Digestion and nutrition (nutrition): content and turnover of nutrients in the body
Digestive: food content and how we take up and translate nutrients such as carbohydrates, fats and protein
We humans are fascinating and the human body is an unprecedented machinery. Every second around the clock, billions of chemical processes are carried out in the body. These chemical processes allow the heart to pump, the brain to think, muscles to work, the intestines to take care of the food, and the kidneys to purify the blood. In other words, chemical processes are ongoing continuously in the body. All chemical processes depend on the following things:
- Fuel — There must be fuel to carry out the processes. On planet Earth, glucose (glucose) is the most common fuel for both animals and plants. Glucose (glucose) is easily accessible and easy for the body to use as fuel. However, we humans can also use fat as an energy source. This will be discussed in detail below.
- Building blocks — The body’s tissues consist of many different building blocks that need to be replaced over time. Our cells are packed with proteins and fats. These proteins and fats participate in chemical reactions and they wear out with time and need to be replaced. To replace them, fats in the food are needed as well as amino acids (the latter for building proteins).
- Machines — The body’s machines are enzymes. Enzymes are proteins that carry out chemical reactions.
In order to maintain these processes, we need to get nutrients and we do this by eating. Most of what we eat must be processed by the mouth, stomach and intestines before the body can absorb the nutrients. For example, the intestines cannot absorb proteins; proteins must be broken down into smaller portions (amino acids) before they can be absorbed by the intestine. Also fats and carbohydrates need to be broken down into smaller portions in order to be absorbed by the intestines. This process, that is, digestion and absorption of nutrients, is called digestion, and therefore the gastrointestinal tract is sometimes called the digestive apparatus.
Understand how the body works to lose weight and live healthy
The gastrointestinal tract is a single long duct that starts in the mouth and ends in the anus. The gastrointestinal tract processes the food and takes up the nutrients. In this chapter we will discuss nutrients in food as well as digestion. The purpose of the chapter is to learn more about what the food contains, how the content is used in the body and how it affects health. The emphasis in this chapter is on understanding how the body works, which is a prerequisite for improving nutrition and thus leading a healthier life. We will focus weight gain, overweight and obesity in detail, as well as how to do to lose weight with scientifically proven and fairly simple methods available to everyone.
Digestion: processing and absorption of nutrients (fat, protein, carbohydrates) in food
Digestion begins already in the mouth when the food is mixed with saliva, and the teeth chew the food into smaller pieces. In saliva there is the enzyme amylase, which breaks down carbohydrates into smaller parts that the intestines can absorb. However, amylase of saliva does not really matter; we do without the amylase of saliva. Saliva also contains the enzyme lipase, which breaks down fats into smaller constituents that the intestines can absorb, but also this enzyme is not of particular importance. This means that the only important digestion that takes place in the mouth is the chewing of the teeth. When we swallow the food, it passes through the esophagus, which is a transport channel to the stomach.
Digestion continues in the stomach (which in medical language is called the “ventricle”). In the stomach, the food is mixed with gastric juice, while the stomach kneads the food. The stomach can knead the food because the wall of the stomach is made up of muscles. It takes about 2-3 hours for the stomach to process all food. To keep food in the stomach (until processing is complete), the stomach has an upper and lower mouth (so-called stomach) that allows the food to go neither up (to the esophagus) nor down (to the small intestine) before it is finished. Hydrochloric acid of the stomach also contributes to digestion. The stomach also has the enzymes amylase and lipase that it releases, but these do not really matter. However, the pepsin enzyme, which is released into the stomach, is important for our absorption of proteins. Proteins must also be cleaved (broken down) into smaller constituents (called amino acids) before they are absorbed into the intestine and pepsin accounts for 15% of this digestion.
After the stomach, food enters the small intestine, which is the most important place for nutrient uptake. Most of our absorption of nutrients takes place in the small intestine. The first part of the small intestine is called the duodenum (which in medical language is called the duodenum) and is connected with the liver and pancreas (which in medical language is called pancreas). The liver and pancreas add liver juice and pancreatic juice to the food. Pancreatic juice contains large amounts of enzymes that split fats, carbohydrates and proteins. These enzymes are the most important for us to take up fat, carbohydrates and protein. In the juice of the liver, among other things, there are bile salts that facilitate the absorption of fat.
Thus, in the small intestine there is an intensive breakdown of fats, protein and carbohydrates into their constituents. These components can then be absorbed into the intestinal mucosa and transported to the body via blood vessels and lymphatic vessels.
The small intestine surrenders to the large intestine. In the large intestine, liquid and minerals are absorbed. As liquid is absorbed, the remains become thicker and eventually only stools remain.
Hormones of the gastrointestinal tract
The gastrointestinal tract is very rich in hormones. Already when we are hungry, hormones are released whose effect is to coordinate both your behavior (e.g. by increasing your hunger so you can find food) as well as the work of the gastrointestinal tract. In fact, hormones are released along the entire gastrointestinal tract. Some of these hormones make sure that the intestines are prepared for food coming, and other hormones signal to the rest of the body that food is on its way. When we eat food, for example, the hormone GLP-1 (glucagon like peptie 1) is released from the small intestine. GLP-1 and the grape sugar absorbed in the intestine have a very strong effect on the pancreas (pancreas). When GLP-1 and glucose (glucose) levels in the blood rise, the pancreas releases insulin. Insulin is a hormone that signals to the rest of the body (especially muscles, fat and liver) that carbohydrates are soon released into the blood and that the organs should be prepared to take up the carbohydrates.
Main nutrients (nutrients) in food: fat, carbohydrates and protein
Fat, carbohydrates and protein are the main nutrients (nutrients) in the food we eat. The body needs all of these, but the body cannot absorb fat, carbohydrates or protein as they are present in the food. This is due to the fact that these nutrients consist of long chains of interconnected molecules (smaller constituents). Our intestines cannot absorb such chain structures, but must first break down the nutrient into smaller constituents. Degradation of nutrients is called “digestion”.
Starch is a very common carbohydrate in the traditional Swedish diet. There are large amounts of starch in potatoes, bread, pasta and rice. In fact, starch consists of pure glucose (glucose) which is interconnected to long chains. See figure below.
Different types of fat in the food
Fat occurs mainly in the following forms in food:
- Triglycerides — These dominate the food’s fat phospholipider
- Steroids — Cholesterol is the most common steroid in food
Thus, triglycerides dominate the fat content of the food. This means that fat in butter, cheese, cream, milk, meat, olive oil, sun oil, rapeseed oil, fish, nuts and so on consists mainly of triglycerides. There are many different types of triglycerides. As mentioned above, we cannot take up the triglycerides themselves in the intestine, but we need to split it into smaller pieces. This is illustrated in the figure below. The triglyceride is thus split into three fatty acids and one monoglyceride (also known as glycerol). These substances can be absorbed by the intestine (intestinal cells). Inside the intestinal cells, the fatty acids and monoglycerides are actually used to rebuild the triglyceride again before it is sent into the body. In the body, triglycerides are absorbed in many different organs, especially in fat and in the liver.
Triglycerides (which is thus what we call “fat”) are used for several purposes in the body. The fat is very energy-rich and contains more than twice as much energy as carbohydrates (sugar). 1 gram of fat contains 9 kilocalories (kcal) of energy, compared to sugar containing 4 kilocalories per 1 gram. But the body would rather use the sugar (glucose) as an energy source if it is in the blood, this because glucose is easier to extract energy from. It is therefore more meagre to use fat as a source of energy and therefore fat is only used when sugar is exhausted. Fat (triglycerides) is therefore our energy reserve in the body. We store the fat under the skin, around our organs, in the muscles and other tissues.
Steroids and phospholipids are used to produce hormones and hormone-like substances.
Part of the fats in the food are essential, which means that they contain fatty acids that the body can not build on its own. We need to get these fatty acids through food in order for the body to function normally. These essential fatty acids, for example, affect blood clotting, immune system and blood pressure.
Fats also provide us with vital, so-called essential, polyunsaturated fatty acids. These fatty acids cannot be produced by the body itself, but we need to get them through the food. The essential fatty acids affect a number of functions in the body, including blood pressure, blood clotting capacity and the immune system.
Carbohydrates in food
Carbohydrates are present in many forms in food and it is quite difficult to separate the difference between them, especially in terms of nutritional content and health effects. Carbohydrates consist of monosaccharides that can be interconnected. One carbohydrate can consist of anything from one to several thousand interconnected monosaccharides. A disaccharide is a carbohydrate consisting of two monosaccharides. One trisaccharide consists of three monosaccharides. An oligosaccharide consists of at least four monosaccharides. The definition of polysaccharide varies, but it can be said that if the carbohydrate consists of at least 10 monosaccharides, it is a polysaccharide. See figure below.
One can combine these three monosaccharides (glucose, fructose, galactose) in different ways to obtain all variants of carbohydrates. In the food we eat, carbohydrates predominantly appear in the form of polysaccharides, monosaccharides and disaccharides most importantly. Monosaccharides and disaccharides are also called ‘sugars’ and these are what we refer to when we talk about ‘sugar’.
There are three types of monosaccharides
- Glucose (also known as “glucose”)
- Fructose (also known as “fruit sugar”)
These three substances make up about 15% of the carbohydrates in normal Swedish food and thus they are sugar.
There are also three types of disaccharides
- Sucrose/sucrose (consists of 1 glucose and 1 fructose)
- Lactose (consists of 1 glucose and 1 galactose).
- Maltose (consists of 2 glucose).
Lactose is also called “milk sugar”. Dairy products contain lactose. Sucrose and sucrose are the same substance, commonly referred to as ‘cane sugar’ or ‘beet sugar’ (this is because sucrose is found in cane and sugar beet). Powdered sugar, bit sugar and icing sugar is sucrose. Note that disaccharides normally account for 35% of the carbohydrates in the food we eat. Maltose is also called ‘malt sugar’.
There are three types of polysaccharides:
- Starch — This polysaccharide dominates among the carbohydrates in the food. In total, starch makes up 50% of carbohydrates in food. Starch consists exclusively of glucose (glucose). Starch is the way of storing glucose by green plants. Potatoes, wheat, corn and rice are rich in starch. Starch breaks down to glucose in the intestines, which transports it to the blood.
- Glycogen — All animals, including man, stores glucose in the form of glycogen. Thus, glycogen is a single long molecule consisting of interconnected glucose. Glycogen is found mainly in the liver. This glycogen can the body use when blood sugar drops. This is done by the liver breaking down glycogen into glucose and sending the glucose to the body.
- Cellulose — Cellulose is what is in the walls of plant cells. Celluosa is a type of fiber and consists only of glucose. But in cellulose, glucose molecules are interconnected by a special type of bond that human enzymes can not break. Therefore, cellulose passes straight through the intestinal tract. Cellulose therefore has no nutritional content for humans. Cellulose is therefore a type of fibre (the term fibre is used for carbohydrates that we do not take up as nutrients). However, bacteria in the rectum can break down cellulose.
As mentioned above, the intestines can only take up monosaccharides, which means that polysaccharides and all other carbohydrates must be broken down to monosaccharides. This is done with the help of the enzyme amylase contained in the pancreas.
Monosaccharides absorbed in the intestine end up in a vessel that goes to the liver first. In the liver, some monosaccharides are taken up (the liver uses one part itself and stores the remainder in the form of glycogen) and the rest goes out into the body to all other organs and tissues.
Degradation of lactose: what is lactose intolerance?
The enzyme that splits lactose into glucose and galactose is called lactase. This enzyme activity tends to decrease as we grow older. This means that we get worse at breaking down lactose with age and it expresses itself as lactose intolerance (you cannot tolerate dairy products). The lactose that does not break down in the small intestine goes on to the colon where the bacteria break it down and it has consequences; you get more gases, bloating, noise, runs, diarrhoea and possibly abdominal pain. These symptoms come within 24 hours after ingestion of milk. Lactose intolerance is very common outside Europe. Swedes, as a rule, have very good enzyme function throughout their lives. Approximately 5% of Swedes have lactose intolerance (compared to 70% of overseas individuals).
Fibers — Which carbohydrates are considered as fibre and what does it mean?
Fibers (dietary fibers) are carbohydrates that our body can not break down and are found in plants. All plants contain fiber but to varying degrees. Cellulose is a common type of fiber in plants. Fibers attract water in the intestines, making it easier for the intestines to handle food. Fiber-rich food slows down gastric emptying, resulting in an increased feeling of satiety and a more stable blood glucose. Fibers can also bind bile acids in the intestine and thus reduce our absorption of fat. Some people with high cholesterol are therefore advised to eat more fibre in the hope that this will lower cholesterol levels. Common to all fibers is that we can not break them down in the small intestine. Therefore, the fibers reach the large intestine virtually unchanged. Fibers therefore contain no energy (nutrition, calories) for us humans.
Fibers are found in greenery, fruits and whole grains. Approximate amount of fiber contained in these foods is as follows:
|Food||Quantity||Fiber content (g)|
|Green Vegetables||0.5 kopp||6.4|
|Orange vegetables (carrot, peppers)||0.5 kopp||2.1|
|Beans (legumes, pea plants)||0.5 kopp||8.0|
|Starch vegetables||0.5 kopp||1.7|
|Other vegetables||0.5 kopp||1.1|
|Whole grain||28 g||2.4|
Fast and slow carbohydrates: what is the glycemic index (GI)?
Monosaccharides and disaccharides are simple for the intestines to absorb. These carbohydrates can be absorbed quickly and therefore provide a rapid increase in blood sugar. Therefore, they are called “fast carbohydrates”. Carbohydrates consisting of longer chains also take longer to process in the intestine and therefore cause a slower increase in blood sugar. Such carbohydrates are called “slow carbohydrates”. Sometimes the term “glycemic index” is used to describe this. High glycemic index means fast carbohydrates, while low glycemic index means slow carbohydrates. It is believed that food with a low glycemic index gives a longer feeling of satiety.
Examples of foods with low glycemic index (GI)
- Whole grain bread (note that finely ground whole grains can have high GI)
- Chickpeas, soybeans, green vegetables, walnuts, beans and more
Examples of foods with high GI:
- Mashed potatoes and potatoes
- Quick rice
- Scones from powdered mix
- Sugarred soft drinks, sweets and other sweets
- Dried fruit
Do we need carbohydrates and if so, how much?
Carbohydrates are not really an essential nutrient, because the body can produce carbohydrates based on amino acids (contained in protein) and glycerol (contained in fat). But we still have to get carbohydrates, otherwise we risk consuming all our fat and proteins. The body prefers to use carbohydrates as a fuel, and only if there is a shortage of carbohydrates, the body begins to consume fat and protein as an energy source. Today, 130 g of carbohydrates per day is recommended, which is about twice as much as the body really needs to cope. A wide range of scientific studies suggest that it is the carbohydrate content of food that is most important for weight, blood pressure, blood lipids, etc. This will be discussed in detail later.
Protein in the food
Protein consists of long chains of amino acids. Man uses 20 different amino acids to build up their proteins. Nine of these amino acids are “essential” which means that we cannot produce them ourselves, but have to get them through food.
Two interconnected amino acids are called “dipeptide”. Three coupled amino acids are called “tripeptide” and so on. A large number of interconnected amino acids (over 10 pieces) is called “polypeptide”. A “protein” usually consists of many more amino acids, not rarely hundreds. In the protein, the amino acids together form a three-dimensional structure that is always the same for that particular protein.
Proteines/peptides cannot be absorbed in the intestine before they are digested into amino acids. They are cleaved in the stomach and small intestine with the help of several enzymes. Once digested into amino acids, the amino acids end up in the same blood vessels that first go to the liver and then to the rest of the body.
Unlike carbohydrates, however, protein cannot be stored in the body. So there is no protein reserve in the body. The organs and tissues of the body take up the amount of amino acids they need to maintain their protein park and any excess (of amino acids) is excreted through the kidneys (and thus ports in the urine).
Energy in the food
Protein, fat and carbohydrates are together called “macronutrients”. These three nutrients are our main sources of energy and building blocks in the body. In addition to macronutrients, the body also needs “micronutrients”, which are vitamins, minerals and trace elements. The micronutrients are needed for the optimal metabolism. For example, several vitamins are needed for proteins to function normally. In Sweden, as in the rest of the world, a lack of micronutrients (vitamins, minerals, trace elements) is not a public health problem. There is also no shortage of macronutrients; on the contrary, in Sweden, like the rest of the world, there is an excess of macronutrients (fat, protein and carbohydrates in food). As far as diabetes, overweight, obesity and cardiovascular disease are concerned, science suggests that we mainly eat too much carbohydrates. This will be discussed in detail later.
How the body uses fat, protein and carbohydrates to extract energy
The body is able to extract energy (fuel) from all macronutrients (fat, carbohydrates, protein). However, the body has a chronological order of use of these and that order is quite sensible:
- In the first place, carbohydrates (glucose) are used as fuel. Glucose is the most accessible fuel on the planet and, moreover, it is easy for the body to use glucose to create energy. Note that glucose is the same as grape sugar.
- Secondly, fat (triglycerides) is used as fuel. Fat is somewhat harder for the body to use as a fuel because it takes more chemical reactions to extract energy from fat. However, each gram of fat contains about twice as much energy as each gram of sugar (glucose)! All cells in the body can use fat as a fuel and it has no significant side effects to use fat instead of sugar.
- Ultimately — i.e. only if the carbohydrates and fats are consumed — the body uses protein to extract energy.
What kind of energy do we get from sugar, fat and carbohydrates?
So far, we have explained that fat, carbohydrates and protein can be used as fuel in the body but this is a little simplification. Actually, these nutrients are used to create the real fuel of the cells, called ATP (adenosine triphosphate). ATP is thus the real fuel and the cells manufacture ATP using fats, sugars and proteins. ATP is used to conduct every chemical process in the body; from the work of the muscles, to the signaling of the nerves and further to the production of insulin.
To create ATP, the cell must “metabolize” sugar, fat or protein. It implies that these nutrients need to undergo several chemical reactions for the cell to finally be able to create ATP. How this is done is rather complicated and not relevant to this discussion. What is however relevant for this discussion is that the route from nutrient to ATP looks different for sugar, fat and protein. Sugar is the easiest to use to create ATP; a couple of chemical reactions are all needed for sugar to trigger ATP. Proteins must first be broken down into amino acids – some of the amino acids can then be used to manufacture substances that can eventually be converted into sugar (glucose), thereby creating ATP. It means that proteins are a rather inefficient source of ATP. Fats (triglycerides) must first be broken down into fatty acids and monoglycerides. The fatty acids can then be used to manufacture ATP and the monoglycerides can actually be used to manufacture sugar (glucose) and thus additional ATP. However, for each gram of fat we get about twice as much energy (ATP) as each gram of sugar (glucose). In other words, we have two energy sources in our fat, namely the fatty acids and the monoglyceride which is converted to sugar (glucose)!
It’s easy to understand why the body is reluctant to use proteins as fuel. Protein is an inefficient source of ATP and also the proteins are needed to maintain all our functions. It would therefore be a waste of using proteins as a fuel, and therefore the body will only do so if the carbohydrates and fat are exhausted.
Micronutrients: vitamins, minerals trace elements
Of course, the food contains other substances in addition to fat, carbohydrates and protein. For example, there are minerals (calcium, iodine, sodium, iron), trace elements (fluorine, copper, selenium, nickel, tin, manganese, chromium, etc.), vitamins and water. These are also essential for our well-being. Minerals participate in many chemical processes in the body. Trace elements and vitamins are often included as important components of proteins and enzymes. Vitamins A, B1, B2, B6, B12, C, D2, D3, E, H, K1, K2, folinic acid, niacinamide and pantothenic acid are all involved in chemical reactions (they are called coenzymes). However, micronutrients are not a public health problem. Swedish food, as a rule, contains all the micronutrients we need and lack of micronutrients is not a big problem. Since this discussion is primarily about diabetes, obesity, overweight and cardiovascular disease — and micronutrients do not play a significant role in those contexts — we will henceforth focus on the macronutrients.