Humans, unlike most mammals, lack the ability to produce vitamin C (ascorbate) themselves. Furthermore, many Americans don’t get the recommended daily allowance (RDA)—60 mg—of ascorbate in the diet, making it important to utilize what we do get very efficiently.
In the Gut
The body absorbs vitamin C throughout the intestine, and then ascorbate moves from the bloodstream into cells via specific pathways. Although passive diffusion—simple movement through tissue—contributes to ascorbate absorption, specific transport mechanisms are much more important. For example, proteins mediate ascorbate transport, requiring adequate cellular energy. The body has a built-in genius: When gut inflammation blocks ascorbate absorption, it oxidizes for transport via another pathway. However, aspirin and other salicylates dramatically decrease bioavailability of ascorbate in the GI tract.Certain hormones increase ascorbate concentration in the bloodstream during bone remodeling. The optimal pH of the blood for ascorbate uptake into cells is an alkaline 7.5, so the ability to regulate systemic pH is important. (Consuming a wholesome diet high in plant foods, rather than refined foods and meat, helps provide the optimal pH balance.) Lacking alkalinizing minerals, the body will steal them from our bones, weakening our skeleton.
High doses of oral vitamin C block its absorption in the gut, but small doses are well absorbed. For this reason, it’s wise to get a little of this vitamin with each meal, rather than megadosing once a day.
It’s often difficult for people with diabetes to maintain adequate ascorbate levels, as high blood sugar inhibits absorption. Because insulin resistance also reduces the ability of the body to transport sugar, the ability to channel ascorbate is likewise reduced. Other antioxidants can help improve muscle glucose utilization and improve insulin sensitivity, moving sugar into cells for fuel. By contrast, oxidative stress—or lack of antioxidants—is considered a contributing cause of insulin resistance.
In the Nervous System
Ascorbate transporters are distributed in nerve tissues as well. Vitamin C is useful in the production of neurotransmitters critical for brain function and mood. Lack of oxygen in the brain causes beta peptide damage, causing calcium-channel dysfunctions that appear as memory problems. Vitamin C can block damage to these calcium channels and allow electron transport in the brain. Combined with iron, ascorbate may activate the calcium channels and protect the brain.
Other Important Uses
Taking 500 mg of vitamin C daily improves dilation of blood vessels in people with atherosclerosis, angina, congestive heart failure, and high blood pressure. Several studies suggest that taking this dosage (along with antihypertensive medicine) helps lower systolic blood pressure, but people with hypertension should never change medication without professional supervision.
C works to safeguard human DNA, helping to prevent certain cancers. This vitamin’s antihistamine properties appear to inhibit inflammation that accompanies allergies. Equally important in our toxic world, supplementation with C results in lower blood levels of lead, although the exact mechanism is not known.
Concentrated in the fluid portion of the eye, vitamin C helps prevent cataracts, reduces the incidence of glaucoma, and protects against age-related macular degeneration. C is needed for collagen synthesis, making it important for healthy joints, ligaments, muscles, and tendons, as well as blood vessels and bones. In its fat-soluble form (ascorbyl palmitate), vitamin C is effective when applied topically to the skin, explaining its popularity in personal care products.