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The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 5 No. 6 • December 2002


Why Antioxidants Do Not Necessarily Prevent Aging Due to Free Radicals

While antioxidants have been shown to have many healthful properties and may reduce the risk of cancer and atherosclerosis, most studies with antioxidants have not found antioxidants to determine maximum lifespan. When tissue antioxidants were directly compared to maximum lifespan potential (MLSP) of various species, 10 out of 12 independent investigations by seven different laboratories found that endogenous antioxidant enzymes and low-molecular weight antioxidants are negatively correlated with maximum longevity, while two other studies found no correlation.1 A review paper proposes that this evidence strongly suggests that the rate of oxygen radical generation in tissues, rather than the antioxidant capacity, is what limits lifespan.1

The review authors provide evidence that supports this hypothesis. They note that in 16 studies on lifelong experimental modifications of antioxidant levels by dietary supplements, pharmacological methods, or transgenic techniques performed in large numbers of animals, four found some increase in MLSP, whereas in the other 12, MLSP did not change.1 They note that an increase in mean lifespan was a much more frequent finding in these and other studies of this type that they cited. Hence, they suggest that antioxidants can nonspecifically prevent causes of death that result in premature death, but are not likely to be effective in extending maximum lifespan.

The rate of generation of free radicals by mitochondria has been identified by these and many other researchers (including Dr. Denham Harman, originator of the free radical theory of aging) as a likely major cause of aging. Indeed, caloric restriction in rodents markedly decreases free radical generation by mitochondria but does not have a consistent effect on antioxidant capacity. Long-lived animals have been found to have constitutively lower levels of antioxidants than short-lived animals because their mitochondria generate low levels of free radicals.1 Hence, a better strategy for increasing maximum lifespan than simply taking antioxidants that sop up excess free radicals is to take supplements that are actually capable of reducing the generation of free radicals by mitochondria.

One way that cells reduce mitochondrial free radical generation is proton leak, which is an uncoupling of the mitochondria from ATP-producing pathways, while allowing protons that would have been involved in those pathways to instead move across the mitochondrial inner membrane, releasing heat. Thermogenesis is not the only purpose of proton leak, however, since even ectotherms such as reptiles have proton leak.* It is proposed in a fascinating paper2 that proton leak is a major mechanism for controlling mitochondrial free radical generation and may be a key to longevity. The paper’s author2 notes that the futile cycle of proton pumping and proton leak is responsible for 20–25% of respiration in rat liver cells, while in perfused rat muscle it is 35–50% of respiration. They also note that the proportion of respiration of mammalian liver cells devoted to proton leak is remarkably constant at about 20% in a wide range of species of diverse sizes.

Proton leak is metabolically expensive, since so much caloric fuel is “wasted” by mitochondria by this means, i.e., it is not used to make ATP, but is dissipated. Hence, this mechanism must serve very important purposes, e.g., to control free radical production, damage, and possibly aging. Mitochondrial uncoupling proteins are part of the regulatory process of this proton pumping. It is known that in brown adipose tissue, uncoupling protein 1 (UCP1) is involved. Whether uncoupling protein 2 or 3 (UCP2 and UCP3), which are UCP1 homologs, are also involved in this process is controversial; they may be mild uncouplers.2,3

Some flavonoids that have been tested have been shown to uncouple mitochondrial respiration.3,4 Quercetin, for example, was shown in one study to be an inhibitor of the mitochondrial membrane permeability transition (MMPT), the opening of an unselective pore elicited by calcium or prooxidants. MMPT is inhibited by uncouplers of oxidative phosphorylation.3 Conjugated linoleic acid (CLA) has been reported to increase the expression of UCP2 in the mammary gland, brown adipose tissue, and white adipose tissue of mice, while UCP3 levels in skeletal muscle were significantly increased.5 Olive oil feeding induced the highest UCP1, UCP2, and UCP3 mRNA expression in rat interscapular brown adipose tissue, as compared to sunflower oil, palm oil, and beef tallow. In fact, olive-oil-fed rats had an increased total-body oxygen consumption as compared to the other oils.6 This is a likely factor in the healthful effects of the “Mediterranean diet.”

The bottom line is that this new perspective does not undermine the free radical theory of aging; in fact, it supports it. Taking the right amounts of the right antioxidants is still a good idea, but in the long run, it appears it will be necessary to reduce the free radicals created by mitochondria to increase maximum lifespan.


*In fact, even plants use this process to protect against the damage that can be caused by absorbing solar energy that exceeds that which can be used in photosynthesis. Under extreme conditions, such as icy winters or scorching summers, many evergreens can, while maintaining their light-absorbing chlorophyll, suspend growth and photosynthesis and thermally dissipate virtually all of the light energy they absorb. See Demmig-Adams and Adams, Antioxidants in photosynthesis and human nutrition. Science 298:2149-53 (2002).


References

  1. Barja. Rate of generation of oxidative stress-related damage and animal longevity. Free Rad Biol Med 33(9):1167-72 (2002).
  2. Brand. Uncoupling to survive? The role of mitochondrial inefficiency in ageing. Exp Gerontol 35:811-20 (2000).
  3. Santos et al. Effect of naturally occurring flavonoids on lipid peroxidation and membrane permeability transition in mitochondria. Free Rad Biol Med 24(9):1455-61 (1998).
  4. Stenlid. Flavonoids as inhibitors of the formation of adenosine triphosphate in plant mitochondria. Phytochem 9:2251-6 (1970).
  5. Ealey et al. Effects of dietary conjugated linoleic acid on the expression of uncoupling proteins in mice and rats. Lipids 37(9):853-61 (2002).
  6. Rodriguez et al. Olive oil feeding up-regulates uncoupling protein genes in rat brown adipose tissue and skeletal muscle. Am J Clin Nutr 75:213-20 (2002).

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