Lipid (fat) metabolism involves the breaking down and building of lipids in cells. Lipids can be stored for energy, or used as structural and functional units like those involved in the construction of cell membranes. Lipids are provided by the diet or synthesised by the liver, most lipids obtained by the diet are found in the body as triglycerides or cholesterol. Other lipids found in the body are membrane lipids and fatty acids.
Lipids to not mix with water and need to be made water-soluble before the can be absorbed through the intestinal walls. Digestion of lipids start with lipase but a majority of the work is performed by bile salts from the liver and pancreatic lipase.
Lipids are broken down into triglycerides and cholesterol and require special proteins to transported in the blood, these are known as lipoproteins. Lipoproteins are classified by density and control the sub-type of lipid they can transfer.
Very-low-density proteins carry triglycerides synthesised by the body, low-density proteins carry cholesterol to peripheral tissues and high-density lipoproteins carry cholesterol back to the liver for removal.
Cholesterols are essential to normal biological function as they can be modified to form different steroid hormones such as progesterone and testosterone. Over 70% of cholesterol synthesis is carried out in the liver.
Other lipids that are important for normal cellular growth and metabolism include:
Steroids – important signalling molecules
Ketone bodies – produce energy
and Glycolipids – maintain the lipid bilayer of the cell membrane
The liver controls the uptake and storage of fat-soluble vitamins. The bile secreted by the liver is necessary for the emulsification of fats so they can be absorbed in the intestines. Impaired bile output will impede the uptake of vitamin A, D, E and K.
Vitamin A (Retinol) – Essential for immune function and cell-cell communication. Levels of vitamin A in the body can be measures by assessing blood retinol levels, but the measurement of vitamin A status is limited as levels do not drop until liver stores are depleted. Vitamin A metabolism is altered in scarring alopecia (cicatrices alopecia)  and it has been determined that a vitamin A levels contribute to severity of scarring. Low dose vitamin A supplementation combined with cysteine supplementation reduces shedding in cases of telogen effluvium  and increased retinol binding expression protein in alopecia areata supports the evidence that vitamin A plays a role in hair loss .
Vitamin D is required for the uptake of calcium in the intestines,. Calcium is an important mediator of the differentiation of keratin cells into mature, robust adequately filamented keratin cells.. Research shows that a low levels of vitamin D are linked to androgenic alopecia and alopecia areata . Low zinc has been reported to be a contributing factor to vitamin D deficiency, when zinc is low vitamin D receptor is unregulated in the liver, hair follicle and other cells in the body. The increase in vitamin D receptor increases demand due to the similar roles zinc and vitamin D play in the role of maintaining immune function. Supplementing with zinc is preferable to supplementing with vitamin D for hair regrowth as this will lower the biological demand for vitamin D. Increasing zinc levels will aid keratin cell synthesis and liver uptake of vitamin D.
A small trial from 2010 found that vitamin E supplements improved hair growth in people with hair loss. It’s thought that the vitamin’s antioxidant properties helped reduce oxidative stress in the scalp. Vitamin E is secreted by the sedbabcous glands
Vitamin K helps to regulate calcium deposition in the body; because of this, it helps prevent blood vessel calcification (as seen in female pattern hair loss).