Carbohydrates: Structure and Function

What Are Carbohydrates and Why Do You Need Them?

Carbohydrates are the most desirable source of energy for the human body, providing 4 calories per gram. Their primary role is to supply fuel, mainly in the form of glucose, to cells, especially the brain and red blood cells. Carbohydrates are mainly derived from plant foods, which synthesize glucose through photosynthesis.

• Photosynthesis Equation: Plants use sunlight, water, and carbon dioxide to produce glucose and oxygen.

• Glucose: The most abundant sugar in nature and the optimal fuel for the body.

• Carbohydrate Categories:

  • Simple carbohydrates: Monosaccharides (glucose, fructose, galactose) and disaccharides (sucrose, lactose, maltose).

  • Complex carbohydrates: Polysaccharides (starch, fiber, glycogen).

Example: Plants store energy as starch, while animals store energy as glycogen.

Types of Carbohydrates

Starch: Storage Form in Plants

Starch is composed of thousands of glucose units linked together. It exists in two forms:

• Amylose: Straight chains of glucose units.

• Amylopectin: Branched chains of glucose units, digested more rapidly due to multiple enzyme attachment sites.

Example: Potatoes, rice, bread, pasta, and cereals are rich in amylopectin; legumes are rich in amylose.

Fiber: Importance and Types

Fiber is a nondigestible component of plant foods. Humans lack the enzyme to break down fiber, so it passes through the digestive tract largely intact.

• Types of Fiber: Cellulose, hemicellulose, lignins, gums, pectin.

• Dietary fiber: Naturally occurring in foods.

• Functional fiber: Added to foods for health benefits (e.g., psyllium).

• Total fiber: Sum of dietary and functional fiber.

• Classification: Soluble/insoluble, viscous, fermentable.

Example: Pectin in apples is dietary fiber; when added to yogurt, it is functional fiber.

Glycogen: Storage Form in Animals

Glycogen is the storage form of glucose in humans and animals, found in the liver and muscle cells. It is highly branched, similar to amylopectin, but is not present in meat after animal death.

Digestion and Absorption of Carbohydrates

Digestive Process

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

• Salivary amylase: Breaks down starch into smaller units in the mouth.

• Pancreatic amylase: Further breaks down starch in the small intestine.

• Brush border enzymes: Maltase, sucrase, and lactase break down disaccharides into monosaccharides.

• Absorption: Monosaccharides are absorbed into the blood and transported to the liver, where fructose and galactose are converted to glucose.

• Fiber: Passes to the large intestine, where some is fermented by bacteria; most is eliminated in stool.

Lactose Malabsorption and Intolerance

Lactose is the main carbohydrate in dairy products. Deficiency of lactase enzyme leads to lactose malabsorption and, in some cases, lactose intolerance.

• Lactose malabsorption: Reduced ability to digest lactose, common after weaning.

• Lactose intolerance: Symptoms include diarrhea, bloating, flatulence, and cramps after consuming lactose-containing foods.

• Management: Most can tolerate small amounts of lactose; lactose-reduced products and lactase pills are available.

Carbohydrate Utilization and Regulation

Hormonal Regulation of Blood Glucose

Hormones regulate blood glucose levels:

• Insulin: Released after meals; promotes glucose uptake by cells and storage as glycogen or fat (glycogenesis).

• Glucagon: Released during fasting; stimulates breakdown of glycogen (glycogenolysis) and creation of glucose from noncarbohydrate sources (gluconeogenesis).

• Epinephrine and norepinephrine: "Fight-or-flight" hormones; stimulate rapid glucose release.

Example: During fasting, glucagon and epinephrine help maintain blood glucose for brain and red blood cells.

Ketosis

After prolonged fasting or low-carbohydrate diets, glycogen stores are depleted, and the body relies on fat and protein for energy. Incomplete fat breakdown produces ketone bodies, leading to ketosis.

• Ketosis: Excess ketone bodies can acidify blood; extended fasting can lead to protein breakdown and death.

Dietary Recommendations and Food Sources

Carbohydrate and Fiber Requirements

The Dietary Reference Intakes (DRIs) recommend:

• Minimum carbohydrate: 130 grams per day for adults and children (to meet brain glucose needs).

• AMDR: 45–65% of total daily calories from carbohydrates.

• Fiber: 14 grams per 1,000 calories consumed.

Example: A 2,000-calorie diet should include 28 grams of fiber.

Best Food Sources of Carbohydrates

• Whole grains: Contain bran, germ, and endosperm; rich in fiber, vitamins, minerals, and phytochemicals.

• Refined grains: Only endosperm; lower in fiber and nutrients. Enriched grains have some nutrients added back.

• Fruits and vegetables: Provide simple sugars, starch, and fiber. Whole fruits are preferable to juices or canned in syrup.

• Legumes, nuts, seeds: Excellent sources of carbohydrates and fiber.

• Dairy: Low-fat and fat-free options provide lactose and nutrients.

• Packaged foods: Can be sources of carbohydrates; choose those with at least 2 grams of fiber per serving and monitor added sugars, salt, and fat.

Table: Fiber Recommendations by Age and Sex

Natural vs. Added Sugars

Definitions and Health Implications

Natural sugars are found in foods such as fruits and dairy, while added sugars are incorporated during processing or preparation. Both taste the same, but natural sugars are found in nutrient-dense foods, whereas added sugars provide "empty calories."

• Health risks of added sugars: Increased LDL cholesterol, triglycerides, risk of heart disease, and weight gain.

• Dental caries: Carbohydrates, especially sticky and sugary foods, promote tooth decay.

• Recommendations: Less than 10% of daily calories from added sugars; American Heart Association suggests 6 teaspoons (100 calories) for females and 9 teaspoons (150 calories) for males.

Table: Common Added Sugars in Foods

Diabetes: Types, Causes, and Management

Definitions and Differences

Diabetes mellitus is a condition characterized by elevated blood glucose due to insufficient insulin production or insulin resistance.

• Type 1 diabetes: Autoimmune destruction of insulin-producing cells; requires daily insulin.

• Type 2 diabetes: Most common; involves insulin resistance and eventual decreased insulin production; often associated with overweight.

• Prediabetes: Blood glucose above normal but not high enough for diabetes diagnosis; increases risk for diabetes and heart disease.

Effects and Complications

• Chronic high blood glucose: Damages nerves, kidneys, eyes, heart, and can cause amputations.

• Hypoglycemia: Low blood glucose; symptoms include dizziness, confusion, weakness.

• Diabetic ketoacidosis: Dangerous accumulation of ketone bodies in poorly managed diabetes.

Management and Prevention

• Diet: Balanced, high-fiber, low-added-sugar diet; Mediterranean, DASH, or plant-based diets recommended.

• Physical activity: Regular exercise improves insulin sensitivity.

• Weight management: Reduces risk and complications.

• Glycemic Index (GI) and Glycemic Load (GL): Classify carbohydrate foods by their effect on blood glucose; lower GI/GL foods are preferable.

Example: Replacing sugar-sweetened beverages with water or calorie-free drinks may help manage weight and reduce diabetes risk.

Table: Comparison of Diabetes Types

Sugar Substitutes

Types and Characteristics

Sugar substitutes are alternatives to sugar that provide sweetness with fewer or no calories. All must be FDA-approved for safety.

• Polyols (sugar alcohols): Sorbitol, mannitol, xylitol, erythritol; slower absorption, do not promote dental caries, may cause digestive upset if consumed in excess.

• Saccharin: Oldest substitute; not metabolized, calorie-free.

• Aspartame: Composed of amino acids; not suitable for individuals with phenylketonuria (PKU).

• Neotame: Similar to aspartame but safe for PKU.

• Acesulfame-K: Contains potassium; not metabolized.

• Sucralose: Modified sucrose; not metabolized.

• Stevia (Rebaudioside A, M, D): Plant-derived, calorie-free.

• Monk fruit: Extracted from luo han guo; intensely sweet.

• Advantame: Derived from aspartame and vanillin; very high intensity.

Table: Comparison of Sugar Substitutes

Health Effects of Fiber

Importance of Fiber in the Body and Diet

Although fiber is not digestible, it has numerous health benefits:

• Constipation and diverticulosis prevention: Fiber increases stool bulk and promotes regularity.

• Obesity prevention: Fiber increases satiation, helping with weight management.

• Heart disease: Viscous fibers lower blood cholesterol by binding bile acids.

• Diabetes: Fiber slows glucose absorption, improving blood glucose control.

• Cancer: Fiber may reduce risk of colorectal and breast cancer by diluting carcinogens, promoting healthy bacteria, and binding bile acids.

Example: A diet rich in whole grains, fruits, vegetables, and legumes is associated with reduced risk of chronic diseases.

Note: Excessive fiber intake can cause digestive discomfort and reduce absorption of some nutrients; increase fiber gradually and drink plenty of fluids.

Table: Health Benefits of Fiber

Summary of Key Points

• Carbohydrates are essential for energy, especially for the brain and red blood cells.

• Carbohydrates are classified as simple (monosaccharides, disaccharides) and complex (polysaccharides).

• Digestion begins in the mouth and continues in the small intestine; fiber is mostly eliminated.

• Hormones (insulin, glucagon, epinephrine) regulate blood glucose.

• Minimum carbohydrate intake: 130 g/day; fiber: 14 g/1,000 kcal.

• Whole grains, fruits, vegetables, legumes, nuts, and seeds are the best sources of nutrient-dense carbohydrates and fiber.

• Added sugars should be limited to less than 10% of daily calories.

• Diabetes is a growing epidemic; type 2 is most common and preventable with healthy diet and lifestyle.

• Sugar substitutes offer sweetness with fewer calories and do not promote dental caries.

• Fiber is vital for digestive health, weight management, and prevention of chronic diseases.