consumption Pattern

• In 2017-2018, the average American diet supplied:
  • About 46% of total calories from carbohydrates
  • Almost 26% of total calories were from complex carbohydrates
  • 20.5% of total calories were from sugars
  • Approximately 18% of total kcal were from added sugars (about 22 tsp)
  • According to the Dietary Guidelines, added sugars intake should be less than 10% of total kcal.

Sugar-Sweetened Beverages

• Added sugar consumption associated with body weight.
• Sugar-sweetened soft drinks are one of the major sources of added sugars in American diets.
• Risks of obesity, type 2 diabetes, heart disease, and stroke are associated with sugar-sweetened soft drink consumption.

Lactose Intolerance

• Inability to digest lactose because of inadequate lactase, common in people of African, Asian, or Eastern European descent.
• Bacteria in the large intestine break down undigested lactose, resulting in:
  • Intestinal cramps, bloating, gas, and diarrhea.
• Treatment involves:
  • Avoiding lactose
  • Using dairy foods that have little or no lactose or have been pretreated with lactase
  • Some people can consume small amounts of foods that contain lactose.
• Lactose intolerance is not the same as milk allergy.

Fiber and Health

• Some forms of fiber swell in the large intestine, leading to softer stools and more regular bowel movements.
• May help prevent or relieve constipation.
• High-fiber diets may reduce the risk of:
  • Obesity
  • Type 2 diabetes
  • Certain intestinal tract disorders
  • Heart disease and stroke
  • Diverticulitis
  • Swollen and inflamed hemorrhoids

Glycemic Index (GI) and Glycemic Load (GL)

• Glycemic Index (GI): Method of classifying carbohydrate-rich food by comparing the rise in blood glucose after eating a portion of food that contains 50g digestible carbohydrate to the rise that occurs after eating 50g of a standard source of glucose.
• Glycemic Load (GL): Considers quantity and quality of carbohydrates in a food.
  • Grams of carbohydrate in a serving of food × the food’s glycemic index; this figure is then divided by 100.

Why Consider GIs and GLs?

• GI and GL values for a particular food may vary depending on:
  • When food is grown
  • Degree of ripeness
  • Extent of processing
  • Other components in the meal.
• Diets with high GIs/high GLs may increase the risk of serious chronic diseases.
• Low GI/GL diets can improve blood fat and cholesterol levels, reduce the risk of cardiovascular disease, and improve HbA1 levels.

Summary

• Carbohydrates are the major source of energy for the body which include monosaccharide, disaccharide, oligosaccharide, and polysaccharide.
• The main difference between a nutritive sweetener and a nonnutritive sweetener is whether it contributes energy after intaking.
• Polysaccharides are complex carbohydrates, which contain >10 monosaccharides bonded together and store energy in plants and animals.