Carbohydrates: Sugars, Starches, and Fiber – Study Notes

Carbohydrates: Sugars, Starches, and Fiber

What Are Carbohydrates and Why Do You Need Them?

Carbohydrates are essential nutrients that serve as the body's primary source of energy. They are a staple in diets worldwide and are especially important for fueling the brain, red blood cells, and nervous system.

• Definition: Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen, typically with the formula (CH2O)n.

• Main Role: Provide energy in the form of glucose.

• Photosynthesis: Plants produce glucose through photosynthesis: $6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2$

• Types of Carbohydrates:

  • Simple Carbohydrates: Monosaccharides (one sugar unit: glucose, fructose, galactose) and disaccharides (two sugar units: maltose, sucrose, lactose).

  • Complex Carbohydrates: Polysaccharides (long chains: starch, fiber, glycogen).

• Starch: Plant storage form of glucose. Includes amylose (straight chains) and amylopectin (branched chains).

• Fiber: Nondigestible carbohydrate. Includes dietary fiber (naturally in foods) and functional fiber (added for health benefits). Total fiber = dietary + functional fiber.

  • Soluble fiber: Dissolves in water, fermented by gut bacteria.

  • Insoluble fiber: Does not dissolve in water, not fermented.

• Glycogen: Animal storage form of glucose, found in liver and muscles.

What Happens to the Carbohydrates You Eat?

Carbohydrate digestion begins in the mouth and continues in the intestines, ultimately resulting in glucose absorption.

• Mouth: Salivary amylase starts breaking down starch into maltose.

• Stomach: Acid inactivates amylase; little carbohydrate digestion occurs.

• Small Intestine: Pancreatic amylase continues starch breakdown. Disaccharides (maltose, lactose, sucrose) are digested and absorbed.

• Fiber: Mostly eliminated in stool; not digested.

• Lactose Intolerance: Inability to digest lactose due to low lactase enzyme.

How Does Your Body Use Carbohydrates?

The body uses carbohydrates for immediate energy, stores excess as glycogen, and relies on hormonal regulation to maintain blood glucose levels.

• Insulin: Hormone that lowers blood glucose by promoting uptake and storage as glycogen (glycogenesis).

• Glycogenolysis: Glucagon hormone stimulates breakdown of liver glycogen to release glucose when needed.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (e.g., amino acids) in liver and kidneys.

• Epinephrine: Stimulates rapid glycogen breakdown during stress or excitement.

• Ketosis: Inadequate carbohydrate intake leads to fat breakdown and production of ketone bodies, causing blood acidity.

• Protein Sparing: Adequate carbohydrate prevents the body from using protein for energy.

How Many Carbohydrates Do You Need and What Are the Best Food Sources?

Carbohydrate needs are based on the minimum required for brain function, with emphasis on complex carbohydrates and fiber-rich foods.

• DRI Recommendation: Minimum 130 grams of carbohydrates per day for adults.

• Best Sources: Whole grains, fruits, vegetables, legumes, nuts, seeds, and low-fat dairy.

• Fiber Intake: Refer to dietary guidelines for age- and gender-specific recommendations.

• Packaged Foods: Can provide carbohydrates but may contain added sugars, salt, and fat. Use Nutrition Facts panels to make healthy choices.

What’s the Difference Between Natural and Added Sugars?

Natural sugars occur in whole foods, while added sugars are incorporated during processing. Both taste the same, but natural sugars are usually accompanied by beneficial nutrients.

• Natural Sugars: Found in fruits, vegetables, and dairy; provide nutrients and fiber.

• Added Sugars: Added during processing/preparation; provide calories but few nutrients.

• Health Effects:

  • Excess added sugar can increase triglycerides, lower HDL cholesterol, and contribute to dental caries.

  • No strong evidence that sugar causes hyperactivity or diabetes directly.

• Label Reading: Ingredients list can help identify added sugars; Nutrition Facts panel does not distinguish natural from added sugars.

• Major Sources: Sweetened soft drinks are the leading source of added sugars in the U.S.

• Recommended Limits:

  • Dietary Guidelines: No more than 5–15% of daily calories from added sugars and solid fats.

  • American Heart Association: Women ≤ 100 kcal (6 tsp) and men ≤ 150 kcal (10 tsp) of added sugar per day.

What Are Sugar Substitutes and What Forms Can They Take?

Sugar substitutes provide sweetness with fewer or no calories and are regulated for safety. They are especially useful for people with diabetes or those managing calorie intake.

• Polyols (Sugar Alcohols): Sorbitol, mannitol, xylitol; found in plants or synthesized. "Sugar free" products may contain polyols but are not always calorie free. Excessive intake can cause diarrhea.

• Saccharin: Oldest substitute; not metabolized, so provides no calories. No longer considered a cancer risk.

• Aspartame: Made from aspartic acid and phenylalanine; 200x sweeter than sugar. Not suitable for people with PKU.

• Neotame: Similar to aspartame but safe for people with PKU.

• Acesulfame-K: Contains potassium; not metabolized by the body.

• Sucralose: Made from sucrose; not absorbed, excreted in urine.

• Rebaudioside A (Stevia): Plant-derived, zero-calorie, does not affect blood glucose.

Sweetener	Source	Relative Sweetness	Calories	Notes
Saccharin	Synthetic	~300x	0	Oldest, not metabolized
Aspartame	Amino acids	~200x	4/g	Not for PKU
Sucralose	Sucrose derivative	~600x	0	Not absorbed
Polyols	Plants/synthetic	0.5–1x	2/g	May cause diarrhea
Stevia (Rebaudioside A)	Stevia plant	~200x	0	Plant-based
Neotame	Amino acids	~7,000–13,000x	0	Safe for PKU
Acesulfame-K	Synthetic	~200x	0	Not metabolized

Why Is Fiber So Important?

Although fiber is not digested, it plays a crucial role in maintaining digestive health and preventing chronic diseases.

• Digestive Health: Insoluble fiber promotes bowel regularity and helps prevent constipation and diverticulosis (formation of pouches in the colon).

• Weight Management: High-fiber foods increase satiety, helping to control calorie intake and support weight loss.

• Chronic Disease Prevention:

  • Soluble fiber lowers blood cholesterol and improves insulin sensitivity.

  • Both types of fiber may reduce risk of heart disease, diabetes, and certain cancers (e.g., colon, breast).

• Excess Fiber: Too much can reduce absorption of some vitamins and minerals and may cause diarrhea.

Type of Fiber	Source	Function
Soluble	Oats, beans, apples	Lowers cholesterol, slows glucose absorption
Insoluble	Whole grains, vegetables	Promotes bowel regularity

Example: Eating an apple (with skin) provides both soluble and insoluble fiber, supporting heart and digestive health.