Pigments in the Human Body - Functions and Health Effects

Pigments are molecules that absorb certain wavelengths of light while reflecting others. When a person looks at a pigmented object, the reflected light waves hit the retina at the back of the person's eyes. The optic nerve then sends nerve impulses from the retina to the brain, which creates an image of the object. The object's color is determined by the wavelengths of light that reach the retina. Each wavelength causes us to perceive a specific color. A dye produces a sensation of color in a similar way to a pigment, but dyes dissolve in the medium that surrounds them, while pigments exist as insoluble particles.

Biological pigments color our bodies and body products, but this is not their primary function. The pigments play vital roles in the daily operation of the body. However, health problems can develop if too much pigment builds up or if too little is made.

Melanin in the Skin

Melanin is the main pigment in skin, where it’s made by cells called melanocytes. There are two forms of melanin in the skin – eumelanin, which appears brown or brown-black, and pheomelanin, whose color ranges from yellow to red. These molecules are present in various proportions in the skin of different people to produce the range of human skin colors. Blood vessels in the skin also contribute to skin color due to the presence of hemoglobin, a red pigment, in the blood.

Melanin absorbs dangerous ultraviolet rays from the sun, converting the light energy to heat energy. Ultraviolet light causes DNA damage in cells and skin cancer, so melanin is an extremely important molecule in skin. However, a tan or darkening of the skin after sun exposure is a sign that the skin has already been injured by UV light. The skin responds to this damage by making more melanin then usual. The extra melanin provides partial (but not complete) protection from further UV damage.

Melanin in the Iris of the Eye

Melanin is found in other areas of the body besides the skin. Eumelanin and pheomelanin give hair its color. Melanin is also present in the inner ear. A dark form of melanin called neuromelanin is found in parts of the brain. In addition, melanin contributes to the color of the iris in the eye.

The thickest layer of the iris is called the stroma. The stroma contains fibers, melanocytes and other cells in a loose arrangement. Iris color is determined by a combination of factors, including the density of the stroma cells, the amount of eumelanin in the melanocytes and the ability of stroma cells to scatter blue light. 

Rods

Several pigments are present in the eye and are essential to its function. Rhodopsin, which is also known as visual purple due to its color, is located in the rod cells of the retina. Rhodopsin functions in dim light and enables us to see shades of grey. In bright light, rhodospin is bleached and breaks up into retinal and a protein called opsin. In darkness the process is reversed and rhodopsin is regenerated.

Since retinal is made from vitamin A, this vitamin is an essential nutrient for night vision. Beta-carotene is a yellow plant pigment which our bodies can convert into vitamin A. This pigment is especially abundant in carrots, so the old “myth” that carrots are good for night vision is actually true! It’s not safe to eat large amounts of pre-formed vitamin A, which is toxic at high levels, but eating a large amount of beta-carotene doesn’t seem to be dangerous. However, research does suggest that while smokers can eat foods containing beta-carotene, they shouldn’t ingest beta-carotene supplements, which may cause an increased risk of lung cancer. The same is true for people who have had a long-term exposure to asbestos fibers.

Another thing to be aware of is that eating large amounts of beta-carotene in food or supplements can result in some of the pigment being deposited in the skin, giving it a yellow color. This condition is called carotenemia and is apparently harmless.

Cones

The cone cells in the retina respond to bright light and enable us to see color and detail. Humans have three types of cone cells, known as the S, M and L cones. Each type of cone responds best to a specific range of wavelengths, although there is some overlap in cone sensitivity. S cones are most sensitive to the shorter wavelengths of light, which produce a blue color, and are sometimes called blue cones. This alternate name is bit confusing, because S cones respond to blue light but are not blue in color. M cones, or green cones, are more sensitive to medium wavelengths, which produce green light. The L cones, or red cones, respond best to long wavelengths, which produce red light. The cone pigment molecules are called iodopsins. Vitamin A is required for the manufacture of the iodopsins, so this vitamin is important for color vision as well as for night vision. Each of the three types of cones contains its own version of iodopsin.

The central part of the retina provides very detailed vision and is known as the macula. When we look directly at something, the reflected light rays from the object strike the macula. The central portion of the macula, which has the best vision in the retina, is called the fovea centralis, or sometimes just the fovea. The fovea contains cones but no rods. This is why at night time when there is little illumination it is useful to look at objects from the side of our visual field rather than looking directly at the objects. This allows reflected light rays from the objects to fall on the outer portion of the retina, which has rods.

Age-related macular degeneration is the leading cause of vision loss in elderly people. As the macular degenerates, it becomes harder to see a clear image. Zeaxanthin and lutein are yellow pigments in the macula. These two pigments belong to the carotenoid family, just as beta-carotene does, and give the macula a yellow appearance. They are thought to help maintain the health of the macula by protecting it from light damage and possibly by reducing oxidative stress.

It's known that when people ingest zeaxanthin and lutein the levels of these pigments in the macula increases. Green leafy vegetables are good sources of zeaxanthin and lutein, and so are eggs and corn. Researchers have found that in people with age-related macular degeneration (AMD) the macula has a lower level of the two pigments than people without AMD. Scientists suspect - but don't know for certain - that ingesting more zeaxanthin and lutein will decrease the chance of AMD development.

Hemoglobin is a red pigment inside red blood cells that transports oxygen around the body. The hemoglobin is responsible for the blood’s red color. One hemoglobin molecule joins to four oxygen molecules, and a normal red blood cell contains 250 million to 300 million hemoglobin molecules. Since there are 4 million to 6 million red blood cells per microliter of blood in a healthy person (one microliter = one millionth of a liter), a lot of oxygen travels through the blood. This oxygen is an essential nutrient for the trillions of cells in the human body. Scientists estimate that an adult human contains between 50 and 100 trillion cells. These cells need oxygen to produce energy from digested food.

Red blood cells live for about 120 days and are then broken down. Their hemoglobin is changed into a green pigment called biliverdin. Biliverdin is then changed into yet another pigment known as bilirubin, which is yellow. Bilirubin is excreted into a liquid called bile in the liver.

The chief function of bile is to emulsify ingested fats in the small intestine. This emulsification prepares the fats for digestion by enzymes. Bile and food that is not digested pass from the small intestine into the large intestine. Here bacteria and chemical reactions change the bilirubin from bile into a brown pigment called stercobilin. Stercobilin leaves the body in the feces, and is responsible for giving feces its brown color.

Some bilirubin is converted into urobilin, a yellow pigment that is absorbed through the intestinal lining into the bloodstream. The kidneys excrete the urobilin in urine, giving urine its typical yellow color.

There are many disorders that are caused by an insufficient amount of a pigment or by too much of the pigment. Three of these disorders are described below.

Vitiligo is a condition in which melanocytes in the skin are destroyed, resulting in white patches that contain no melanin. The cause of vitiligo is unknown, but researchers suspect that it results from the inheritance of specific genes and that it's an autoimmune disease. The body’s immune system mistakenly attacks the melanocytes.

Hyperbilirubinemia is a condition in which bilirubin becomes too concentrated in the body. As a result, bilirubin collects in the skin, producing a yellow color known as jaundice. Hyperbilirubinemia may develop if too many red blood cells are destroyed, which would result in too much hemoglobin breakdown and excess production of bilirubin. It may also develop due to liver damage that prevents release of bilirubin into the small intestine, or due to an obstruction in the passageways that transport bile.

Red blood cell and hemoglobin destruction, an insufficient amount of hemoglobin in the red blood cells or the production of abnormal hemoglobin can cause a number of disorders, including several types of anemia. The anemia may be mild or severe. The most common type of anemia is called iron-deficiency anemia. Hemoglobin contains iron, and can't be made without this element. If the body lacks hemoglobin an insufficient number of red blood cells will be produced, and an inadequate amount of oxygen will be delivered to the body’s tissues. Iron-deficiency anemia can arise due to a diet that is low in iron, inadequate absorption of iron, perhaps due to intestinal damage, or blood loss.