Vitamin A plays an essential role in the development of osteoblasts, the bone-building cells that lay down new bone. A deficiency in vitamin A also limits calcium absorption and metabolism, which can result in poor bone growth. Overall, low vitamin A levels are associated with osteoporosis and increased risk of fracture.
On the other hand, there is some controversy as to whether high vitamin A intakes are actually helpful, or more of a hindrance to bone health. Some studies suggest high vitamin A can be bone-damaging, but this relates only to the active forms of vitamin A, or retinoids. The jury is still out on how — or even if — excess vitamin A intake actually increases risk of osteoporosis and bone fracture. But in the meantime, we recommend limiting intake of supplemental vitamin A — that is, preformed vitamin A, or retinoid forms — to 5000 IU per day. This is still well below the known tolerable upper limit (UL) of preformed vitamin A — around 10,000 IU per day.
What do we mean by “preformed” vitamin A? There are many forms of vitamin A, with retinoids and carotenoids being the two main categories.
- Retinoids — natural, fat-soluble forms of vitamin A that are available for immediate use in the body. Derived from animal sources, retinoids are a smaller class than the carotenoids, and include retinol, retinal, and retinoic acid, among others.
- Subgroup: Retinol — also referred to as “preformed vitamin A,” retinol is regarded as the main active form for vitamin A in the body. It is found naturally in some animal tissues, such as liver, which makes liver a good dietary source of this vitamin.
- Carotenoids — a large class of natural, fat-soluble pigments found principally in deeply-colored plant foods. Carotenoids, sometimes referred to as provitamins, are dietary precursors to the active forms of vitamin A in the body. More than 600 carotenoids have been identified to date.
- Subgroup: Beta-carotene — the most well-known plant precursor source of active vitamin A. Our bodies can convert beta-carotene into active vitamin A when needed, storing or eliminating any extra.
A full description of the hundreds of forms and functions of vitamin A is beyond the scope of this article, so here are a few key points to remember:
- Most of the vitamin A in our diets comes from plants in the form of beta-carotene, which, again, is a precursor that is safely stored in our body fat and liver, where it gets converted into active vitamin A forms as needed.
- Retinol, the primary form of active vitamin A, can be toxic if consumed at very high levels.
- Thanks to an inbuilt mechanism that shuts off our body’s conversion of beta-carotene into retinol when levels are adequate, high intake of beta-carotene is generally not of concern.
- Some recent research has, however, linked high-dose beta-carotene supplements to increased risk of lung cancer among smokers — the opposite seems to be the case among nonsmokers!
- Though there may be different thoughts on what constitutes “high-dose beta-carotene,” amounts over 25,000 IU are typically considered to be in the higher range.
- Science is always evolving, but Better Bones products have been well-formulated and contain useful, nontoxic amounts of both forms as part of supplement regimen.
Today, for the approximately 44% of the US population that under-consume vitamin A in their food, supplementing with both the above-described forms of vitamin A is a wise move for bone health.
I’m Dr. Susan Brown. I am a nutritionist, medical anthropologist, writer, and speaker. Get my free weekly newsletter here.