📚 Human Nutrition (Topic 302): Diet, Digestion & Absorption | 人体营养学（专题302）：饮食、消化与吸收
‘Nutrition’ refers to the intake and utilization of food by an organism. In IGCSE biology, understanding human nutrition is essential because it combines biological chemistry, physiology, and health education into a single coherent process.
“营养”是指生物体对食物的摄入与利用。在IGCSE生物课程中，理解人体营养至关重要，因为它将生物化学、生理学与健康教育融为一体，形成一个连贯的过程。
1. Essential Nutrients for Humans | 人体必需的主要营养素
Humans require a range of macronutrients and micronutrients to sustain life. Macronutrients include carbohydrates, proteins, and lipids, while micronutrients include vitamins and minerals. Water is also vital for all metabolic reactions.
人类需要多种常量营养素和微量营养素来维持生命。常量营养素包括碳水化合物、蛋白质和脂肪；微量营养素包括维生素和矿物质。水对一切代谢反应同样至关重要。
| Nutrient / 营养素 | Main Function / 主要功能 | Common Sources / 常见来源 |
| Carbohydrates / 碳水化合物 | Provide energy; store energy as glycogen / 提供能量；储存为糖原 | Rice, bread, potatoes, fruits / 米饭、面包、马铃薯、水果 |
| Proteins / 蛋白质 | Growth, repair, and maintenance of cells / 细胞的生长、修复与维持 | Meat, fish, eggs, beans, nuts / 肉、鱼、蛋、豆类、坚果 |
| Lipids (Fats) / 脂肪 | Insulation, protection, and long-term energy store / 保温、保护器官及长期储能 | Butter, oil, margarine, cheese / 黄油、食用油、人造黄油、奶酪 |
| Vitamin C / 维生素C | Maintains healthy skin, gums, and connective tissue / 维持皮肤、牙龈和结缔组织健康 | Citrus fruits, green vegetables / 柑橘类水果、绿叶蔬菜 |
| Calcium / 钙 | Builds strong bones and teeth; needed for blood clotting / 构建强健骨骼与牙齿；参与凝血 | Milk, cheese, green leafy vegetables / 牛奶、奶酪、绿叶蔬菜 |
| Iron / 铁 | Forms haemoglobin in red blood cells / 构成红细胞中的血红蛋白 | Red meat, liver, spinach, lentils / 红肉、肝脏、菠菜、扁豆 |
2. Balanced Diet and Energy Requirements | 均衡饮食与能量需求
A balanced diet contains all the required nutrients in the correct proportions. The amount of energy released by food is measured in kilojoules (kJ). Different activities consume different amounts of energy, so energy intake must match energy expenditure to maintain a constant body weight.
均衡饮食是指以正确比例含有所有必需营养素的饮食。食物释放的能量以千焦（kJ）为单位。不同活动消耗的能量不同，因此能量摄入必须与能量消耗相匹配，才能维持恒定的体重。
能量摄入 (Energy Intake) = 能量消耗 (Energy Expenditure)
Basal metabolic rate (BMR) is the minimum energy required by the body at rest to keep vital functions working. Factors such as age, sex, and physical activity affect an individual’s daily energy requirement.
基础代谢率是身体在静息状态下维持生命活动所需的最低能量。年龄、性别和体力活动等因素都会影响个人每日的能量需求。
3. Malnutrition and Diet Choices | 营养不良与饮食选择
Malnutrition is caused by a diet that is unbalanced, either lacking or exceeding certain nutrients. For example, a shortage of vitamin C leads to scurvy, while a lack of protein causes kwashiorkor. Excess intake of fats and sugars can lead to obesity.
营养不良是由于饮食不均，缺乏或过多摄入某类营养素所引起的。例如，缺乏维生素C会导致坏血病，缺乏蛋白质会引起夸希奥科病（恶性营养不良）。过量摄入脂肪和糖类则会导致肥胖。
Obesity increases the risk of diabetes, heart disease, and high blood pressure. Poor diet choices often involve high levels of processed food, saturated fats, and refined sugars, which provide ’empty’ calories with little nutritional value.
肥胖会增加患糖尿病、心脏病和高血压的风险。不良饮食选择通常涉及大量加工食品、饱和脂肪和精制糖，这些食物提供几乎没有营养价值的“空”热量。
4. The Alimentary Canal | 消化道及其结构
The alimentary canal is the long tube through which food passes. It is adapted for digestion and absorption, and the overall pathway in humans is clear and specific.
消化道是一条输送食物的长管道。它适应了消化和吸收的功能。人体的食物通路明确且固定。
Mouth 口腔 → Oesophagus 食道 → Stomach 胃 → Small Intestine 小肠 → Large Intestine 大肠 → Anus 肛门
Along this tract, several accessory organs, including the salivary glands, liver, gallbladder, and pancreas, help by secreting digestive juices or storing substances that break down food.
沿着消化道，还有唾液腺、肝脏、胆囊和胰腺等附属器官，它们分泌消化液或储存相关物质，帮助分解食物。
5. Mechanical Digestion vs Chemical Digestion | 物理消化与化学消化
Mechanical digestion involves physically breaking food into smaller pieces. This occurs in the mouth through chewing (teeth) and churning in the stomach. It increases the surface area for enzyme action.
物理消化是指将食物机械性地分解成更小的碎片。这发生在口腔中，通过牙齿的咀嚼以及胃部的蠕动搅拌来完成。这个过程增大了酶作用的表面积。
Chemical digestion involves enzymes breaking down large, insoluble molecules into smaller, soluble molecules. This allows nutrients to pass through the gut wall into the bloodstream. Both processes work together to achieve efficient digestion.
化学消化则是通过酶将大的、不溶性的分子分解成小的、可溶性的分子。这使得营养素能透过肠壁进入血液。这两种过程相互配合，实现高效的消化。
6. Enzymes in Digestion | 消化系统中的酶
Enzymes are biological catalysts that speed up chemical reactions. Each enzyme is specific to a particular substrate. In digestion, three main enzymes act on the three main macronutrients.
酶是能加速化学反应的生物催化剂。每种酶只对特定底物起作用。在消化过程中，三种主要的酶分别作用于三大类常量营养素。
| Enzyme / 酶 | Site of Action / 作用部位 | Substrate → Product / 底物 → 产物 |
| Amylase / 淀粉酶 | Mouth & Small Intestine / 口腔、小肠 | Starch → Maltose / 淀粉 → 麦芽糖 |
| Protease / 蛋白酶 | Stomach & Small Intestine / 胃、小肠 | Protein → Amino Acids / 蛋白质 → 氨基酸 |
| Lipase / 脂肪酶 | Small Intestine / 小肠 | Lipids → Fatty Acids + Glycerol / 脂肪 → 脂肪酸 + 甘油 |
Enzymes work best at their optimum temperature and pH. For example, pepsin in the stomach works best in acidic conditions, while pancreatic enzymes in the small intestine require an alkaline environment.
酶在各自适宜的温度和pH值下发挥最佳作用。例如，胃中的胃蛋白酶在酸性条件下活性最高，而小肠中的胰酶则需要碱性环境。
7. Digestion of Carbohydrates | 碳水化合物的消化
Starch is a large, insoluble polysaccharide. In the mouth, salivary amylase begins the breakdown of starch into the disaccharide maltose. In the small intestine, maltase (an enzyme on the intestinal lining) breaks maltose into glucose.
淀粉是一种大的、不可溶的多糖。在口腔中，唾液淀粉酶开始将淀粉分解为二糖麦芽糖。在小肠中，肠壁上的麦芽糖酶将麦芽糖分解为葡萄糖。
Starch (C₆H₁₀O₅)ₙ →(Amylase 淀粉酶) Maltose (C₁₂H₂₂O₁₁) →(Maltase 麦芽糖酶) Glucose (C₆H₁₂O₆)
Glucose is a soluble monosaccharide that is readily absorbed into the bloodstream across the small intestine. Excess glucose is stored in the liver and muscles as glycogen.
葡萄糖是可溶的单糖，能通过小肠壁迅速被血液吸收。多余的葡萄糖会在肝脏和肌肉中以糖原的形式储存。
8. Digestion of Proteins and Lipids | 蛋白质与脂肪的消化
Protein digestion begins in the stomach, where hydrochloric acid helps denature proteins and activates the enzyme pepsin, which breaks proteins into polypeptides. In the small intestine, trypsin (from pancreatic juice) continues the breakdown into shorter chains, eventually yielding amino acids.
蛋白质消化始于胃，胃酸能帮助蛋白质变性并激活胃蛋白酶，将蛋白质分解为多肽。在小肠中，胰蛋白酶（来自胰液）继续将其分解为短链，最终产生氨基酸。
Lipid digestion primarily occurs in the small intestine. Bile salts, made by the liver and stored in the gallbladder, emulsify fats into tiny droplets, increasing the surface area for lipase to act. Lipase then hydrolyses lipids into fatty acids and glycerol.
脂肪消化主要发生在小肠。肝脏分泌并由胆囊储存的胆汁盐能将脂肪乳化成微小液滴，增加脂肪酶作用的表面积。脂肪酶随后将脂肪水解为脂肪酸和甘油。
Lipids 脂肪 + Water 水 →(Bile + Lipase 胆汁+脂肪酶) Fatty Acids 脂肪酸 + Glycerol 甘油
9. Roles of the Liver, Gallbladder & Pancreas | 肝脏、胆囊与胰腺的作用
The liver is a major metabolic organ. In digestion, it produces bile, which neutralises stomach acid and emulsifies lipids. The gallbladder is a small organ that stores and concentrates bile produced by the liver until it is needed.
肝脏是重要的代谢器官。在消化中，它分泌胆汁，用以中和胃酸并乳化脂肪。胆囊是一个储存和浓缩胆汁的小器官，在需要时将胆汁释放入小肠。
The pancreas is an accessory gland that secretes pancreatic juice, containing amylase, proteases, and lipases, directly into the duodenum. It also secretes sodium hydrogencarbonate, which neutralises acidic chyme from the stomach, providing optimal conditions for pancreatic enzymes.
胰腺是附属腺体，它分泌含淀粉酶、蛋白酶和脂肪酶的胰液，直接进入十二指肠。同时它还分泌碳酸氢钠，以中和来自胃的酸性食糜，为
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