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Chapter 4: Carbohydrates, Diabetes - Lecture Review

What Are Carbohydrates?

  • One of the three macronutrients; important energy source, especially for nerve cells.

  • Composed of carbon (C), hydrogen (H), and oxygen (O).

  • Plant-derived sources include fruits, vegetables, and grains.

  • Glucose is the most abundant carbohydrate; produced by plants via photosynthesis; preferred energy source for the brain and important energy source for all cells.

ext{1 g carbohydrate} = 4\ \text{kcal}

Simple Carbohydrates

  • Simple carbohydrates contain one or two molecules.

  • Monosaccharides (one molecule): glucose, fructose, galactose, ribose.

  • Disaccharides (two molecules): lactose, maltose, sucrose.

Disaccharides (examples)

  • Lactose = glucose + galactose (milk sugar).

  • Maltose = glucose + glucose (forms when starch is broken down; involved in starch digestion).

  • Sucrose = glucose + fructose (found in sugar cane, sugar beets, honey).

Complex Carbohydrates

  • Include starch, glycogen, and dietary fiber.

  • Amylose and amylopectin are the two forms of starch.

  • Glycogen is the storage form of glucose in animals; stored in liver and muscles; not found in foods.

  • Dietary fiber: nondigestible part of plants; functional fiber: nondigestible form added to foods with known health benefits; total fiber = dietary fiber + functional fiber.

  • Common fiber sources include cellulose, guar gum, pectin, psyllium.

Starch, Glycogen, and Fiber Details

  • Starch: plants store glucose as polysaccharides (amylose and amylopectin) in grains, legumes, and tubers; we digest starch to glucose.

  • Glycogen: stored in liver and muscles; not present in foods; not a dietary carbohydrate source.

  • Fiber types:

    • Dietary fiber: nondigestible part of plants.

    • Functional fiber: nondigestible form added to foods; has health benefits.

    • Total fiber = dietary fiber + functional fiber.

Fiber: Soluble vs Insoluble

  • Soluble fiber:

    • Dissolves in water; viscous and fermentable; easily fermented by colonic bacteria.

    • Sources: citrus fruits, berries, oats, beans.

    • Health benefits: lowers blood cholesterol and glucose; reduces risk of cardiovascular disease and type 2 diabetes.

    • ext{Soluble fiber}
      ightarrow ext{lower cholesterol/glucose}

  • Insoluble fiber:

    • Generally does not dissolve in water; sources include whole grains (wheat, rye, brown rice) and many vegetables.

    • Benefits: promotes regular bowel movements, reduces risk of diverticulosis.

Why Do We Need Carbohydrates?

  • Energy for daily activity and exercise.

  • Helps preserve protein for other uses.

  • If diet lacks carbohydrates, gluconeogenesis converts proteins to glucose.

Carbohydrate Use by Exercise Intensity

  • Light exercise: about

    • Carbohydrate: ~12.5% of energy

    • Fat: ~87.5%

  • Moderate exercise:

    • Carbohydrate: ~45%

    • Fat: ~55%

  • Intense exercise:

    • Carbohydrate: ~67%

    • Fat: ~33%

Carbohydrate Energy and Ketosis

  • Each gram of carbohydrate provides 4\ \text{kcal} of energy.

  • Red blood cells rely entirely on glucose for energy.

  • Carbohydrates and fats both supply energy for daily activities; glucose is especially important during exercise.

  • Sufficient carbohydrate intake prevents production of ketones as alternative energy; excessive ketones can cause metabolic acidosis.

  • Ketoacidosis is a state of high blood acidity that can damage tissues.

Carbohydrate and Fiber Benefits

  • Fiber may reduce colon cancer risk; promotes bowel health and regularity; reduces risk of hemorrhoids.

  • Fiber may reduce risk of heart disease and type 2 diabetes; may aid in weight management; may lower diverticulosis risk.

Diverticulosis (concept drawn in class diagram)

  • Diverticula are small pouches that can form in the wall of the colon.

  • Diverticulosis refers to the condition where these diverticula are present.

Fiber and Cholesterol: How Fiber Impacts Bile and Cholesterol

  • The liver synthesizes bile from cholesterol; bile is stored in the gallbladder and released into the small intestine to aid fat digestion.

  • A high-fiber diet increases bile binding in the intestine; bile is excreted in feces.

  • With less bile available for reabsorption, less cholesterol is recycled; blood cholesterol levels fall.

Digestion of Carbohydrates

  • Primary digestion occurs in the small intestine.

  • Pancreatic amylase digests starch into maltose in the small intestine.

  • Mucosal enzymes (maltase, sucrase, lactase) digest disaccharides into monosaccharides.

  • Monosaccharides are absorbed by enterocytes and enter the bloodstream.

  • In the liver, monosaccharides are converted to glucose and supplied to body cells; excess glucose is stored as glycogen in liver and muscle.

Digestive Organs and Steps (summary)

  • Mouth: Chewing stimulates saliva; salivary amylase digests starch into shorter polysaccharides and maltose.

  • Stomach: Salivary amylase is inactivated by stomach acid; no carbohydrate digestion occurs here.

  • Small intestine: Pancreatic amylase continues starch digestion to maltose; disaccharidases in enterocytes convert disaccharides to monosaccharides.

  • Large intestine: Some carbohydrates pass through undigested; bacteria ferment some undigested carbs; fiber remains and is excreted.

  • Portal circulation: Monosaccharides travel to liver via portal vein; liver converts to glucose; glucose is released into bloodstream for energy; excess stored as glycogen.

The Process of Glycogenesis

  • Dietary carbohydrates are digested to monosaccharides and transported to the liver as glucose.

  • In the liver, glucose is stored as glycogen (glycogenesis).

  • Muscle glycogen also serves as a glucose reserve for muscle cells.

  • Blood glucose levels are maintained for brain and other cells.

Regulation of Blood Glucose: Insulin

  • Insulin is a hormone secreted by the pancreas; travels in the bloodstream.

  • Function: helps transport glucose from the blood into cells; stimulates liver and muscle uptake and conversion of glucose to glycogen (glycogenesis); stimulates storage as triglycerides in adipose tissue (lipogenesis).

  • High blood glucose after a meal triggers insulin release from beta cells.

  • Cellular uptake: insulin promotes translocation of glucose transporters to the cell membrane, facilitating glucose entry into cells.

  • In summary: Insulin lowers blood glucose and promotes storage as glycogen and fat.

Regulation of Blood Glucose: Glucagon

  • Glucagon is another pancreatic hormone secreted by alpha cells.

  • Function: stimulates glycogen breakdown (glycogenolysis) in the liver to release glucose; stimulates gluconeogenesis (production of glucose from amino acids).

  • Low blood glucose triggers glucagon release.

Regulation of Blood Glucose: Fructose

  • Fructose does not stimulate insulin release.

  • Metabolized differently from glucose; absorbed further down in the small intestine.

Glycemic Index and Glycemic Load

  • Glycemic index (GI): a measure of a food’s ability to raise blood glucose levels after consumption.

  • Low GI foods cause small to moderate fluctuations in blood glucose.

  • Glycemic load (GL): amount of carbohydrate in a food multiplied by its GI; considered a more useful tool than GI alone.

Examples: Glycemic Index (GI) Illustrations (as shown in the slide)

  • High GI examples (approximate values from chart):

    • Instant mashed potatoes ~56

    • Rice Krispies ~85

    • Jelly beans ~82

    • White bread ~78

    • Ice cream ~73

    • White rice ~61

    • Glucose reference ~100

  • Moderate/low GI examples (approximate):

    • Banana ~52

    • Oat bran bread ~47

    • Orange ~42

    • Apple ~38

    • Kidney beans ~28

  • Note: Lower GI and higher fiber foods are generally better for blood glucose control.

Blood Glucose Levels and Diet Quality

  • Meals with lower GI/GL are generally better for people with diabetes.

  • Lower GI meals are often higher in fiber.

  • Lower GI/GL meals may reduce risk of heart disease and colon cancer; may be linked to reduced risk of prostate cancer.

How Much Carbohydrate Should We Eat?

  • RDA for carbohydrate: 130 g per day (to supply brain).

  • AMDR (Acceptable Macronutrient Distribution Range) for carbohydrate: 45\% \le\text{AMDR} \le 65\% of total daily energy intake.

  • Added sugars: recommended to be 25% or less of total daily energy intake.

  • Focus on foods high in fiber and low in added sugars.

Dietary Recommendations (Key Points)

  • RDA: ext{RDA}_{carbs} = 130\ \text{g/day}.

  • AMDR: 45\% \le \text{AMDR}_{carbs} \le 65\% of total daily energy.

  • Added sugar target: ext{Added sugar energy} \le 0.25 \times \text{Total energy} (i.e., ≤ 25% of total energy).

  • Dietary pattern should emphasize vegetables, fruits, whole grains, legumes, and healthy fats;
    include whole grains as kernels retaining bran, endosperm, and germ.

How Much Carbohydrate Should We Eat? (Continued)

  • Most Americans consume too much added sugar.

  • Common sources of added sugars include soft drinks, cookies, candy, and fruit drinks; there are other sources like flavored rice mixes, salad dressings, peanut butter, etc.

  • Added sugars are not chemically different from natural sugars but often come with fewer nutrients.

Forms of Sugar on Food Labels (Table 4.2 Synopsis)

  • Common names for sugars include brown sugar, cane sugar, confectioner’s sugar, corn syrup, high-fructose corn syrup, honey, invert sugar, maple sugar, molasses, raw sugar, sugar alcohols, sorbitol, turbinado sugar, and others.

  • Also includes terms for natural sweeteners such as fructose and alternative sweeteners.

  • Non-nutritive (alternative) sweeteners provide little to no energy; nutritive sweeteners provide about 4 kcal/g; sugar alcohols provide about 2–3 kcal/g.

  • Non-nutritive sweeteners include saccharin, aspartame, and sucralose; acesulfame-K is another example.

  • Saccharin has no established Acceptable Daily Intake (ADI) in some contexts; others have set ADIs for various sweeteners.

Alternative Sweeteners: ADI and Safety (Tables 4.4 and 4.2/4.3 context)

  • Nutritive sweeteners: ~4 kcal/g; examples include sucrose, fructose, honey, brown sugar, etc.

  • Sugar alcohols: ~2–3 kcal/g; can reduce glycemic response and dental caries risk.

  • Non-nutritive sweeteners: provide little/no energy; designed to sweeten foods without typical risks.

  • ADIs (Acceptable Daily Intakes) have been established for several artificial sweeteners (e.g., acesulfame-K, aspartame, sucralose).

  • Saccharin: historically no ADI for some uses; later regulatory updates may vary by region.

  • Table 4.4 illustrates rough daily intakes needed to exceed the ADI for aspartame in a 50-lb child vs a 150-lb adult using common foods (e.g., diet beverages, gelatin desserts, packets of tabletop sweetener).

Fiber Content and Whole Grains: Labels and Intake

  • Whole grains include bran, endosperm, and germ; whole-wheat flour is unrefined and contains all three components.

  • On nutrition labels, you’ll see information about total carbohydrate, dietary fiber, and sugars; ingredients may include various sugars and sweeteners.

  • Terms describing grains on labels (Table 4.3):

    • Brown bread: may or may not be whole-grain; often colored to appear brown.

    • Enriched (fortified): nutrients added back to refined foods.

    • Refined: coarse parts removed; refined white flour is common.

    • Stone ground: involves grinding with limestone; does not guarantee whole-grain content.

    • Unbleached flour: refined but not bleached; similar to refined white flour.

    • Whole-grain flour / Whole-wheat flour: refers to flour milled from whole grain; not necessarily the same as “brown bread.”

Fiber Content of Common Foods (Conceptual Reference)

  • A variety of foods (breads, vegetables, fruits, legumes) provide dietary fiber; foods and serving sizes are listed to illustrate fiber content ranges.

  • Typical serving FS (grams) vary; major idea: higher fiber foods include legumes, whole grains, vegetables, and certain fruits; lower fiber options include refined grains and many processed foods.

Alternative Sweeteners: Practical Notes

  • Nutritive sweeteners provide energy; sugar alcohols offer lower energy and may reduce glycemic response.

  • Non-nutritive sweeteners offer little to no energy and are used to replace sugar in many products.

  • ADIs help regulate safe daily intakes; tabled values depend on regulatory bodies and product types.

Quick Reference Formulas and Key Numbers

  • Energy from carbohydrate: 4\ \text{kcal/g}

  • RDA for carbohydrate: ext{RDA}_{carbs} = 130\ \text{g/day}

  • AMDR: 45\% \le \text{AMDR}_{carbs} \le 65\% of total daily energy

  • Added sugars: ext{Added sugar energy} \le 0.25 \times \text{Total energy} (i.e., ≤ 25% of total energy)

  • AI for fiber: AI = 14\ \text{g per 1000 kcal}; adults: women ~25 g/day, men ~38 g/day

  • GI: range 0–100; foods are categorized as low, moderate, or high GI based on their effects on blood glucose

  • GL: GL = \text{carbohydrate (g)} \times GI

Connections to Health and Real-World Relevance

  • Emphasizes the role of carbohydrates in energy provision, especially for the brain and during exercise.

  • Shows how fiber benefits align with heart health, digestive health, and cancer risk reduction.

  • Highlights the importance of controlling dietary sugars for energy balance, dental health, and chronic disease risk.

  • Provides framework for evaluating foods via GI and GL, labeling terms for grains, and recognizing fiber content on Nutrition Facts panels.

Ethical, Philosophical, and Practical Implications

  • Balancing nutrient intake: emphasize nutrient-dense carbohydrates (whole grains, fruits, vegetables, legumes) over foods high in added sugars.

  • Industry labeling and marketing practices (brown colorings, enrichment) influence consumer choices; understanding labels is important for informed decisions.

  • Public health recommendations aim to reduce sugar consumption while promoting fiber-rich foods to support long-term health outcomes.