Metabolism of the main nutrients Part I — Lipids
Along with carbohydrates and proteins, fats are among the main components of our diet. Even though they are not very popular among fitness-conscious people, fats are essential for the body’s survival. Because fats not only serve as energy suppliers and stores: they are also important components for the construction of cells, important hormones and bile acids, which in turn are essential for fat digestion.
Fat metabolism in a nutshell for you!
- The body makes one to two grams of cholesterol itself every day.
- Only 15% of our blood cholesterol comes from food.
- Whether high cholesterol levels require treatment or not depends on the individual risk factors.
- According to current guideline or target values, a total cholesterol value of below 200 mg/dl (below 5.2 mmol/l) is desirable.
- Cholesterol and triglyceride levels can be better regulated with a carbohydrate-reduced and rather protein- and fat-emphasized diet than with a fat-reduced diet.
- Caffeine has an excellent lipolytic effect, i.e. it increases the release of free fatty acids.
How do fats get into our bodies?
There are different types of fats. We absorb triglycerides for the most part from food. In addition, there are phospholipids, cholesterol, free fatty acids and fat-soluble vitamins, such as vitamin A. However, most of the fats that are essential for life are produced by the body itself. Exceptions are the so-called essential fatty acids such as linoleic acid and alpha-linolenic acid, as well as the three fat-soluble vitamins A, E and K. These must be taken in through food, since the body is not capable of producing them itself. Linoleic acid is found in sunflower oil, for example, and alpha-linolenic acid in walnuts, flaxseed, rapeseed and soybeans. Whole grains and various vegetables cover the vitamin requirement.
A variety of mechanisms ensure that the body can also use these fats. During digestion, the fatty components of food are first broken down and mixed by rhythmic movements in the stomach and intestines. In the small intestine, bile ensures that the fat droplets are broken down even further and made more accessible to enzymes, which now break down the fats into their smallest components — the triglycerides and fatty acids. Broken down in this way, the fats can be absorbed via the cells of the small intestine to enter the bloodstream. However, since the blood consists mainly of water and can only dissolve fats with difficulty, fats are transported further to the body cells with the help of special proteins (lipoproteins).
What are lipoproteins?
Lipoproteins are transport proteins with the task of transporting fat in the blood. Since water repels fat, the fatty substances (especially cholesterol and triglycerides) must be bound to these carrier proteins for transport in the blood. There are lipoproteins that transport cholesterol and lipoproteins that are specialized in the transport of triglycerides. The amount of the different lipoproteins in the blood is a measure of the level of fats in the blood. For example, the well-known HDL cholesterol level (“the good cholesterol”) is actually not a cholesterol level, but the amount of lipoprotein that transports healthy cholesterol. Since this “good” transport protein has a particularly high chemical density, it is called HDL (high density lipoprotein).
Where does cholesterol actually come from?
Cholesterol is a natural component of almost all body tissues — human and animal. Among other things, it is needed to stabilize cell membranes and to produce bile acids, vitamin D and hormones. There are about 140 grams of cholesterol in the entire body, although about 95% of it is in the cells, i.e. it does not travel in the blood. The body produces one to two grams of cholesterol itself every day. The main production sites are the liver and the intestinal mucosa. Another part is absorbed with food, while cholesterol is only found in animal-based products. However, only 15% of blood cholesterol comes from food.
This makes it clear that the effectiveness of a change in diet can be limited, especially if cholesterol levels are highly elevated. The maxim that used to apply, “In the case of elevated cholesterol levels, the first treatment measure must be a change in diet,” is therefore now also viewed critically by many scientists.
However, this does not change the fact that a healthy diet low in cholesterol is absolutely recommended in case of high cholesterol levels (and of course also otherwise). In the case of cholesterol, however, it should not be forgotten that other causes can also be involved in the elevated cholesterol levels (for example, genetically determined peculiarities in the metabolism).
What is “good” and what is “bad” cholesterol?
In order for fat-soluble cholesterol to be transported in the blood, the lipoproteins already mentioned above are necessary. There are low-density lipoproteins (LDL) and high-density lipoproteins (HDL). LDL transports cholesterol stored in the liver to the organs via the bloodstream. If the amount of cholesterol in the bloodstream exceeds the requirements of the organs, the fat can be deposited in the vessel walls and lead to arteriosclerosis. LDL is therefore also referred to as “bad” cholesterol. HDL, on the other hand, transports cholesterol that is not needed from the tissues and vessels to the liver. There it is converted into bile acids, which are excreted with the bile. HDL thus transports the fat from the blood circulation back to the liver and therefore counteracts arteriosclerosis. This is therefore referred to as “good” cholesterol.
How high can cholesterol levels be?
Whether high cholesterol levels require treatment or not depends on the individual risk factors. The initial panic that even slightly elevated cholesterol levels required treatment has been followed by an increasing number of “critics” who consider the risk of heart attack or stroke due to high cholesterol to be overestimated and set threshold values for treatment individually according to age, lifestyle and previous illnesses. If there is no risk of vascular calcification or cardiovascular disease, many physicians now dispense with drug treatment (for the time being) and simply monitor cholesterol levels. The situation is different for people with pronounced risk factors (overweight, lack of exercise, high blood pressure, diabetes, etc.) or people who already have heart disease. In these cases, it is essential to keep cholesterol levels as low as possible. According to current guideline or target values, a total cholesterol value of below 200 mg/dl (below 5.2 mmol/l) is desirable. Up to 240 mg/dl (up to 6.2 mmol/l) is considered borderline cholesterol. Values above this are considered too high.
What role do triglycerides play?
Regardless of the cholesterol level, elevated triglyceride levels are a risk factor for vascular calcification (atherosclerosis). It is not uncommon for both cholesterol and triglyceride levels to be elevated. This is called combined dyslipidemia or combined hyperlipidemia. Triglycerides are neutral fats that come largely from food, but are also produced in the body from excess energy (e.g., from excessive sugar and alcohol intake). Three fatty acids are attached to the molecule glycerol, from which the name is derived. Triglycerides are stored in fatty tissue and form the body’s most important energy reserve. The fat reserves in subcutaneous fatty tissue and muscle alone could be sufficient for over 23 marathons lasting three hours.
Less fat or less sugar?
It may seem paradoxical, but cholesterol and triglyceride levels can be better regulated with a carbohydrate-reduced and rather protein- and fat-emphasized diet than with a fat-reduced diet. This is because, in particular, quickly digestible white flour and starch products and those containing refined sugar are converted into fat in excess in the body. To further influence the body fat percentage, endurance training with low intensity and a total energy expenditure of at least 300 kcal per training session several times a week is not to be missed. For cardiovascular health, a weekly energy turnover through sports or physical activity of 2000 to 3000 kcal is scientifically proven. A greater training load is not necessary, unless the athletic performance is to be increased. Moreover, the fat metabolism enzymes in the fat cells of the body and the muscles can only be increased in their activity by longer loads of low to medium intensity, and not by short-term anaerobic loads with high lactate formation.
Can the fat metabolism be trained?
The load intensity (power, speed) determines which energy carriers are used to generate energy during exercise. At low exercise intensity (25 to 50 percent of maximum oxygen uptake/VO2max), up to 70 percent of free fatty acids can be burned in total energy metabolism. The free fatty acids, triglycerides and lipoproteins in the blood plasma are the decisive energy carriers here. If the load intensity is about 65 percent of the maximum oxygen uptake, then the free fatty acids contribute about 50 percent to the energy gain. During long-term exercise, the neutral fats (triglycerides) stored in the muscles are used to provide energy in addition to the free fatty acids from the blood. At load intensities of 85 percent of the maximum oxygen uptake and above, fat burning decreases significantly because the lactate produced in the process impedes the burning of the free fatty acids. These high exercise intensities deplete glycogen stores in the liver and muscle. At a load intensity that produces more than 7 mmol/l lactate, the breakdown of free fatty acids is suppressed in competitive athletes. Energy conversion then occurs exclusively via aerobic and anaerobic breakdown of glycogen (sugar stores).
How can fat metabolism activity be influenced?
- Do not eat before training
Numerous studies have been conducted in recent years to assess the influence of fasting and carbohydrate intake on fat metabolism. Athletes who exercised in a fasting state showed much higher concentrations of free fatty acids in the blood, while in athletes who ate a high-carbohydrate diet, the concentration of free fatty acids in the blood was even lower than before breakfast. In short, if food is eaten before exercise, fats are not used for energy production.
The activation of the fat metabolism can therefore be influenced by the food intake. If the loaded musculature has only little glycogen stored, it comes to an intensified fat splitting and consequently to an increase of the free fatty acids. In starvation metabolism, i.e. when glucose availability is low, fat metabolism is significantly activated after just 30 minutes of aerobic exercise. However, care must be taken to avoid intense training or training lasting several hours during the starvation metabolism, as premature hypoglycemia can occur during exercise. The stress hormones, especially adrenaline, increase significantly. The first limited carbohydrate intake (energy bar, dried fruit) should be taken after 60 minutes of exercise at the latest. This prevents hypoglycemia and a drop in performance. - Drink coffee before training
Caffeine has an excellent lipolytic effect, i.e. it increases the release of free fatty acids. Drinking coffee increases their release and thus indirectly increases their breakdown. In addition, the feeling of hunger after drinking coffee can be postponed for about one to two hours.
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