Comprehensive Notes on Macronutrients: Focus on Carbohydrates
Macronutrients Overview
Macronutrients provide energy (calories) to the body.
Fat: 9 \text{ kcal/g}
Carbohydrate: 4 \text{ kcal/g}
Protein: 4 \text{ kcal/g}
Ethyl Alcohol: 7 \text{ kcal/g}
Dietary Carbohydrates: Sugars and Starches
Importance:
Form the basis of most modern diets, providing > 1/2 of kilocalories (kcals) consumed by Americans.
Provide a readily available source of energy: 4 \text{ kcal} per gram.
When consumed in "whole food" form, they provide a wide variety of nutrients essential for health.
Energy Density Comparison: 1 \text{ g} of carbohydrates = 4 \text{ kcal}, while 1 \text{ g} of fat = 9 \text{ kcal}.
Sources of Carbohydrates
Carbohydrates are found in a wide array of foods including:
Grains: Oatmeal, spaghetti, whole-wheat bread, corn, potatoes.
Fruits: Orange, kiwi, banana.
Vegetables: Carrot, broccoli.
Legumes: Kidney beans.
Dairy: Low-fat plain yogurt, 1\% milk (though the primary macronutrient is not carbohydrate).
Many other sources listed in the provided chart (e.g., nuts, seeds, some processed foods).
Whole versus Refined Carbohydrates
Unrefined (Whole-Grain) Products:
Contain three key components: typically the bran, germ, and endosperm.
Retain many essential nutrients naturally found in the food.
Refined Grains:
Undergo processing that separates carbohydrates from many essential nutrients.
May be enriched with thiamin, riboflavin, niacin, and iron.
May be fortified with folate.
However, they do not contain the magnesium, vitamin E, some B vitamins, or other beneficial nutrients present in whole grains.
Trends in Dietary Macronutrient Intake (US Adults, 1999-2016)
Data from NHANES surveys showed:
A significant decrease in estimated percentage of energy intake from total carbohydrates (P < .001 for trend).
A significant increase in estimated percentage of energy intake from total protein (P < .001 for trend).
A significant increase in estimated percentage of energy intake from total fat (P < .001 for trend).
These trends indicate changes in eating patterns among US adults over this period.
High-Fructose Corn Syrup (HFCS) and Obesity
A visual correlation was presented between the increase in HFCS consumption and the prevalence of obesity among U.S. adults from 1970 to 2010. While the graph illustrates a parallel increase, the lecture did not explicitly state causation.
Added versus Naturally Occurring Sugar
Naturally occurring sugars (e.g., in fruits like kiwi) come packaged with additional nutrients like Vitamin C, folate, and calcium.
Added sugars (e.g., in red licorice) often provide calories primarily from carbohydrates with fewer other beneficial nutrients.
Types of Carbohydrates
Simple Carbohydrates:
Monosaccharide: Made up of a single sugar unit.
Disaccharide: Made up of two sugar units.
Complex Carbohydrates:
Oligosaccharides: Short chains containing 3-10 monosaccharides.
Polysaccharides: Long chains of many sugar units.
Monosaccharides
Definition: The basic unit of a carbohydrate is a single sugar molecule.
Common Dietary Monosaccharides:
Glucose: Also known as "blood sugar," it is the most important carbohydrate fuel for the body and the main circulating sugar.
Fructose: Found in fruits and honey.
Galactose: Part of milk sugar (lactose).
Glucose Regulation:
Under tight regulatory control to maintain blood glucose at a constant level.
Produced from the liver and disposed of in muscle.
The primary fuel for the central nervous system.
Low blood sugar (hypoglycemia) can occur if levels drop too low.
Sources of Glucose:
Dietary carbohydrate.
Gluconeogenesis: Production of glucose from non-carbohydrate sources (e.g., amino acids, glycerol) primarily in the liver.
Precursor amino acids can come from dietary protein or the breakdown of body protein (muscle).
Disaccharides
Definition: Made of two monosaccharides linked together.
Examples:
Sucrose (Table Sugar): Formed by linking glucose with fructose.
In the U.S., sucrose is the only sweetener that can be legally labeled "sugar."
Lactose (Milk Sugar): Glucose + Galactose.
Maltose (Malt Sugar): Glucose + Glucose.
Making and Breaking Sugar Chains
Hydrolysis Reaction: Breaks sugar molecules apart (e.g., during digestion).
Dehydration Reaction: Links two sugar molecules together (e.g., during synthesis of larger carbohydrates).
Complex Carbohydrates (Polysaccharides)
Glycogen:
Found in animals.
The storage form of glucose in the body.
Primarily stored in the liver and muscle cells.
Liver glycogen increases after a meal and is depleted by an overnight fast.
Starch:
Found in plants.
The storage form of carbohydrates in plants (e.g., potato starch granules).
Composed of amylose and amylopectin.
Fiber (Cellulose):
Found in plants.
Human digestive enzymes cannot break it down.
Adds bulk to the diet, increasing stool volume.
Dietary Fiber
Definition: Cannot be digested or absorbed by humans but is crucial for the digestive process and GI tract health.
Two Types of Fiber:
Soluble Fiber: Dissolves in water.
Good sources: Legumes, prunes, apricots, raisins, oranges, bananas, oats, apples, eggplant, flaxseed.
Can bind dietary cholesterol and reduce its absorption, thus reducing the risk of heart disease.
Insoluble Fiber: Doesn't dissolve in water.
Good sources: Wheat bran, whole-wheat bread, broccoli, corn, eggplant, apple skins, nuts and seeds.
Applications: Fibers can be added to processed foods to thicken them and reduce fat and calories.
Effect on Digestion: Fiber slows down digestion and absorption of nutrients.
Carbohydrate Digestion and Absorption
Mouth: Salivary amylase begins breaking starch into shorter polysaccharides.
Stomach: Salivary amylase is inactivated by stomach acid; no significant carbohydrate digestion occurs.
Small Intestine: Pancreatic amylase further breaks down starch into disaccharides and oligosaccharides.
Villi of Small Intestine: Enzymes attached to the microvilli complete the digestion of disaccharides and oligosaccharides into monosaccharides. These monosaccharides are then absorbed into the capillaries and transported to the liver.
Large Intestine: Fiber is partially broken down by bacteria, forming short-chain fatty acids and gas. The remaining fiber is excreted in the feces.
Lactose Intolerance
Cause: Insufficient enzyme lactase in the small intestine to digest the milk sugar lactose.
Mechanism: Undigested lactose passes into the large intestine where it is rapidly metabolized by intestinal bacteria, producing acids and gas.
Symptoms: Abdominal distention, flatulence, cramping, and diarrhea.
Prevalence: Varies significantly by ethnicity, with higher rates observed in Native Alaskans, African Americans, and American Indians (51-100\%$%) compared to some other populations.
Blood Glucose Regulation
The liver and pancreas tightly regulate blood glucose concentration to ensure a steady supply of glucose to body cells.
High Blood Glucose Levels (After a meal):
The pancreas releases insulin.
Insulin stimulates the uptake of glucose by muscle and fat cells.
Insulin promotes glycogenesis (glucose storage as glycogen in liver and muscle) and lipogenesis (conversion of excess glucose to fat).
Result: Blood glucose levels are reduced.
Low Blood Glucose Levels (Several hours after a meal):
The pancreas releases glucagon.
Glucagon stimulates the liver to break down glycogen (glycogenolysis) into glucose.
Glucagon also stimulates the liver to synthesize new glucose molecules (gluconeogenesis) from non-carbohydrate precursors.
Result: Glucose is released into the blood, raising blood glucose levels.
Glycemic Response
Glycemic Response: How quickly and how high blood glucose rises after carbohydrates are consumed.
Glycemic Index (GI):
A ranking of how a food affects the glycemic response, relative to a standard food (usually white bread or pure glucose) within two hours after consumption.
A GI of 70 means the food causes 70\% of the blood glucose response observed with the same amount of carbohydrate from pure glucose.
Categories:
Very Low GI ($\le 40$): Raw apple, lentils, soybeans, kidney beans, cow's milk, boiled carrots, barley, fructose.
Low GI ($41-55$): Noodles and pasta, apple juice, raw oranges/orange juice, raw banana, specialty grain bread, strawberry jam, sweet corn, chocolate.
Intermediate GI ($56-70$): Brown rice, rolled oats, soft drinks, pineapple, sucrose (table sugar), honey.
High GI ($> 70$): Bread (white or wholemeal), boiled potato, cornflakes, French fries, mashed potatoes, white rice (low amylose or "sticky rice"), rice crackers.
Glycemic Load (GL): Calculated by multiplying a food’s glycemic index by the amount of available carbohydrate in a serving of that food. It considers both the quality and quantity of carbohydrates.
Effect of Refined vs. Unrefined Carbohydrates: Unrefined carbohydrates tend to have a lower and slower impact on blood glucose levels compared to refined carbohydrates.
Metabolic Fate of Carbohydrates
Glycogen Storage: Glucose is stored as glycogen in the liver and muscles.
Oxidation (Conversion to Energy): This metabolic pathway uses 6 molecules of oxygen ($O2$) to convert 1 molecule of glucose ($C6H{12}O6$) into 6 molecules of carbon dioxide ($CO2$), 6 molecules of water ($H2O$), and approximately 38 molecules of ATP (adenosine triphosphate) for energy.
Lipogenesis (Fat Synthesis):
Very little glucose (from starches like rice, bread, pasta) is converted to fat for storage in the body under normal conditions.
However, fructose has no storage capacity in the body, and its excess intake can more readily lead to lipogenesis (fat synthesis).
Diabetes Mellitus
Definition: A group of metabolic diseases characterized by high blood glucose levels.
Three Main Types:
Type 1 Diabetes: The body no longer produces insulin; an autoimmune condition.
Type 2 Diabetes: Insulin is present, but the cells do not respond effectively to it (insulin resistance).
Gestational Diabetes: Occurs during pregnancy.
Insulin Resistance: A key feature of Type 2 Diabetes, occurring in:
Skeletal Muscle: The primary site of glucose disposal. Insulin is needed to transport glucose into muscle cells. In insulin resistance, a greater secretion of insulin is required, leading to decreased glucose disposal and elevated blood glucose.
Liver: Responsible for glucose storage and release to maintain blood glucose levels. Insulin normally shuts down hepatic glucose production after a meal. In insulin resistance, insulin has a diminishing effect, resulting in overproduction of glucose by the liver and elevated blood glucose.
Glycosylation:
Glucose is "sticky" and binds to proteins in the body.
High blood glucose levels result in glycosylated hemoglobin (HbA1c) and many other glycosylated proteins.
Hemoglobin A1C: A measure reflecting average blood glucose levels over the past 2-3 months. A high HbA1c indicates poor glucose control.
Consequences: Cataract formation in the eye is a consequence of glycosylation.
Symptoms and Complications:
Immediate Symptoms: Excessive thirst, frequent urination, blurred vision, weight loss.
Long-Term Complications: Damage to the heart, blood vessels, kidneys, eyes, and nervous system. Infections are more common, and amputations may be necessary in severe cases.
Components of Daily Energy Expenditure
Basal Metabolic Rate (BMR): The energy expended for basic life-sustaining functions.
Body Composition: Muscle mass is the most important determinant of BMR.
Energy Balance: A hypocaloric diet (caloric restriction) can reduce metabolic rate.
Thermic Effect of Feeding (TEF): The energy expended to digest, absorb, and metabolize food.
Fat is far less thermogenic than carbohydrate.
Protein is more thermogenic than carbohydrate.
Physical Activity: Energy expended through movement.
Voluntary: Exercise and daily activities.
Involuntary: Fidgeting (Non-Exercise Activity Thermogenesis, NEAT).
Myths vs. Realities Regarding Macronutrients and Weight
Myth: "All Calories contribute equally to energy balance – A Calorie is a Calorie."
Reality: While a calorie is a unit of energy, the Thermic Effect of Food (TEF) differs among macronutrients:
Protein > Carbohydrate > Fat (meaning more energy is expended to process protein than carbohydrates, and more for carbohydrates than fat).
Myth: "Carbohydrate causes an increase in fat." or "Carbohydrates increase the risk of diabetes." or "Carbohydrates should be eliminated from the diet."
Reality (from studies presented):
Induced Obesity Study (Sims et al.): It took 5 times as many kcals to produce the same weight gain with a mixed diet compared to a high-fat diet, suggesting fat is more easily stored as body fat than mixed macronutrients.
Lipogenesis from a Large Amount of Sugar (Acheson et al.): Consumption of 500 \text{ g} (2,000 \text{ kcal}) of glucose resulted in only 5 \text{ g} (45 \text{ kcal}) of lipid being made. The metabolic fate was primarily oxidation for energy and glycogen storage.
Dietary Intake, Oxidation, and Storage of Macronutrients (Jequier et al.): In eucaloric conditions (no weight gain/loss), lipid intake often matches oxidation. In hypercaloric conditions, the body tends to store excess lipid more readily than carbohydrate or protein.
High Carbohydrate, Low Fat Diet and Weight Management
High Carbohydrate, Low Fat Ad Libitum Study (Hays et al., 2004):
Participants: Groups on a control diet ($40\%$ fat, $40\%$ CHO, $20\%$ Pro), a high-carbohydrate (HICHO) diet ($20\%$ fat, $60\%$ CHO, $20\%$ Pro) without exercise, and a HICHO diet with exercise.
Findings:
No change in energy intake in any group and no difference in energy intake at completion.
The high-carbohydrate, ad libitum diet (with or without aerobic exercise) led to approximately 1 \text{ lb/wk} of weight loss.
No reduction in basal metabolic rate was observed, despite no attempt at kcal restriction.
Significant improvement in insulin sensitivity, decreasing the risk of type 2 diabetes.
Body fat percentage decreased significantly in both HICHO groups compared to the mixed diet group.
Low-fat Dietary Pattern and Weight Change Over 7 Years: The Women's Health Initiative Dietary Modification Trial (Howard et al., 2006):
Participants: 48,835 postmenopausal women were divided into an intervention group ($29,294$ women aiming to decrease fat and increase fruit/grain intake) and a control group ($19,541$ women).
Findings:
Women in the intervention group lost an average of 2.2 \text{ kg} ($P<0.001$) and maintained a lower weight compared to the control group for 7.5 years.
Weight loss was greatest among women in either group who decreased their percentage of energy from fat.
Food Environment and Portions
Examples from the "Restaurant Hall of Shame" highlight extremely high-calorie and high-fat foods offered as single servings (e.g., Cheese fries with ranch dressing: 3,010 \text{ kcal}, 217 \text{ g} fat).
Soda fountain drink sizes have drastically increased over the years (e.g., McDonald's Large Soda: 7 \text{ oz} in 1955 to 32 \text{ oz} in 1990; 7-11 Big Gulp: 40 \text{ oz}$$).
Carbohydrates and Heart Disease
Diets high in whole grains have been consistently found to reduce the risk of heart disease.
Water-soluble fiber plays a crucial role by binding dietary cholesterol and reducing its absorption in the body.