There comes a point in the study of antioxidant supplementation as a means to extend healthy life, after decades of work and thousands of scientific studies in which all the more rigorous results and meta-analyses indicate no effect or negative effects, at which one has to conclude than this is not merely an ambiguous or poorly understood outcome, but rather the case that in fact antioxidant supplementation has no effect or negative effects. Those studies in which some benefit is shown can be written off as the effects of inadvertent calorie restriction, an issue that is very prevalent in studies run prior to about ten years ago and still quite common now. Alternately, they used model organisms and other experimental situations in which the metabolic biochemistry was of little relevance to a healthy human.
Antioxidant therapies can be helpful in treating some medical conditions, and researchers are discovering that mitochondrially targeted antioxidants - still something that you can't obtain from a store - have
Hope springs eternal, of course, and the voice of the scientific community is soft in this matter when compared to the marketing efforts of companies selling antioxidants. The level of funding that flows into science from that direction also has its distorting effects. There are certainly scientists who talk about the present balance of evidence as ambiguous, and will cheerfully do so in a paper that lists scores of studies that show no great or relevant benefit.
From my point of view it seems as though focusing on the study of commonplace antioxidant supplements and health in this day and age is an avoidance of those fields of research that might actually meaningfully extend healthy human life spans. Antioxidants aren't going to achieve that goal, not even the impressive new mitochondrially targeted compounds. The way to the future is rather to be found via technologies such as gene therapy, the creation of engineered bacterial enzymes to clear out metabolic waste, immune therapies, stem cell treatments and other regenerative medicine, and a panoply of further modern approaches to repairing the damage of aging.
The following paper is largely a litany of antioxidant studies in which no relevant benefits were observed, and yet the researchers mark the field as ambiguous and finish with a comment that more work is needed to build a better way of delivering antioxidant supplements - quite missing their own conclusion, it seems. Hope springs eternal, but it is way past time to move on to better and more promising science in this modern age of biotechnology and progress.
Organic compounds and structures composed of them are thermodynamically unstable in an oxygen-containing atmosphere. Molecular oxygen, in its triplet basal state, is rather unreactive due to the spin restriction. However, formation of oxygen free radicals and other reactive oxygen species (ROS) opens the gate for potentially deleterious oxidative reactions of oxygen. Seen from that perspective, the "Free Radical Theory of Aging" (FRTA), now more commonly termed the oxidative damage theory of ageing, seems to address a key facet of intrinsic biological instability of living systems. The basic idea of the FRTA is that free radicals and other ROS, formed unavoidably in the course of metabolism and arising due to the action of various exogenous factors, damage biomolecules, and accumulation of this damage are the cause of age-related diseases and aging.
If FRTA is true, antioxidants should slow down aging and prolong lifespan. This apparently obvious conclusion has stimulated enormous number of studies aimed at finding a relationship between levels of endogenous antioxidants and lifespan of various organisms on the effects of addition of exogenous antioxidants on the course of aging and lifespan of model organisms. Pubmed provides more than 13300 hits for conjunction of terms "antioxidant" and "aging or ageing." However, in spite of the plethora of studies, the answer to the question if exogenous antioxidants can prolong life is far from being clear.
Generally, the effects of antioxidant supplementation in model organisms are disappointing. Many studies showed no effect or even negative effects on the lifespan. Only in some cases considerable prolongation of lifespan was obtained and in organisms which are evolutionarily quite distant from mammals. In some cases, mean but not maximal lifespan was affected, which may be caused by reduction of mortality due to diseases rather than interference with the aging process itself. An apparently obvious conclusion from the plethora of studies could be that antioxidants cannot be expected to prolong significantly the lifespan, especially of mammals, which does not support the FRTA.
In summary, while beneficial effects of antioxidant supplements seem undoubtful in cases of antioxidant deficiencies, additional studies are warranted in order to design adapted prescriptions in antioxidant vitamins and minerals for healthy persons.