Carotene

Class of plant pigments involved in photosynthesis and human nutrition

Epsilon-carotene

β-Carotene represented by a 3D molecular diagram

Carotene is responsible for the orange color of carrots and the pigmentation of various fruits, vegetables, and some animals.

Lesser Flamingos in the Ngorongoro Crater, Tanzania. Their pink hue comes from carotenoids in their diet, primarily from brine shrimp.

Introduction

Carotene, also known as carotin, is a class of unsaturated hydrocarbons that are responsible for the yellow, orange, and red pigmentation in many plants, fungi, and some animal species. These fat-soluble compounds belong to the carotenoid family and are crucial for photosynthesis in plants and vitamin A production in animals.

Carotenes are synthesized exclusively by plants, algae, and certain bacteria, while animals must obtain them through their diet. They serve multiple biological functions, from acting as antioxidants to being precursors of retinol (vitamin A).

Molecular Structure and Types

Chemically, carotenes are tetraterpenes composed of 40 carbon atoms, forming long polyunsaturated hydrocarbon chains. They may contain cyclic end groups, but they do not include oxygen atoms, distinguishing them from xanthophylls.

The most well-known types of carotenes include:

• Alpha-carotene (α-carotene)
• Beta-carotene (β-carotene) – The most biologically active form, a precursor to vitamin A.
• Gamma-carotene (γ-carotene)
• Delta-carotene (δ-carotene)
• Epsilon-carotene (ε-carotene)
• Zeta-carotene (ζ-carotene)

Beta-carotene is especially significant because it contains two retinyl groups, which can be cleaved in the intestine to form retinol (vitamin A). Carotenes are fat-soluble, meaning they are best absorbed when consumed with dietary fats.

Dietary Sources of Carotenes

Carotenes are found abundantly in colorful fruits and vegetables, particularly those with red, orange, or yellow hues. Some of the richest dietary sources include:

• Carrots
• Sweet potatoes
• Pumpkins
• Mangoes
• Apricots
• Papayas
• Cantaloupe
• Tomatoes
• Spinach and other leafy greens
• Bell peppers
• Winter squash
• Goji berries

Carotenes are best absorbed when consumed with dietary fats (such as olive oil or avocado) and when cooked or processed, as cooking breaks down cell walls, enhancing bioavailability.

Role in Photosynthesis

In plants, carotenes play a vital role in photosynthesis, acting as accessory pigments that help in light absorption and energy transfer to chlorophyll. Additionally, they serve as photoprotective agents by:

• Absorbing excess light energy to prevent damage to plant cells.
• Quenching reactive oxygen species (ROS) produced during photosynthesis.
• Enhancing the efficiency of light capture in low-light conditions.

Carotenes and Vitamin A Production

Carotenes are provitamin A compounds, meaning they can be converted into retinol (vitamin A) in the body. This process occurs in the small intestine, where the enzyme β-carotene 15,15'-monooxygenase cleaves β-carotene to form two molecules of retinal, which is then reduced to retinol.

Not all carotenes exhibit vitamin A activity. The most effective are:

• β-Carotene – Converts efficiently to vitamin A.
• α-Carotene and γ-Carotene – Convert to a lesser extent.
• Lycopene (found in tomatoes) does not contribute to vitamin A production but has powerful antioxidant properties.

Excess β-carotene is stored in fat tissues and the liver, serving as a reserve for vitamin A production when needed.

Carotenes in Animal Pigmentation

Carotenes also contribute to pigmentation in animals, particularly in birds, fish, and crustaceans. Examples include:

• Flamingos and salmon – Their pink coloration comes from astaxanthin, a carotenoid found in shrimp and algae.
• Chickens and egg yolks – The yellow pigment in yolks is due to dietary xanthophylls, derived from carotenes.
• Beeswax – Contains carotenoids, giving it a yellow hue.

Some animals lack the enzyme needed to convert carotenes to vitamin A, such as cats and ferrets, which require preformed vitamin A from animal sources.

Health Benefits of Carotenes

Carotenes provide multiple health benefits due to their antioxidant properties and role as vitamin A precursors. These include:

• Supporting vision and eye health – Helps prevent night blindness and macular degeneration.
• Enhancing immune function – Boosts resistance against infections.
• Protecting skin from UV damage – Reduces risk of sunburn and photoaging.
• Reducing oxidative stress – Acts as an antioxidant, neutralizing free radicals.
• Lowering risk of cardiovascular disease – Associated with reduced LDL oxidation.

Carotenes and Disease Prevention

Research suggests that diets rich in β-carotene and other carotenoids are linked to:

• Lower rates of heart disease – Due to antioxidant effects.
• Reduced cancer risk – Particularly for lung and prostate cancer.
• Improved cognitive function – Protects against age-related neurodegeneration.

However, high-dose β-carotene supplementation in smokers has been linked to an increased risk of lung cancer, indicating that carotenoids are most beneficial when obtained from whole foods rather than supplements.

See Also

• Vitamin A
• Chlorophyll
• Photosynthesis
• Lycopene
• Beta-carotene

External Links

• NCBI – Carotenoids and Human Health
• WHO – Vitamin A and Carotenoids