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    Home / Science lessons / Nutrition & Digestion
    • Nutrition & Digestion

      Your kitchen is a great place to study science!  Cooking involves chemical reactions that combine molecules in a mixture, or change them through chemical reactions. When you eat the tasty food you've cooked, your body performs another series of chemical reactions allowing the nutrients in the food to nourish you.  It is an amazing process:  read on to find out how our bodies interact with different foods. Then try out some kitchen science experiments!

      Nutrients

      Carbohydrates, fats, and proteins are the main nutrients that provide our bodies with energy. (Vitamins and minerals are also important, as they are helpful to chemical processes.) Carbohydrates, including sugars and starches, are the main provider of energy. Protein is important for building and repairing tissues, especially muscles. Fats also build tissue, manufacture cell membranes, and carry vitamins like A and D. A well-balanced diet includes all of these nutrients in some form.

      Carbohydrates are carbon, hydrogen, and oxygen compounds. Simple carbohydrates (sugars like fructose, lactose, and sucrose) don't have vitamins or minerals; they are 'empty carbs' that just give you a jolt of energy. Complex carbohydrates (starches) contain vitamins, minerals, and sometimes fiber, which is plant material that doesn't get broken down during digestion. (Fiber helps clear your digestive system.)  Although some diets are low-carb, your body usually needs carbohydrates to work properly. Your brain, for instance, needs the sugar glucose in order to function. Potatoes, rice, fruits, vegetables, and bread are good sources of carbs. 

      Every living cell contains protein, and it is the main component of your muscles, organs, and glands.  Proteins are complex molecules, built with amino acids. Your body produces some of these, but there are also some essential amino acids that you can only get through food. You need all of these amino acids if your body is going to work properly. Some foods have only incomplete proteins; they don't contain all of the essential amino acids. It is a good idea to combine these foods with others that will provide the rest of the amino acids you need (rice, for example, is an incomplete protein, but you can 'complete' it by adding beans).

      Since proteins don't get stored in your body, you have to regularly eat enough of them for your body to use in building muscle and other tissue. Too little protein can result in stunted growth. Lack of protein can also cause edema, a condition where water leaks into tissue resulting in swelling of hands, feet, and bellies. (This is the reason starving children sometimes have protruding bellies.)

      Fats are an important high-energy source for the body.  They provide fatty acids that are necessary for many body functions, including controlling blood pressure and clotting, transporting vitamins through the bloodstream, maintaining body temperature, and building healthy cells.  Fats are especially essential for the growth and development of babies.  Your body stores the fat that you don't use, so eating too much can result in obesity and other health problems.  Fats are usually found in meat and dairy products, but many plants turn carbohydrates into fat and store them in seeds (peanuts are a good example). 

      Carbohydrates, proteins, and fats all provide your body with energy, but you also need vitamins and minerals to help use that energy.  Vitamins are chemical compounds and minerals are chemical elements (you can  find them on a periodic table). Vitamins are necessary for some chemical reactions inside us. Among other things, they help make our bones, skin, and hair healthy, as well as strengthening our immune system. Our bodies also use many minerals in tiny amounts; for example, iron is used in making hemoglobin, which carries oxygen through our blood. 

      Malnutrition is an imbalance between a body's nutrient intake and the energy it needs to function. It is still a major cause for child deaths in developing countries, especially in Asia and sub-Saharan Africa. (This UNICEF report gives more information.)

      Calories

      Calories are the measure of energy contained in something - food, fuel, etc. In chemistry, a calorie is the amount of heat energy needed to raise the temperature of 1 gram of water by 1 °Celsius. A food calorie is actually 1000 calories, or one kilocalorie. Protein has four calories per gram, carbohydrates have the same amount, and fats have nine calories per gram. In other words, a small amount of fat will give you more energy than the same amount of carbohydrates.

      The amount of energy (and calories) that you need varies. Your heart, lungs, and other organs are always using energy; physical activity increases your energy usage, and so does digesting food in order to get more energy. Factors like height, weight, and age also influence how much energy you need to take in. Since calories are just a measure, the energy in one calorie in a piece of cake and in a carrot is the same - one isn't worse for you, it's just that one kind of food has more calories. Eating too much of a high calorie food can be harmful, since extra calories get turned into fat and stored.

      Metabolism is the process of breaking food down to get the energy from it.  Although your metabolism slows down when you sleep, you're still using energy. So when you wake up in the morning, it's important to eat something in order to 'refuel' your body for the day.

      Digestion

      The nutrients in our food are made up of molecules that are too large for our bodies to use.  Digestion is the process of breaking these large molecules into smaller ones that our bodies can absorb.  Proteins need to be broken down into their individual amino acids; fats need to be reduced to separate fatty acids; and carbohydrates need to be converted into the simplest sugars, like glucose.  This is a complex process involving many organs. 

      First, food is mixed with saliva in the mouth.  Saliva is mostly made up of water, but it also contains antibodies to protect against disease, lysozyme to kill bacteria, and a digestive enzyme called amylase to help break down starch.   

      After you swallow the food in your mouth, it travels through the esophagus and into the stomach.  The stomach is a temporary storage chamber for food.  It mixes the food with highly acidic gastric juices that contain digestive enzymes, proteins that speed up chemical reactions, in this case reactions that break down food molecules.  The body can absorb water and alcohol through the stomach, but most other nutrients can only be absorbed through the small intestine.  After the food is mixed by the stomach, it is called chyme and travels next to the small intestine.

      The small intestine is where most digestion happens.  It contains many glands that secrete various digestive enzymes that break down the chyme as far as possible.  This organ is almost 20 feet long, if you were to measure it end to end!  This length, as well as tiny folds and projections on the intestine wall (called intestinal villi), provides lots of surface area for nutrients to be absorbed into the blood stream. 

      After all the nutrients are absorbed from the chyme in the small intestine, it goes to the large intestine.  This organ is much shorter than the small intestine, but it is called 'large' because of its bigger diameter.  In the large intestine most of the water in the chyme is absorbed back into the bloodstream. The remaining waste forms feces and is expelled from the body. 

      That's the simple path food takes through your body, but there are also other organs involved.  The liver produces bile that aids the digestion of  fat, and the gall bladder concentrates the bile and secretes it into the small intestine.  The liver also converts the nutrients into forms your body needs: if there is too much glucose in the blood, the liver will convert the excess into glycogen and store it.  The pancreas is also important in digestion, as it produces and secretes essential enzymes. 

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