Introduction to Carbohydrates
Welcome to your study notes for Carbohydrates, an essential part of AS 1: Principles of Nutrition. Carbohydrates are our body's primary fuel source. In this chapter, we will unpack how carbohydrates are built, how they are classified nutritionally, their vital roles in the human body, the official government targets (DRVs), and how they impact health.
Don't worry if nutrition chemistry feels tricky at first! We will break everything down step-by-step using clear memory tricks and straightforward examples.
1. Chemical Composition & Classification
All carbohydrates are made from three simple chemical elements:
• Carbon (C)
• Hydrogen (H)
• Oxygen (O)
Chemically, carbohydrates are divided into three main groups depending on how many basic sugar units (monomers) are joined together:
A. Monosaccharides (Single Sugar Units)
These are the simplest carbohydrate units. They cannot be broken down any further during digestion and are absorbed directly into the bloodstream.
• Glucose: The primary fuel for cellular respiration and energy in the human body.
• Fructose: The sugar found naturally in fruits, vegetables, and honey.
• Galactose: A component of the sugar found in milk.
B. Disaccharides (Double Sugars)
Disaccharides are formed when two monosaccharides join together. Every disaccharide contains at least one molecule of glucose!
• Sucrose = Glucose + Fructose (Common table sugar, extracted from sugar cane or sugar beet)
• Lactose = Glucose + Galactose (The sugar naturally present in mammalian milk and dairy products)
• Maltose = Glucose + Glucose (Malt sugar, formed during the breakdown/germination of barley and starch)
Memory Trick: Remember "S-L-M" (Sucrose, Lactose, Maltose) and remember that Glucose is the VIP guest present in all three!
C. Polysaccharides (Complex Carbohydrates)
Polysaccharides are long chains consisting of many monosaccharide units joined together.
• Starch: The storage form of carbohydrate in plants. Starch consists of two structural polymers: Amylose (a straight chain) and Amylopectin (a branched chain). Examples include potatoes, rice, pasta, and bread.
• Glycogen: Known as 'animal starch', this is the storage form of carbohydrate in humans and animals, stored specifically in the liver and muscle tissue.
• Non-Starch Polysaccharides (NSP / Dietary Fibre): Structural components of plant cell walls (e.g., cellulose) that human digestive enzymes cannot break down.
• Pectin: A complex polysaccharide found in fruits (like apples and citrus fruits) that forms a gel when heated with sugar and acid.
Key Takeaway for Chemical Classification: Monosaccharides = 1 unit (Glucose, Fructose, Galactose); Disaccharides = 2 units (Sucrose, Lactose, Maltose); Polysaccharides = many units (Starch [Amylose & Amylopectin], Glycogen, NSP/Fibre, Pectin).
2. Modern Nutritional Classification (SACN 2015)
In modern nutritional science and for your CCEA exam, carbohydrates are classified according to the guidelines set out in the Scientific Advisory Committee on Nutrition (SACN) 2015 report ('Carbohydrates and Health').
Free Sugars
Free sugars are defined as:
• All monosaccharides and disaccharides added to foods and drinks by the manufacturer, cook, or consumer.
• Plus sugars naturally present in honey, syrups, and unsweetened fruit juices.
Important Exam Distinction: Free sugars do NOT include sugars naturally present within the intact cellular structure of whole fresh fruits and vegetables, nor do they include lactose naturally present in milk and dairy products!
Dietary Fibre
Under the SACN 2015 definition, dietary fibre includes all carbohydrates that are neither digested nor absorbed in the small intestine and have a degree of polymerization (DP) of \(3\) or more monomeric units. In the UK, this is officially measured using the AOAC method.
Key Takeaway for Nutritional Classification: Free sugars include added sugars, honey, syrups, and fruit juices, but exclude dairy lactose and intact cellular sugars in whole fruit. Fibre refers to non-digestible carbs with \(\text{DP} \ge 3\).
3. Functions of Carbohydrate in the Body
Carbohydrates carry out four major physiological functions:
1. Primary Source of Energy
Carbohydrates are the most accessible source of energy for the body and the only preferred fuel for the brain and central nervous system. In UK nutritional science and food labelling, carbohydrate provides precisely \(3.75\text{ kcal}\) (\(16\text{ kJ}\)) per gram.
2. Protein-Sparing Action
When you consume an adequate supply of carbohydrates, your body uses them for energy rather than breaking down dietary protein or muscle tissue. This 'spares' protein, allowing it to be used for its primary function: growth and repair.
3. Complete Metabolism of Fats
Carbohydrates are necessary for the complete breakdown and oxidation of fats. If carbohydrate intake is insufficient, the liver cannot oxidize fatty acids completely, leading to the accumulation of acidic compounds called ketone bodies, which results in a state called ketosis.
4. Energy Storage
Surplus dietary glucose is converted and stored as glycogen in the liver and skeletal muscles. When blood glucose drops, glycogen is converted back into glucose to supply steady energy.
Key Takeaway for Functions: Provides energy (\(3.75\text{ kcal/g}\)), spares protein, ensures complete fat oxidation (prevents ketosis), and provides storage as glycogen in liver and muscles.
4. Dietary Reference Values (DRVs)
The UK government and SACN provide specific targets for daily carbohydrate and fibre intake:
• Total Carbohydrate: Should provide approximately \(50\%\) of total dietary energy.
• Free Sugars: Should provide no more than \(5\%\) of total dietary energy for individuals aged \(2\) years and older.
Dietary Fibre DRVs (AOAC Method)
• Adults: \(30\text{ g}\) per day
• Children aged \(11\text{--}15\) years: \(25\text{ g}\) per day
• Children aged \(5\text{--}11\) years: \(20\text{ g}\) per day
• Children aged \(2\text{--}5\) years: \(15\text{ g}\) per day
Quick Review: Make sure you memorize the adult fibre target of \(30\text{ g}\) and the free sugar limit of \(\le 5\%\) of dietary energy—these are tested frequently in CCEA exam questions!
5. Health Impacts: Excess and Deficiency
Effects of Excess Free Sugars
• Dental Caries: Bacteria in dental plaque ferment free sugars, producing acid that demineralizes tooth enamel, causing tooth decay.
• Excess Energy Intake and Obesity: Foods and beverages high in free sugars are often energy-dense. Consuming more energy than the body expends leads to weight gain and obesity.
• Type 2 Diabetes: Excess free sugar intake contributes directly to weight gain, and obesity is a major risk factor for developing Type 2 Diabetes.
Health Benefits of Dietary Fibre
• Cardiovascular Disease (CVD): High fibre intake helps reduce blood cholesterol levels and regulates blood pressure, lowering the risk of coronary heart disease and stroke.
• Type 2 Diabetes: Fibre slows the digestion and absorption of carbohydrates, preventing sharp spikes in blood glucose levels and improving insulin sensitivity.
• Colorectal Cancer: Fibre adds bulk to the stool, dilutes potential carcinogens in the bowel, and speeds up transit time through the colon, reducing the risk of bowel cancer.
• Satiety and Weight Management: Fibre-rich foods take longer to chew and digest, creating a feeling of fullness (satiety) and reducing overall calorie overconsumption.
Effects of Carbohydrate Deficiency
• Hypoglycaemia: A drop in blood glucose levels resulting in weakness, dizziness, confusion, and shakiness.
• Fatigue and Lethargy: Insufficient immediate fuel for muscles and the brain.
• Ketosis: Incomplete breakdown of fats produces ketone bodies, causing nausea, fatigue, and bad breath.
Key Takeaway for Health: Excess free sugars cause tooth decay, obesity, and diabetes risk. Fibre protects against CVD, Type 2 diabetes, and colorectal cancer while increasing satiety. Deficiency causes hypoglycaemia, fatigue, and ketosis.
6. Common Exam Pitfalls & Examiner Tips
Watch out for these common student mistakes when answering CCEA exam questions:
• Pitfall 1: Confusing "Free Sugars" with "Total Sugars"
Exam Tip: Lactose in milk and natural sugars inside intact cellular walls of whole fruits are not free sugars. Never classify milk or an uncut apple as a source of free sugars!
• Pitfall 2: Incorrect Energy Values
Exam Tip: In CCEA A Level Nutrition, the precise UK standard value for carbohydrate energy is \(3.75\text{ kcal}\) (\(16\text{ kJ}\)) per gram. Do not write \(4\text{ kcal}\) unless an exam question specifically directs you to round up.
• Pitfall 3: Using Outdated Terminology
Exam Tip: Use Dietary Fibre (as defined by SACN 2015 and measured by AOAC) rather than relying solely on older terms like NSP.
• Pitfall 4: Oversimplifying Fibre's Role
Exam Tip: Do not just state that fibre "prevents constipation". High-scoring exam answers explain how dietary fibre reduces the risk of chronic conditions such as cardiovascular disease, Type 2 Diabetes, and colorectal cancer.