Retinyl palmitate remains the main chemical form of vitamin A storage and is mainly located in hepatic stellate cells HSCs in lipid droplets resembling those found in adipose cells. White adipose tissue WAT , is essentially involved in the regulation of lipid metabolism, through its role in lipid storage, and might also be considered as a vitamin A storage and metabolism site. WAT contains all the intracellular equipment for vitamin A metabolism and signaling pathways which allows retinol to be metabolized into retinoic acid, known to control genomic expression in WAT.
The description of molecular mechanisms involved in the activation of HSCs and the differentiation of preadipocytes reveal similar cellular and molecular mechanisms. They do not dissolve in water. Vitamins help the body function effectively. There are two types: water-soluble and fat-soluble vitamins. The water-soluble vitamins are vitamins B and C. Most vitamins come from food, but sunshine contributes to vitamin D.
Some people need or choose to take supplements that provide extra vitamins. This article looks at the types, functions, and sources of fat-soluble vitamins, and what can happen if a person has too much or too little. Vitamin A helps maintain healthy vision.
Without vitamin A, a person could experience vision problems and possibly vision loss. Vitamin A is not a single vitamin but a collection of compounds known as retinoids. Retinoids occur naturally in the human body, and they are present in some dietary sources. Some foods provide retinols, which the body can use directly as vitamin A. Others provide provitamin A, compounds that the body converts into vitamin A. Learn more here about why we need vitamin A. Plant sources provide carotenoids, such as beta-carotene , which is a powerful antioxidant.
The body can convert these into vitamin A. Find out more here about dietary sources of vitamin A. Food packages usually show the amounts in IU. Anyone who has concerns about their vitamin A intake should seek advice from a health professional, who will help them understand these measures.
The — Dietary Guidelines for Americans recommend consuming the following amounts each day. Amounts are in mcg RAE. In severe cases, coma and death can result. Vitamin A supplements are available for purchase online. However, people should speak to a doctor before taking these or other supplements. The time a person takes a vitamin can affect its impact. Find out more here about when to take different vitamins. People obtain vitamin D :. The body obtains the compounds it needs to make vitamin D from food.
It also produces vitamin D when ultraviolet UV light meets the skin. Find out more about the health benefits of vitamin D. A person can obtain some vitamin D from the sun, but most people will also need to use other sources, too. Throughout history, there have been many instances when food was scarce.
Our ability to store excess caloric energy as fat for future usage allowed us to continue as a species during these times of famine. So, normal fat reserves are a signal that metabolic processes are efficient and a person is healthy. Lipids are a family of organic compounds that are mostly insoluble in water. Composed of fats and oils, lipids are molecules that yield high energy and have a chemical composition mainly of carbon, hydrogen, and oxygen.
Lipids perform three primary biological functions within the body: they serve as structural components of cell membranes, function as energy storehouses, and function as important signaling molecules. The three main types of lipids are triacylglycerols, phospholipids, and sterols. Triacylglycerols also known as triglycerides make up more than 95 percent of lipids in the diet and are commonly found in fried foods, vegetable oil, butter, whole milk, cheese, cream cheese, and some meats.
Naturally occurring triacylglycerols are found in many foods, including avocados, olives, corn, and nuts. As with most fats, triacylglycerols do not dissolve in water.
The terms fats, oils, and triacylglycerols are discretionary and can be used interchangeably. In this chapter when we use the word fat, we are referring to triacylglycerols.
Phospholipids make up only about 2 percent of dietary lipids. They are water-soluble and are found in both plants and animals. In fact, phospholipids are synthesized in the body to form cell and organelle membranes. In blood and body fluids, phospholipids form structures in which fat is enclosed and transported throughout the bloodstream. Sterols are the least common type of lipid. Cholesterol is perhaps the best well-known sterol. Though cholesterol has a notorious reputation, the body gets only a small amount of its cholesterol through food—the body produces most of it.
Cholesterol is an important component of the cell membrane and is required for the synthesis of sex hormones, vitamin D, and bile salts.
Later in this chapter, we will examine each of these lipids in more detail and discover how their different structures function to keep your body working. The excess energy from the food we eat is digested and incorporated into adipose tissue, or fatty tissue. Most of the energy required by the human body is provided by carbohydrates and lipids. As discussed previously , glucose is stored in the body as glycogen. While glycogen provides a ready source of energy, lipids primarily function as an energy reserve.
As you may recall, glycogen is quite bulky with heavy water content, thus the body cannot store too much for long. Alternatively, fats are packed together tightly without water and store far greater amounts of energy in a reduced space.
A fat gram is densely concentrated with energy—it contains more than double the amount of energy than a gram of carbohydrate. Energy is needed to power the muscles for all the physical work and play an average person or child engages in. Unlike other body cells that can store fat in limited supplies, fat cells are specialized for fat storage and are able to expand almost indefinitely in size.
An overabundance of adipose tissue can result in undue stress on the body and can be detrimental to your health. A serious impact of excess fat is the accumulation of too much cholesterol in the arterial wall, which can thicken the walls of arteries and lead to cardiovascular disease.
Thus, while some body fat is critical to our survival and good health, in large quantities it can be a deterrent to maintaining good health. Triacylglycerols also help the body produce and regulate hormones. For example, adipose tissue secretes the hormone leptin, which regulates appetite.
In the reproductive system, fatty acids are required for proper reproductive health; women who lack proper amounts may stop menstruating and become infertile. Omega-3 and omega-6 essential fatty acids help regulate cholesterol and blood clotting and control inflammation in the joints, tissues, and bloodstream.
Fats also play important functional roles in sustaining nerve impulse transmission, memory storage, and tissue structure. More specifically in the brain, lipids are focal to brain activity in structure and in function.
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