Carbohydrates: Composition, Classification, Functions, Sources, and Recommended Daily Allowances
Introduction
Carbohydrates are the primary source of energy for the body. They are organic compounds made up of carbon (C), hydrogen (H), and oxygen (O), usually in a 1:2:1 ratio. Carbohydrates are essential for energy production, brain function, and metabolism.
Composition of Carbohydrates
Carbohydrates are composed of three main types based on their chemical structure:
1. Sugars (Simple Carbohydrates)
Definition: Small molecules that provide quick energy.
Examples: Glucose, fructose, sucrose, lactose.
Sources: Fruits, honey, milk, table sugar.
2. Starches (Complex Carbohydrates)
Definition: Large molecules made up of multiple sugar units.
Examples: Amylose, amylopectin.
Sources: Rice, wheat, potatoes, corn.
3. Cellulose (Dietary Fiber)
Definition: A type of indigestible carbohydrate found in plant cell walls.
Function: Promotes digestion and prevents constipation.
Examples: Raffinose, Stachyose (found in beans and legumes).
Polysaccharides (More than 10 sugar units)
Examples: Starch (grains, tubers), Glycogen (stored in liver and muscles), Cellulose (fiber from plant cell walls).
3. Digestibility-Based Classification
Available Carbohydrates (Digestible) – Provide energy (e.g., glucose, starch).
Non-Available Carbohydrates (Indigestible) – Provide fiber (e.g., cellulose, hemicellulose).
Classification of Carbohydrates
Introduction
Carbohydrates are organic compounds composed of carbon (C), hydrogen (H), and oxygen (O) in a ratio of 1:2:1. They are the primary source of energy for the body and are classified based on their complexity, structure, and digestibility.
Classification of Carbohydrates
Carbohydrates are classified into three major types based on their structure and composition:
1. Simple Carbohydrates (Sugars)
Contain one or two sugar units (monosaccharides or disaccharides).
Easily digestible and provide quick energy.
Sources: Fruits, honey, milk, sugar, candies.
a. Monosaccharides (Single Sugar Molecules)
Definition: The simplest form of carbohydrates that cannot be broken down further.
General Formula:C₆H₁₂O₆
Examples:
Glucose – Primary energy source (found in fruits, honey).
Fructose – Sweetest sugar (found in fruits, honey).
Galactose – Found in milk products.
b. Disaccharides (Two Sugar Units)
Definition: Formed by the combination of two monosaccharides.
Maltose = Glucose + Glucose (Found in germinating grains).
2. Complex Carbohydrates (Polysaccharides)
Contain many sugar units linked together.
Provide sustained energy and are harder to digest.
Sources: Whole grains, rice, potatoes, legumes.
a. Oligosaccharides (3-10 Sugar Units)
Definition: Short-chain carbohydrates found in plant foods.
Examples: Raffinose, Stachyose (found in beans, legumes).
Function: Act as prebiotics (support gut bacteria).
b. Polysaccharides (More than 10 Sugar Units)
Definition: Long-chain carbohydrates that serve as energy storage or structural components.
Examples:
Starch – Energy storage in plants (Rice, wheat, potatoes).
Glycogen – Energy storage in animals (Stored in liver and muscles).
Cellulose – Structural carbohydrate in plants (Fiber found in fruits, vegetables).
Chitin – Found in fungi and insect exoskeletons.
3. Digestibility-Based Classification
Carbohydrates can also be classified based on whether the human body can digest them or not.
Type
Function
Examples
Available Carbohydrates (Digestible)
Provide energy, easily absorbed
Glucose, Starch
Non-Available Carbohydrates (Indigestible)
Provide fiber, aid digestion
Cellulose, Hemicellulose
Comparison of Different Types of Carbohydrates
Type
Structure
Digestibility
Sources
Function
Monosaccharides
1 sugar unit
Easily digested
Fruits, honey
Quick energy
Disaccharides
2 sugar units
Easily digested
Milk, sugarcane
Energy supply
Oligosaccharides
3-10 sugar units
Partially digestible
Beans, legumes
Gut health
Polysaccharides
Many sugar units
Slowly digested or indigestible
Rice, wheat, vegetables
Energy storage, digestion
Functions of Carbohydrates
Primary Source of Energy
Provides 4 kcal per gram.
Essential for brain, muscles, and nervous system.
Spares Proteins
Prevents protein breakdown for energy.
Supports Fat Metabolism
Prevents ketosis (accumulation of ketone bodies).
Maintains Blood Sugar Levels
Glucose homeostasis ensures constant energy supply.
Aids Digestion and Gut Health
Fiber (cellulose) prevents constipation and supports gut bacteria.
Enhances Satiety
Fiber-rich foods reduce hunger and control weight.
Sources of Carbohydrates
Type
Food Sources
Simple Carbohydrates (Sugars)
Fruits, honey, milk, sugar, candy
Complex Carbohydrates (Starches)
Rice, wheat, potatoes, corn, legumes
Fiber (Cellulose & Hemicellulose)
Whole grains, vegetables, nuts, seeds
Recommended Daily Allowance (RDA) for Carbohydrates
The RDA for carbohydrates varies based on age, gender, and activity level.
Age Group
Carbohydrate Requirement (grams/day)
Infants (0-6 months)
60 g
Children (1-3 years)
130 g
Adolescents (14-18 years)
130-150 g
Adults
130-150 g
Pregnant Women
175 g
Lactating Women
210 g
Athletes
200-400 g (depending on activity level)
Carbohydrate Intake Guidelines
55-65% of daily calorie intake should come from carbohydrates.
At least 50% should be from complex carbohydrates (whole grains, vegetables).
Sugar intake should be limited to less than 10% of total calories.
Dietary fiber intake: 25-30 g/day.
Functions of Carbohydrates
Introduction
Carbohydrates are the primary source of energy for the human body. They play a crucial role in metabolism, digestion, brain function, and overall health. Apart from providing energy, carbohydrates also contribute to various physiological processes.
Major Functions of Carbohydrates
1. Primary Source of Energy
Carbohydrates provide 4 kcal per gram.
Glucose is the main fuel used by the brain, muscles, and nervous system.
During physical activity, stored glycogen in muscles is converted to glucose for energy.
2. Protein-Sparing Action
When sufficient carbohydrates are available, the body does not break down proteins for energy.
This helps proteins to focus on tissue repair, enzyme production, and growth.
3. Aids in Fat Metabolism
Carbohydrates are essential for the complete breakdown of fats.
Insufficient carbohydrates lead to ketosis (accumulation of ketone bodies), which can cause metabolic imbalances.
4. Maintains Blood Sugar Levels
Glucose homeostasis is regulated by insulin (lowers glucose) and glucagon (raises glucose).
Complex carbohydrates (whole grains, fiber-rich foods) help in gradual absorption and stable blood sugar.
5. Supports Brain and Nervous System Function
The brain relies on glucose as its primary fuel.
A lack of carbohydrates can lead to fatigue, mental confusion, and reduced cognitive function.
6. Provides Dietary Fiber for Digestion
Cellulose and fiber from plant foods aid in digestion, prevent constipation, and promote gut health.
Soluble fiber (found in oats, legumes) helps in cholesterol control.
Insoluble fiber (found in whole grains, vegetables) supports bowel movements.
Balanced carbohydrate intake helps in maintaining a healthy weight.
8. Supports Immune Function
Some carbohydrates (glycoproteins) are involved in immune response and cell signaling.
They help in the recognition of pathogens and the formation of antibodies.
9. Acts as a Structural Component
Certain carbohydrates (ribose, deoxyribose) are essential components of DNA and RNA, which are responsible for genetic information.
Glycoproteins and glycolipids are involved in cell communication and membrane structure.
10. Storage of Energy (Glycogen)
Excess glucose is stored as glycogen in the liver and muscles for later use.
During fasting or intense exercise, glycogen is converted back to glucose to supply energy.
Summary Table of Carbohydrate Functions
Function
Description
Example
Energy Source
Provides 4 kcal per gram
Glucose, Starch
Protein-Sparing
Prevents protein breakdown
Sufficient carbohydrate intake
Fat Metabolism
Prevents ketosis by aiding fat breakdown
Balance of fats and carbs
Blood Sugar Regulation
Maintains stable glucose levels
Insulin, Glucagon
Brain Function
Glucose is the main fuel for the brain
Cereals, Fruits
Digestive Health
Fiber aids digestion and prevents constipation
Whole grains, Vegetables
Weight Management
Increases satiety and reduces overeating
Fiber-rich foods
Immune Function
Supports immune response
Glycoproteins
Structural Role
Part of DNA, RNA, and cell membranes
Ribose, Glycolipids
Energy Storage
Stored as glycogen in liver and muscles
Glycogen
Energy in Nutrition: Unit of Energy (Kcal) –
Introduction
Energy is essential for the growth, development, and functioning of the human body. In nutrition, energy refers to the capacity of food to fuel bodily functions such as movement, digestion, circulation, and metabolism. The body derives energy from macronutrients (carbohydrates, proteins, and fats), which are broken down during metabolism.
Unit of Energy in Nutrition: Kilocalorie (Kcal)
Definition of Kilocalorie (Kcal)
A kilocalorie (Kcal) is the unit of measurement for energy provided by food.
1 Kcal = 1,000 calories (small calories).
In simple terms, 1 Kcal is the amount of heat required to raise the temperature of 1 kilogram of water by 1°C.
In nutrition, the term “Calorie” with a capital C is often used interchangeably with Kcal.
Energy Yield from Macronutrients
Different nutrients provide different amounts of energy:
Nutrient
Energy Value (Kcal per gram)
Examples
Carbohydrates
4 Kcal/g
Rice, wheat, fruits, potatoes
Proteins
4 Kcal/g
Meat, fish, eggs, legumes
Fats
9 Kcal/g
Butter, oil, nuts, seeds
Alcohol(not a nutrient)
7 Kcal/g
Beer, wine, spirits
Sources of Energy
Primary Energy Source: Carbohydrates (55-65% of total energy intake).
Secondary Energy Source: Fats (long-term energy storage).
Protein as Energy: Used when carbohydrate and fat intake is insufficient.
Importance of Energy in Nursing and Nutrition
Energy is crucial for bodily functions in all individuals, especially in clinical settings where nutrition plays a key role in patient recovery and health maintenance.
1. Energy for Basic Metabolic Functions
Basal Metabolic Rate (BMR): The minimum energy required for vital functions such as breathing, circulation, and cell repair.
Resting Energy Expenditure (REE): Similar to BMR but includes minimal activity.
2. Energy for Physical Activity
Light Activity: 30-50% of BMR (e.g., walking, light household work).
Moderate Activity: 50-70% of BMR (e.g., brisk walking, exercise).
Heavy Activity: 70-100% of BMR (e.g., running, manual labor).
3. Energy for Growth and Development
Children and Adolescents: Require higher energy intake for rapid growth.
Pregnancy and Lactation: Increased Kcal needs for fetal development and milk production.
4. Energy for Disease Recovery
Malnutrition and Wasting Conditions: Require high-calorie diets to restore energy balance.
Surgical and Trauma Patients: Need additional energy for wound healing and tissue repair.
Energy Requirements in Different Groups
The Recommended Daily Allowance (RDA) for energy varies based on age, gender, and activity level.
Age Group
Energy Requirement (Kcal/day)
Infants (0-6 months)
500-550 Kcal
Children (1-3 years)
1000-1300 Kcal
Adolescents (14-18 years)
2000-2800 Kcal (Males), 1800-2400 Kcal (Females)
Adults (Sedentary)
2000-2500 Kcal (Males), 1600-2000 Kcal (Females)
Pregnant Women
+300 Kcal
Lactating Women
+500 Kcal
Elderly (60+ years)
1800-2200 Kcal
Energy Balance in Nursing Practice
Energy balance is critical in patient care, ensuring that calorie intake meets energy expenditure.
1. Positive Energy Balance
Calorie intake > Energy expenditure
Leads to weight gain and obesity.
Seen in sedentary lifestyles and high-fat diets.
2. Negative Energy Balance
Calorie intake < Energy expenditure
Leads to weight loss, muscle breakdown, and fatigue.
Seen in malnutrition, eating disorders, and critical illness.
3. Clinical Applications in Nursing
Assessing Nutritional Status: Using BMI, dietary history, and calorie counting.
Monitoring Malnutrition Risks: Identifying underweight or obese patients.
Providing Diet Counseling: Recommending nutritionally balanced diets for patients with chronic diseases like diabetes, heart disease, and obesity.
Basal Metabolic Rate (BMR):
Introduction
Basal Metabolic Rate (BMR) is the minimum amount of energy required by the body to perform its basic vital functions while at rest. These functions include:
Breathing
Circulation (heart pumping)
Cellular metabolism
Body temperature regulation
Brain function
Muscle tone maintenance
Since BMR accounts for 60-75% of total daily energy expenditure (TDEE), it plays a crucial role in energy balance, weight management, and nutrition planning in nursing care.
Definition of BMR
Basal Metabolic Rate (BMR) refers to the rate at which the body burns calories to maintain basic physiological functions at complete rest, in a neutral temperature environment, and after an overnight fast.
It is measured in kilocalories per day (Kcal/day).
Key Characteristics of BMR
Measured under resting conditions (fasting for 12 hours, complete physical rest).
Represents energy used for vital organs (heart, lungs, liver, kidneys, brain, muscles).
Accounts for 60-75% of total energy expenditure in most individuals.
Influences caloric intake and weight management in patients.
Factors Affecting BMR
1. Age
BMR is highest in infants and children due to rapid growth.
Declines with age due to loss of muscle mass and metabolic slowdown.
2. Gender
Males have a higher BMR than females due to greater muscle mass.
Females have lower BMR because of a higher fat percentage.
3. Body Composition
More muscle mass = Higher BMR (muscle is metabolically active).
More fat mass = Lower BMR (fat requires less energy).
Larger body size = Higher BMR due to increased energy requirements.
6. Climate and Environmental Temperature
Cold environments increase BMR to generate heat.
Hot climates slightly raise BMR due to sweat production.
7. Physical Activity and Exercise
Regular exercise increases muscle mass, which boosts BMR.
Sedentary lifestyle reduces BMR over time.
8. Nutritional Status and Diet
Starvation and fasting lower BMR as the body conserves energy.
Protein-rich diets slightly increase BMR due to the thermic effect of food.
9. Illness and Fever
BMR increases by 7% for every 1°C rise in body temperature.
Chronic diseases (e.g., cancer, infections) may elevate BMR.
How to Measure BMR?
1. Direct Calorimetry
Measures heat production from the body in a controlled environment.
Highly accurate but expensive and rarely used.
2. Indirect Calorimetry (Most Common)
Measures oxygen consumption and carbon dioxide production.
Uses a metabolic cart in hospitals or research settings.
3. Predictive Equations (Estimated BMR)
Several formulas help estimate BMR based on age, weight, height, and gender.
a. Harris-Benedict Equation (Most Common)
Used to calculate BMR based on body weight, height, and age.
For Men:BMR=88.36+(13.4×weight in kg)+(4.8×height in cm)−(5.7×age in years)BMR = 88.36 + (13.4 \times \text{weight in kg}) + (4.8 \times \text{height in cm}) – (5.7 \times \text{age in years})BMR=88.36+(13.4×weight in kg)+(4.8×height in cm)−(5.7×age in years)
For Women:BMR=447.6+(9.2×weight in kg)+(3.1×height in cm)−(4.3×age in years)BMR = 447.6 + (9.2 \times \text{weight in kg}) + (3.1 \times \text{height in cm}) – (4.3 \times \text{age in years})BMR=447.6+(9.2×weight in kg)+(3.1×height in cm)−(4.3×age in years)
b. Mifflin-St Jeor Equation (More Accurate)
For Men: BMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)+5BMR = (10 \times \text{weight in kg}) + (6.25 \times \text{height in cm}) – (5 \times \text{age in years}) + 5BMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)+5
For Women: BMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)−161BMR = (10 \times \text{weight in kg}) + (6.25 \times \text{height in cm}) – (5 \times \text{age in years}) – 161BMR=(10×weight in kg)+(6.25×height in cm)−(5×age in years)−161
BMR and Total Daily Energy Expenditure (TDEE)
To determine the total calories a person needs daily, BMR is multiplied by an activity factor.
Pregnant women need extra calories to support fetal growth.
Elderly require fewer calories due to reduced BMR.
5. ICU and Critical Care Nutrition
Patients on ventilators or post-surgery require precise calorie intake based on BMR.
BMR and Energy Requirements for Different Age Groups
Age Group
BMR Range (Kcal/day)
Infants (0-6 months)
500-600
Children (1-3 years)
900-1200
Adolescents (14-18 years)
1400-2000
Adults (Sedentary)
1400-1800 (Females), 1600-2200 (Males)
Elderly (60+ years)
1200-1600
Summary Table of BMR Concepts
Factor
Effect on BMR
Age
Decreases with aging
Gender
Higher in males due to muscle mass
Muscle Mass
Increases BMR
Fat Mass
Decreases BMR
Hormones (Thyroid)
Increases or decreases BMR
Climate
Cold climates increase BMR
Physical Activity
Increases BMR
Factors Affecting Basal Metabolic Rate (BMR)
Introduction
Basal Metabolic Rate (BMR) is the amount of energy the body requires at rest to maintain essential functions such as breathing, circulation, temperature regulation, and cellular metabolism. It accounts for 60-75% of total daily energy expenditure (TDEE) and varies among individuals based on several factors.
Understanding factors affecting BMR is crucial in nutrition and nursing for dietary planning, weight management, and clinical care.
Factors Affecting BMR
BMR is influenced by biological, physiological, and environmental factors.
1. Age
BMR is highest in infants and children due to rapid growth and high metabolic activity.
BMR gradually decreases with age due to a decline in muscle mass and hormonal changes.
Elderly individuals have a lower BMR because of reduced physical activity and muscle loss (sarcopenia).
2. Gender
Men have a higher BMR than women because of greater muscle mass and lower body fat percentage.
Women have a lower BMR due to higher fat composition and hormonal differences (e.g., estrogen effects).
During pregnancy and lactation, BMR increases due to higher energy demands.
3. Body Composition (Muscle Mass vs. Fat)
Muscle tissue is metabolically active, requiring more energy for maintenance.
Higher muscle mass = Higher BMR (e.g., athletes, bodybuilders).
Fat tissue requires less energy, so individuals with higher fat percentages have a lower BMR.
4. Body Size and Weight
Larger body size = Higher BMR due to increased metabolic activity to maintain organ function.
Overweight individuals may have a slightly higher BMR due to greater body mass, but the rate of calorie burn per kilogram is lower than in lean individuals.
5. Hormonal Influence
Thyroid hormones (T3, T4) play a crucial role in regulating metabolism: