Fundamentals of Nutrition: Understanding the body's energy sources, pathways of energy expenditure, and functions of macronutrients
Human survival requires a constant supply of nutrients from the external environment; these substances are called nutrients. The functions of nutrients can be summarized as follows: providing the energy needed for various bodily activities and maintaining body temperature; providing the materials needed for growth, development, and cell tissue renewal; and regulating physiological activities to ensure coordinated metabolism within the body. The nutrients required by the human body can be divided into five major categories: carbohydrates, proteins, lipids, vitamins, macroelements, and microelements. Some advocate including water and dietary fiber, thus reaching seven categories.
Carbohydrates, proteins, and lipids are required in large quantities and are therefore called macronutrients. These three are the three major nutrients that provide energy to the human body. Vitamins and macro- and micronutrients are required in relatively smaller quantities and are collectively called micronutrients. There are more than 40 essential nutrients for the human body. This chapter will only focus on the three major energy nutrients and several micronutrients that are often insufficiently consumed in the diets of Chinese residents.
energy
Just as a car needs fuel to run, the human body also needs energy to sustain life and perform various activities. Even during sleep, energy is required for the heart to beat, for breathing to continue, and for glands to secrete fluids. (I) Sources of Energy for the Human Body. The human body's energy supply mainly comes from the three major nutrients in food: 1 gram of carbohydrates, after digestion and absorption, produces 16.7 kilojoules (4 kilocalories) of energy through oxidation in the body; 1 gram of protein produces 16.9 kilojoules (4 kilocalories) of energy through oxidation in the body; and 1 gram of fat produces 37.56 kilojoules (9 kilocalories) of energy through oxidation in the body. In addition, 1 gram of alcohol produces 29.29 kilojoules (7 kilocalories) of energy in the body.
(II) Energy Consumption of the Human Body
Adults expend energy through three pathways: First, basal metabolism, which refers to the energy consumed by the body to maintain basic life functions such as body temperature, heartbeat, and respiration when awake, fasting, and at rest. Second, the thermic effect of food (also known as the specific dynamic action of food), which refers to the energy consumed during the digestion, absorption, and transport of food after ingestion. The amount of energy consumed in this way is related to the composition of the food. Foods high in protein have the greatest thermic effect, meaning the body releases the most heat after consuming high-protein foods, while the thermic effect of fats and carbohydrates is lower than that of protein.
The third pathway to energy expenditure is physical activity. The greater the intensity and duration of physical activity, the more energy is consumed. While there's little subjective control over energy expenditure from basal metabolism and the thermic effect of food, energy expenditure from physical activity is manageable. For those trying to lose weight, increasing physical activity can achieve the desired energy expenditure. In addition to these three pathways, infants, children, and adolescents also use energy for physical growth and development.
carbohydrate
Carbohydrates are a class of compounds composed of carbon, hydrogen, and oxygen. They are widely found in both plants and animals and are a major source of energy. (I) Classification of Carbohydrates. Based on the number of monosaccharides they contain, carbohydrates in food can be classified into three categories: sugars, oligosaccharides, and polysaccharides. 1. Sugars. These include monosaccharides, disaccharides, and sugar alcohols. Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed, such as glucose, fructose, and galactose. Disaccharides are composed of two monosaccharide molecules. Sugar alcohols, such as sorbitol and mannitol, are naturally occurring glucose alcohols found in some fruits.
2. Oligosaccharides. Also known as oligosaccharides, they are composed of 3 to 9 monosaccharide molecules. Examples include fructooligosaccharides and soybean oligosaccharides. They can be used as sweeteners and can also promote the growth of beneficial intestinal bacteria such as Bifidobacteria. 3. Polysaccharides. Polysaccharides are a class of carbohydrates composed of 10 or more monosaccharides, including starch, dietary fiber, and glycogen. Glycogen, also known as animal starch, is a starch polysaccharide found in the liver and muscles of animals.
(II) Physiological functions of carbohydrates
1. Energy Supply. Energy from carbohydrates is the primary source of dietary energy. One gram of digestible carbohydrate provides approximately 16.7 kilojoules (4 kilocalories). The energy released from carbohydrates in the body is the main energy source for the nervous system and muscles (including the cardiac muscle). Blood glucose is the most convenient and rapid energy source for the human body, providing energy quickly, but in limited quantities. Therefore, a larger "energy reserve" is needed for continuous replenishment; this reserve is glycogen.
2. Constituting cells and tissues. Carbohydrates are distributed in the cell membrane and cytoplasm in the form of glycolipids and glycoproteins. Brain and nerve tissue contain large amounts of glycolipids. Riboside is an essential component of nucleic acids and nucleotides. 3. Anti-ketogenic effect (helping to completely oxidize fats). Ketone bodies produced by the oxidation of fats in the body must combine with oxaloacetate produced by carbohydrate metabolism to be completely oxidized. If carbohydrate intake is insufficient, ketone bodies generated from fat oxidation will accumulate in the body. 4. Protein-saving effect. When the body has sufficient carbohydrates to provide energy, it does not need to use protein to provide energy, thus saving protein.