Don't Take Your Vitamins, New York Times, 8 June 2013
"Scientists have known for years that large quantities of supplemental vitamins can be quite harmful indeed.I wrote about this a little in 2009, in Antioxidant Supplements Might Increase Risk For Diabetes
Some studies have shown that people who eat more fruits and vegetables have a lower incidence of cancer and heart disease and live longer. The logic is obvious. If fruits and vegetables contain antioxidants, and people who eat fruits and vegetables are healthier, then people who take supplemental antioxidants should also be healthier. It hasn’t worked out that way.
Free radicals aren’t as evil as advertised. (In fact, people need them to kill bacteria and eliminate new cancer cells.)
As a result [of vitamin industry lobbying], consumers don’t know that taking megavitamins could increase their risk of cancer and heart disease and shorten their lives; they don’t know that they have been suffering too much of a good thing for too long."
How Antioxidants Interfere With Glucose Metabolism
When we exercise (or when we eat, or when we perform many metabolic tasks) we generate free radicals. This is normal. However, free radicals can harm healthy cells; they can damage DNA and cause fats in our body to oxidize, or "go rancid" as cooking oils do. DNA damage and oxidized fats are factors in the development of cancer and atherosclerosis (and so heart disease). It is this negative aspect of free radicals, their power to oxidize, that gave rise to supplemental anti-oxidants. Antioxidants prevent oxidation by neutralizing free radicals (by donating electrons, or "reducing" them).
Since production of free radicals is a natural occurrence, you'd think we've evolved mechanisms to mitigate their damage. We have. Free radicals (which usually include an oxygen molecule and are known collectively as Reactive Oxygen Species (ROS)), act as signals telling cells to make more of our own, in-house (endogenous) antioxidants - like the powerful enzymes superoxide dismutase and glutathione peroxidase.
But before free radicals are neutralized ...
- Free radicals partake in a positive feedback loop giving us more energy. Muscles need oxygen and glucose to work. When we increase the work of our muscles, e.g. when we exercise, we increase the muscle's need for glucose and oxygen. Byproducts of muscle work are ROS ... ROS act as signals to produce more mitochondria. Mitochondria are energy-producing factories inside cells that use the glucose and oxygen the muscles take up. More mitochondria lead to --> more glucose uptake, lead to --> more energy.
- Free radicals spur the expression of genes that increase sensitivity to insulin - Done through stimulation of PPARs and other compounds. (The thiazolidiediones, a class into which the diabetes drugs Avandia and Actos fall, also stimulate PPARs.)
- Free radicals stimulate the release of the hormone adiponectin. Adiponectin is produced by our fat cells and has been shown to increase insulin sensitivity. Low levels of it correlate with increased diabetes risk.
All of these beneficial actions (endogenous antioxidant production, mitochondria manufacture, gene expression, hormone release), and others, are blocked when we consume too many antioxidants. At least, that's where the evidence is leading us.