 In order to survive, the human organism needs to take in oxygen, water, and food. We can survive only about three minutes without air, three days without water, and three weeks without food. Because food is so important to our survival, it's been studied extensively. And since 1827, when a British physician by the name of William Prout first proposed that humans need three macronutrients to survive, physicians, scientists, and increasingly the general public have been trying to figure out exactly how much of each of these nutrients will optimize our health. But to some extent, the focus on nutrients rather than food has confused many people. And we tend to get lost in this conversation about nutrients good and bad. And it's understandable because scientists need to reduce things to a single variable to study them. And the variable in food would appear to be the nutrient. But we also have lots of studies that show that simply removing or boosting nutrients or turning them into supplements doesn't seem to work. That food is much more complicated. It's a system. A basic understanding of the nutrients can give us a helpful background for our discussions about food. So we'll review the nutrients and how the body uses them. But most of this course will focus on discussions about food and health because ultimately people eat food, not nutrients. Dietary carbohydrates are combinations of sugar units that come in both simple and complex forms. Simple carbohydrates include the monosaccharides or single sugar units like glucose and fructose, as well as the disaccharides or two sugar units like sucrose or table sugar. Complex carbohydrates or polysaccharides include the dietary starches that our body can break down and digest, and also the indigestible polysaccharides that make up dietary fiber. During the process of digestion, carbohydrates are broken down and converted to glucose, which can then be metabolized by the body to produce usable energy in the form of ATP. If energy demands are low, glucose can be stored, and most of the time it's stored as adipose tissue. Dietary proteins are also broken down into their component parts, amino acids, during the process of digestion. And these amino acids can be used to build and repair lean tissues in the body and perform many other important functions. But amino acids can also be broken down and used for energy, and if they're consumed in excess, they can contribute to fat stores in the body as well. Dietary fats can also be broken down into smaller components and used for energy, or they can be stored as adipose tissue depending on our energy needs. Fats are the most energy-dense storage form, providing 9 calories of energy for every gram, and alcohol provides 7 calories per gram. In contrast, carbohydrates and proteins provide only 4 calories per gram. This is one of the reasons we've evolved to store excess nutrients as adipose tissue, and this brings us to the underlying physiologic cause of overweight and obesity. Many calories that aren't converted into usable energy in the form of ATP are stored in the body for later use. Most of the time, they're stored as adipose tissue. So disturbing the energy balance to favor energy expenditure over energy storage needs to be one of the priorities for people who are trying to lose excess weight. This can be achieved by consuming fewer calories, burning more calories through exercise, or a combination of both. But in addition, when long-term health is the end goal, the quality of our food matters just as much as the number of calories we consume. Within each category of macronutrient, we need to choose foods that will support our health rather than working against it.