Our liver is amazing. As the largest gland in the body, it circulates about 1.5 L of blood per minute through its four lobes. This blood continuously carries essential chemicals through the liver and to the rest of the body so that thousands of metabolic reactions can take place all day long. Like the skin, the liver acts as one of our major “detox” organs, but it houses a vast number of other key processes as well. Some of these include the conversion of nutrients from our food and the environment (i.e. the sun) into the chemicals our body can utilize for fuel and nutrient storage. The liver stores all of our fat-soluble vitamins – vitamins A, D, E, and K – as well as vitamin B12 and various minerals, including zinc, iron, copper, and magnesium. Many of these nutrients not only play major roles in liver health, but they also play major roles in the health of our skin. Here, we explore Vitamin A and its major functions for both liver and skin health.
In our bodies, Vitamin A takes on roles in growth, gene expression, vision, and other cell functions, while its precursor molecules- the carotenoids- can act as antioxidants. Retinoic acid, a vitamin A derivative, directs immature skin cells, called keratinocytes, to grow into mature skin cells. When we have a deficiency of vitamin A in the body, our skin can become extremely dry (xerosis). Many dermatologists have used retinoids topically to treat photoaged and photodamaged skin for decades. However, the vitamin A our body needs for its daily metabolism must come from the diet.
Metabolism and Food Sources
Both vitamin A and the carotenoids come from foods and provide us nutritional benefit. The term carotenoid is an umbrella term inclusive of different pigment molecules found in plants, which provide their yellow, red, and orange colors. These molecules, or phytochemicals, include alpha-carotene, beta-carotene, and beta-cryptoxanthin, which act as pro-vitamin A compounds in our bodies. Lycopene is another carotenoid and may have partial pro-vitamin A metabolism.[4,5] There also exists lutein and zeaxanthin, which do not make any pro-vitamin A compounds. A study comparing human skin to blood levels of carotenoids showed that the skin concentrations are comparable to blood levels indicating that the carotenoids find their way to the skin once they are in the bloodstream.
Function in Skin
Beta-cryptoxanthin: Beta-cryptoxanthin is abundantly found in squashes such as butternut and winter varieties. Vegetables typically have a lower amount of beta-cryptoxanthin compared to alpha-carotene and beta-carotene. It is reported to have higher absorption after ingestion than alpha-carotene or beta-carotene.[8,9] It is a prodrug to vitamin A and is converted to retinol. Studies in mice showed that beta-cryptoxanthin inhibited the activity of the skin’s pigment producing cells (known as melanocytes) that had been exposed to ultraviolet light. Their interactions in the skin of humans still needs further research. .
Alpha-carotene: Alpha-carotene is most abundant in pumpkin, carrots, and different varieties of squash. Many foods containing significant levels of alpha-carotene also contain beta-carotene (Table 1). Few research studies have assessed alpha-carotene alone. Alpha-carotene appears to have a high antioxidant value but more studies are needed to see how it is helpful to the skin.
Beta-carotene: Beta-carotene is the most well-known of the pro-vitamin A carotenoids and is largely found in sweet potato, kale, and dark leafy greens. Beta-carotene has twice the potency of alpha-carotene in its vitamin A conversion capacity, meaning it takes half the amount of beta-carotene compared to alpha-carotene to produce an equal amount of vitamin A in our bodies. A single-blind randomized crossover trial assessed dietary intake of carotenoids including beta-carotene and found that higher consumption of carotenoid-rich fruits and vegetables increased plasma carotenoid levels and enhanced skin yellowness. These results were true for both beta and alpha-carotene. Research reminds us that it is important to eat dietary carotenoids with a source of fat, such as avocado, to enhance their absorption capacity.
Some of the best sources of vitamin A include liver, butter, eggs, and fish like sardines, herring, and tuna. These animal sources provide us vitamin A in the form of retinyl esters. We get carotenoids from plant foods. Both alpha-carotene and beta-carotene exist in many of the same fruits and vegetables, which include pumpkins, squash, carrots, collard greens, kale, yams, and more (Table 1). Across almost all foods, beta-carotene exists in higher quantities than alpha-carotene and beta-cryptoxanthin. Beta-cryptoxanthin is found in some similar foods containing alpha and beta-carotene but also exists in butternut squash, red bell peppers, oranges, papayas, tangerines, and others.
Once ingested, the carotenoids, namely beta-carotene, get converted into vitamin A in the small intestine and liver and utilized for our body’s chemical reactions. We only absorb about 5-50% of the carotenoids we ingest, whereas we absorb 70-90% of the vitamin A from animal sources we ingest (in the form of retinyl esters). The conversion of beta-carotene into vitamin A may be less efficient in someone who has better vitamin A status, meaning they have built-up stores of vitamin A in their bodies already and thus do not need as much of this conversion. To make matters even more challenging, the plant-bound carotenoids must first be released from the cell matrix of their plant foods in order to be absorbed. This release from the matrix may be done in various ways, including cooking, steaming and chopping the plant foods. Dietary carotenoids require the presence of fats for absorption. This is why carotenoid supplements that are packaged in oil – already released from the matrix and in the presence of fat- are regarded as better absorbed than their counterpart plant sources.
Whether or not a person eats foods from animal sources, he or she may find good sources of dietary vitamin A. In order to have a sufficient amount of vitamin A to keep our skin, liver, and metabolism healthy, it is crucial to remember the different ways our body can absorb and use vitamin A from different foods.
Table 1. Comparison of foods containing different carotenoids in decreasing order of abundance, per 100g portions.
|Butternut Squash||Pumpkin||Sweet Potato|
|Red Bell Pepper||Butternut Squash||Dandelion Greens|
|Sweet Yellow Corn||Grape Leaves||Mustard Greens|
|Dandelion Greens||Dandelion Greens||Spinach|
|Jalapeño Peppers||Hot Peppers||Turnip Greens|
|Kale||Green Snap Beans||Collard Greens|
|Hot Peppers||Collard Greens||Parsley|