There still is no cure for debilitating cognitive conditions such as Alzheimer?s and Parkinson?s. But natural agents hold promise for supporting brain health and at least delaying the onset of many of these diseases. David Perlmutter, MD, researches the field
The end of the 1990s marked the completion of the so-called Decade of the Brain, a title bestowed upon this period by researchers and clinicians in the neurosciences. Their intent was both to enhance awareness of the various neurodegenerative diseases and to encourage research into the diverse genetic, infectious, environmental, traumatic and lifestyle influences on their development. But despite the commendable advances in our understanding of the causes of these maladies, lack of significant progress from a therapeutic perspective may mean that we will leave the Decade of the Brain to usher in the Century of Brain Dysfunction.
For example, our understanding of the possible underlying causes of Alzheimer?s disease—including genetic predisposition, educational level, electromagnetic radiation, aluminum exposure and oestrogen insufficiency—has advanced dramatically. But despite the encouraging claims in medical journal advertisements, none of the currently marketed Alzheimer?s drugs offers any significant benefit for this disease. Indeed, as the highly respected Journal of the American Medical Association recently reported in a comprehensive review of the effectiveness of tacrine (Cognex), one of the most aggressively marketed drugs for Alzheimer?s disease, ?Despite being licensed in several countries, the efficacy of tacrine in treating the symptoms of Alzheimer?s disease remains controversial, and government approval for its use has been refused in several countries.?1
These are sobering comments from a journal looked upon as the sounding board of American medicine, especially in light of the fact that there are an estimated 4.5 million Alzheimer?s patients in the United States today, and caring for these individuals costs the nation more than $60 billion annually. But these statistics pale when confronting projections for the next 30 years. Alzheimer?s, being primarily a disease of the aged, will have an ever-increasing negative impact on societies, because the elderly represent the most rapidly growing segment of the population. As seen in the chart to the right, by 2030 more than 9 million Alzheimer?s patients will be in America, with close to half the population over 85 years old carrying the diagnosis.2
The situation is much the same with other degenerative diseases of the nervous system. Between 1955 and 1986, the mortality from Parkinson?s disease in the US rose 411 per cent, while deaths from Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig?s disease) increased 328 per cent in only the nine-year period from 1977 to 1986.3
But is the rapid increase in incidence of these diseases simply a reflection of the fact that we are now living longer and not succumbing to other processes? Or are there other influential factors in our society that have a bearing on these degenerative conditions? An attempt to answer these questions appeared in a 1993 editorial in The Lancet in which the author stated, ?Modern civilisation is not the cause of our chronic diseases, it merely unveiled what our genes had lurking in store for us for centuries, if not millennia, as we now live long enough to see these genes massively expressing themselves.?4
The idea that our health is entirely determined by our genes suggests that our fate with respect to neurodegenerative conditions is predetermined at the time of our conception. In this perspective, environmental, nutritional and emotional aspects of our lives have little influence. Progression of disease is inherent in this plan, allowing perhaps only a role for treating symptoms—paying attention only to the smoke and not the underlying fire.
But in the past several years, the vigorous explorations of the fundamental biochemical events involved in the progressive deterioration of the nervous system characteristic of the neurodegenerative diseases have revealed that these conditions do not simply represent a linear predetermined progression. The unbending rules of genetic determinism have been replaced by the paradigm of genetic predisposition—a concept that recognises inherited risk as only one of many factors that influences an individual?s ability to resist the entropic nature of the tenuous personal capacities that maintain nervous system integrity.
Underlying Oxidative Stress
The molecular mechanism underlying nervous system decay that is characteristic of these disorders shares a final common pathway with the ageing process. The key players mediating this degeneration belong to a group of chemicals commonly known as free radicals, or more generally, pro-oxidants.
Despite the negativity associated with these molecules in popular health texts, free radicals mediate processes central to the perpetuation of life itself. In humans, these pro-oxidants direct the formation of our various body parts, initiate and modulate immune activity, ?kill? invading bacteria, and destroy cancer cells.
Pro-oxidants are generated during the normal processes of cellular metabolism and are produced in increased amounts when physiologic systems are stressed. Stressors associated with increased pro-oxidant production may be intrinsic, as occurs for example when emotion induces the production of ?stress hormones,? or when blood supply is compromised. Extrinsic influences known to stimulate pro-oxidant production include such diverse events as infection, trauma and exposure to toxins.
It is when the controlling mechanisms that normally limit the ?life span? or activity of pro-oxidants become inadequate that the destructive nature of these short-lived molecules becomes manifest. This occurs when excessive pro-oxidants either overwhelm the body?s protective antioxidant chemicals, or antioxidant insufficiency prevents adequate protection from even the ?normal? level of pro-oxidant production.
Within each living cell in the human body there exist microscopic particles whose sole purpose is energy production. These inclusions, the mitochondria, are responsible for utilising fuel to provide power for all of the cell?s various activities. But despite their critical role in maintaining the life of the cell, mitochondria are unusually susceptible to being damaged by pro-oxidants. When damaged, mitochondrial energy production suffers, rendering the entire cell, including the mitochondria themselves, even more susceptible to oxidative damage. And to make matters worse, this entire process becomes self-perpetuating: Damaged mitochondria produce excessive amounts of pro-oxidants.
The process may be initiated by any number of factors, but ultimately culminates in cell death. Alzheimer?s disease is characterised by an inherent defect in mitochondrial energy production, an inflammatory response to one or more brain proteins, increased amounts of brain iron, and ineffective antioxidant activity.5,6 All of these factors contribute to a hostile environment for delicate brain neurons by enhancing pro-oxidant-mediated damage.
Parkinson?s disease has much the same etiology, also featuring a primary flaw in mitochondrial energy production, plus increased concentrations of brain iron, a catalyst for oxidative stress.7,8 Inflammation is less of a factor, but unique to this disease are defects in the ability of the liver to detoxify various environmental toxins. These flaws allow accumulation of diverse chemical agents that directly compromise mitochondrial function and enhance the destructive potential of pro-oxidants.
Multiple sclerosis also is marked by the hallmark excessive pro-oxidant activity, in this case as a consequence of inflammation induced by an uninhibited immune reaction directed at myelin, the fragile protective insulation surrounding brain neurons.9 The damaging effects of this misdirected immune response are compounded by antioxidant inadequacies characteristic of this disease as well.10
ALS fundamentals remain somewhat more obscure. Increased incidence in individuals with a history of pesticide exposure suggests abnormalities of liver detoxification, as is seen in Parkinson?s disease.11 Although the focus of intensive research during the 1980s, the idea that ALS may represent a misdirected immune reaction is now less popular, while excessive pro-oxidant production from damaged mitochondria is now well accepted.12
While each of the many neurodegenerative disorders is characterised by a unique set of symptoms, two biochemical flaws underlie the process of brain destruction in all of these disorders: defective antioxidant protection and reduced mitochondrial energy production.
Thus, therapeutic strategies targeting these fundamental elements are applicable to all the neurodegenerative conditions. These strategies have yielded intriguing results.
Vitamin E, for instance, is the subject of two promising studies. In a prominent 1997 study published in the New England Journal of Medicine, 1,000IU of all-rac [dl]-alpha-tocopherol, twice daily for two years, was reported to be equally effective to the amphetamine precursor drug selegiline in delaying the progression of Alzheimer?s.13 In the study, vitamin E was shown to delay the onset of certain end points such as requiring assistance with activities of daily living and nursing home placement. While these are not specific measures of cognitive function, they are meaningful parameters nonetheless.
In addition, as part of the Nurses? Health Study, researchers tested the cognitive function of almost 15,000 women, ages 70-79. The tests included memory, immediate and delayed recall of lists of words, and tests of verbal fluency. For example, one of the tests asked women to name as many animals as they could in one minute (responses ranged from two to 38 animal names). Test scores were compared with the subjects? use of vitamin C and E supplements. Women who had been taking both supplements for at least ten years had significantly better cognitive performance than women who had never taken those supplements. The benefits were less consistent among women who had taken vitamin E alone, and no benefits were associated with vitamin C alone.14
Alpha-lipoic acid may revolutionise the treatment of neurodegenerative diseases because it has the unique attributes of being rapidly absorbed from the gut, readily crosses the blood-brain barrier, and has free radical-scavenging activity.
In addition to its own antioxidant activity, alpha-lipoic acid also facilitates the regeneration of all the other major brain antioxidants including vitamins C and E and glutathione. Lipoic acid acts as a heavy metal chelator, enhancing the excretion of brain toxic metals including lead, cadmium and mercury.15 It is no wonder that this sulfur-containing antioxidant is the subject of intensive scientific evaluation.16,17
Glutathione is one of the fundamental players in the entire process of mitochondrial energy production. There is currently a great deal of debate as to whether oral administration of glutathione has any significant effect on raising blood levels of glutathione,18,19 but clearly the production of glutathione can be dramatically enhanced by providing the nutritional supplement N-acetyl-cysteine (NAC).20
NAC has the ability to increase availability of glutathione, an ability that is enhanced by the presence of adequate amounts of vitamins C and E.21 In addition, NAC itself is an antioxidant that has been demonstrated to reduce the formation of the free radical nitric oxide (NO) in vitro in cultured rat brain cells.22 NO has been implicated as having a causative role in Parkinson?s disease, multiple sclerosis, Alzheimer?s disease and other neurodegenerative conditions.22,23,24,25 These research studies clearly support the causative role of NO in these diseases, and strategies to decrease NO are being studied worldwide with the goal of having an impact on these diseases.
Co-enzyme Q10, present in all living plant and animal cells, is another important player in energy production. The brain is dependent upon adequate energy availability for such processes as the production of neurotransmitters, the brain?s chemical messengers. Interestingly, M Flint Beal, MD, and colleagues at the Massachusetts General Hospital have found that Parkinson?s patients demonstrate a profound deficiency of coenzyme Q10 in plasma, which may explain why their brains produce an inadequate supply of the neurotransmitter dopamine.26 Beal?s laboratory further revealed in a pilot study of 15 Parkinson?s patients that 200mg Co-Q10, formulated as a chewable wafer, administered two, three or four times daily for one month, is readily absorbed, is well tolerated and measurably increases cellular energy production.27
These qualities, coupled with its antioxidant properties, prompted what has become a landmark study published in October 2002 in which Beal and principal investigator Cliff Shults, MD, demonstrated that the rate of worsening of 80 early-stage Parkinson?s patients could be slowed by more than 40 per cent with the addition of oral Co-Q10.28 The multi-centre, randomised, parallel-group, placebo-controlled, double-blind trial compared placebo with 300mg/day, 600mg/day or 1,200mg/day, for 32 months.
Although Co-Q10 appeared to slow the progressive deterioration in Parkinson?s cases, it should be noted that the best response was with the highest dosage—and 1,200mg/day Co-Q10 can fairly be called a cost-intolerable dose for most consumers.
Finally, recognising the importance of Co-Q10 makes it critical to identify any factors that may suppress its de novo synthesis. Unfortunately, two of the most commonly prescribed cholesterol-lowering drugs, pravastatin (Pravachol) and lovastatin (Mevacor), dramatically lower serum Co-Q10 levels.29 What remains to be seen is whether and how much exogenous Co-Q10 can compensate for this effect as well as increase intracellular concentrations of Co-Q10. While there is no direct evidence so far, in our centre, patients taking statin drugs are provided Co-Q10 at a dosage of 100mg per day.
Mevacor is dwarfed by the sales of simvastatin (Zocor)?one 12-week study found simvastatin to elicit significant reductions in both alpha-tocopherol and Co-Q10 concentrations in serum.30 However, an open-label study involving six months of simvastatin therapy found no change in muscle Co-Q10 concentrations—muscle indicators of energy production—and no change in LDL Co-Q10 or LDL antioxidant contents, when expressed as a function of plasma LDL concentrations.31 Still, many researchers and raw materials marketers are starting to think of working with pharmaceutical interests to incorporate Co-Q10 into statins.
Acetyl-L-carnitine serves a critical role in brain energetics by acting as a co-factor in the process by which mitochondria convert fatty acids into energy. This has important implications in the neurodegenerative diseases, as deficient energy production is the hallmark of these conditions.
In a 1995 study, researchers demonstrated the ability of acetyl-L-carnitine to completely prevent Parkinsonism in laboratory animals. When laboratory animals are exposed to the brain toxin MPTP, they immediately develop full-blown Parkinsonism as a consequence of enhanced production of destructive free radicals specifically in the brain area that produces dopamine. Pre-treating the animals with acetyl-L-carnitine prior to MPTP exposure offered complete protection—none of the animals developed Parkinsonism. This study suggested the therapeutic potential of acetyl-L-carnitine as a brain protectant specifically useful in Parkinson?s disease.32
Acetyl-L-carnitine functions primarily as a shuttle, transporting critical fuel sources into the mitochondria, the energy-producing machinery of the neuron. Its second task is to facilitate the removal of the toxic by-products of brain metabolism. Because of these functions, acetyl-L-carnitine has a pivotal role in facilitating the fundamental processes necessary for brain cell survival.
In addition, acetyl-L-carnitine is readily converted into an important neurotransmitter known as acetylcholine, which is known to be profoundly deficient in the brains of Alzheimer?s patients.
It is for these reasons that acetyl-L-carnitine has been evaluated in dementia studies. Researchers at the University of California, San Diego, found a striking reduction in the rate of mental decline in younger Alzheimer?s patients taking 3g/day acetyl-L-carnitine hydrochloride over the one-year evaluation.33 However, late-onset Alzheimer?s patients tended to progress more rapidly.
These differing results have confounded researchers. Two separate 2003 meta-analyses from London had different conclusions regarding acetyl-L-carnitine?s efficacy for dementia, particularly for Alzheimer?s patients. In one, researchers analysed double-blind, placebo-controlled, prospective, parallel group comparison studies of at least three months? duration for mild cognitive impairment and mild (early) Alzheimer?s disease. Doses of alpha-lipoic acid ranged from 1.5?3g/day. A significant advantage was seen compared to placebo at three months on both the clinical scales and the psychometric tests.34
Conversely, the second recent meta-analysis of 11 trials concluded that ?the evidence does not suggest that ALC is likely to prove an important therapeutic agent.?35
Ginkgo biloba has been used for centuries and is described in a traditional Chinese pharmacopoeia. In France, extracts of Ginkgo biloba are administered orally and intravenously and are among the most commonly prescribed pharmaceutical drugs. This also holds in Germany, where ginkgo is licensed for the treatment of a variety of brain disorders including headache, tinnitus, vertigo and memory disorders.
Perhaps the most convincing validation of the effectiveness of Ginkgo biloba comes from a 1997 study published in the Journal of the American Medical Association. In this placebo-controlled study, the progress of 309 Alzheimer?s patients was evaluated over a one-year period. At the completion of the study, the placebo group showed a progressive decline in mental function on a standardised psychological test, whilst the group receiving ginkgo, on average, actually improved. Similar results were also noted in independent evaluations of social skills. The authors concluded that Ginkgo biloba is ?safe and appears capable of stabilising and, in a substantial number of cases, improving for six months to one year the cognitive performance and the social functioning of demented patients.?36
The effectiveness of Ginkgo biloba may be explained by several mechanisms including enhancement of mitochondrial energy production, increasing blood flow and perhaps most importantly, its ability to reduce the damaging activity of free radicals.36
However, two more recent trials found no benefit for ginkgo for age-associated memory loss. A 2000 study—since criticised for its design flaws—was a 24-week, double-blind, placebo-controlled trial of 214 participants and found no benefit with a dose of 160mg or 240mg/day.37 The second, in 2002, was a six-week study of 230 healthy individuals older than 60 who took 120mg/day with no significant improvement in mental function.38
Docosahexaenoic acid (DHA) is a primary component of the mitochondrial membrane, site of cellular energy production. It?s therefore not surprising that levels of this omega-3 fatty acid are markedly increased in more metabolically active areas of the brain. DHA makes up an impressive 25 per cent of total human brain fat, and deficiencies of DHA have been linked to behavioural problems, visual dysfunction, and a variety of other neurological disorders including Alzheimer?s disease.39
While synthesis of DHA from precursors does take place in humans, deficiencies of the enzymes required for this process may cause inefficiency, resulting in low levels of this critical brain fat. Fortunately, the consumption of fish or fish oils provides adequate amounts of preformed DHA. In July 2003, a highly publicised study was published in the Archives of Neurology demonstrating that the key reason consuming fish at least once a week lowered Alzheimer?s disease risk was DHA. In fact, it was shown that of the 815 subjects studied, those consuming the most DHA had a remarkable 60 per cent reduction in Alzheimer?s risk.40
But the idea of simply increasing fish consumption to reap the benefits of DHA is now being scrutinised. A recent study reported by the Associated Press has had a profound effect on the public awareness of the potential hazards of fish consumption. The report summarised research demonstrating that having as little as two servings of fish monthly led to toxic levels of mercury in 89 per cent of the 116 subjects studied.41,42 This strengthens the case for the use of supplements of highly refined fish oils free of mercury and other heavy metals as well as PCBs.
As we are now firmly entrenched in the ?information age,? the knowledge we so desperately need to maintain health and meet the challenges of disease is now within our grasp. Our dependence upon a system in which standard of care is dictated by pharmaceutical advertising is giving way to a new standard of care—one that recognises the utility of a wide spectrum of well-studied, scientifically validated interventions not solely disseminated by the prescription pad.
It has been said that knowledge is power, and clearly in this context, knowledge is health.
David Perlmutter, MD, is a board-certified neurologist. He is author of BrainRecovery.com and medical advisor for Nordic Naturals.
1. Qizilbash N, et al. Cholinesterase inhibition for Alzheimer?s disease ? a meta-analysis of the Tacrine trials. JAMA 1998; 280L:1777-82.
2. Cummings JL. Current perspectives in Alzheimer?s disease. Neurology 1998;51(suppl 1):1S.
3. Riggs JE. The ageing population ? implications for the burden of neurologic disease. In Riggs J, editor. Neurologic clinics, Philadelphia: W.B. Saunders;1998. p 556.
4. Editorial: rise and fall of diseases. Lancet 1993;341:151-2.
5. Eckert A, et al. Effects of EGb 761(R) Ginkgo biloba extract on mitochondrial function and oxidative stress. Pharmacopsychiatry 2003 Jun;36:Suppl 1:15-23.
6. Yamamoto A, et al. Iron (III) induces aggregation of hyperphosphorylated tau and its reduction to iron (II) reverses the aggregation: implications in the formation of neurofibrillary tangles of Alzheimer's disease. J Neurochem 2003 Sep;86(6):1568.
7. Greenamyre JT, et al.The rotenone model of Parkinson's disease: genes, environment and mitochondria. Parkinsonism Relat Disord 2003 Aug;9.
8. Faucheux BA, et al. Neuromelanin associated redox-active iron is increased in the substantia nigra of patients with Parkinson's disease. J Neurochem 2003 Sep;86(5):1142-8.
9. Vojdani A, et al. Antibodies to myelin basic protein, myelin oligodendrocytes peptides, alpha-beta-crystallin, lymphocyte activation and cytokine production in patients with multiple sclerosis.J Intern Med 2003 Oct;254(4):363-74.
10. Besler HT, Comoglu S. Lipoprotein oxidation, plasma total antioxidant capacity and homocysteine level in patients with multiple sclerosis. Nutr Neurosci 2003 Jun;6(3):189-96.
11. Brooks BR. Risk factors in the early diagnosis of ALS: North American epidemiological studies. ALS CARE Study Group. Amyotroph Lateral Scler Other Motor Neuron Disord 2000 Mar;1 Suppl 1:S19-26.
12. Rothstein JD, et al. Decreased glutamate transport by the brain and spinal cord in amyotrophic lateral sclerosis. N Eng. J Med 1992;236:1464-8.
13. Sano M, et al. A controlled trial of selegeline, alpha-tocopherol, or both as treatment for Alzheimer?s disease. N Engl J Med 1997;336:1216-22.
14. Grodstein F, et al. High-dose antioxidant supplements and cognitive function in community-dwelling women. Am J Clin Nutr 2003;77:975-84.
15. Persson HL, et al. Prevention of oxidant-induced cell death by lysosomotropic iron chelators. Free Radic Biol Med 2003 May 15;34(10):1295-305.
16. Lovell MA, et al. Markesbery WR. Protection against amyloid beta peptide and iron/hydrogen peroxide toxicity by alpha lipoic acid.J Alzheimers Dis 2003 Jun;5(3):229-39.
17. Scott BC, et al. Lipoic and dihydrolipoic acids as antioxidants. A critical evaluation. Free Radic Res 1994 Feb;20(2):119-33.
18. Witschi A, et al. The systemic availability of oral glutathione. Eur J Clin Pharmacol 1992;43(6):667-9.
19. Erick Valencia; Gil Hardy. Practicalities of Glutathione Supplementation in Nutritional Support. Current Opinion In Clinical Nutrition And Metabolic Care 2002;5:321-6.
20. Roes, EM, et al. Effects of oral N-acetylcysteine on plasma homocysteine and whole blood glutathione levels in healthy, non-pregnant women. Clin Chem Lab Med 2002;40(5):496-8.
21. Lean JM, et al. A crucial role for thiol antioxidants in estrogen-deficiency bone loss. J Clin Invest 2003 Sep;112(6):915-23.
22. Pahan K, et al. N-acetyl cysteine inhibits induction of NO production by endotoxin or cytokine stimulated rat peritoneal macrophages, C6 glial cells and astrocytes. Free Rad Biol Med 1998;24(1):39-48.
23. Hirsch EC, et al.The role of glial reaction and inflammation in Parkinson's disease. Ann N Y Acad Sci 2003 Jun;991:214-28.
24. Kayhan B, et al. Glatiramer acetate (Copaxone(R)) regulates nitric oxide and related cytokine secretion in experimental autoimmune encephalomyelitis. Immunol Lett 2003 Sep 8;88(3):185-92.
25. Selley ML. Increased concentrations of homocysteine and asymmetric dimethylarginine and decreased concentrations of nitric oxide in the plasma of patients with Alzheimer's disease. Neurobiol Aging 2003 Nov;24(7):903-7.
26. Shults CW, et al. A possible role of coenzyme Q10 in the etiology and treatment of Parkinson's disease. Biofactors 1999;9(2-4):267-72.
27. Shults CW, et al. Absorption, tolerability and effects on mitochondrial activity of oral coenzyme Q10 in Parkinsonian patients. Neurology 1998;50:793-5.
28. Shults CW, et al. Effects of coenzyme Q10 in early Parkinson?s disease. Arch Neurol 2002 Oct;59:1541-50.
29. Mortensen SA, et al. Dose-related decrease of serum coenzyme Q10 during treatment with HMG-CoA reductase inhibitors. Mol Aspects Med 1997;18(Suppl):137S-44S.
30. Jula A, et al. Effects of diet and simvastatin on serum lipids, insulin, and antioxidants in hypercholesterolemic men: a randomized controlled trial. JAMA 2002 Feb 6;287(5):598-605.
31. Laaksonen R, et al. The effect of simvastatin treatment on natural antioxidants in low-density lipoproteins and high-energy phosphates and ubiquinone in skeletal muscle. Am J Cardiol 1996 Apr 15;77(10):851-4.
32. Steffen V, et al. Effect of intraventricular injection of 1-methyl-4-phenylpyridinium protection by acetyl-L-carnitine. Human Exp Toxicol 1995;14:865-71.
33. Thal LJ, et al. A 1-year multicenter placebo-controlled study of acetyl-L-carnitine in patients with Alzheimer?s disease. Neurology 1996;47:705-11.
34. Montgomery SA, et al. Meta-analysis of double blind randomized controlled clinical trials of acetyl-L-carnitine versus placebo in the treatment of mild cognitive impairment and mild Alzheimer?s disease. Int Clin Psychopharmacol 2003 Mar;18(2):61-71.
35. Hudson S, Tabet N. Acetyl-L-carnitine for dementia. Cochrane Database Syst Rev 2003;(2):CD003158.
36. Le Bars P, et al. A placebo-controlled, double-blind randomized trial of an extract of Ginkgo biloba for dementia. JAMA 1997;278(16):1327-32.
37. van Dongen MC, et al. The efficacy of Ginkgo for elderly people with dementia and age-associated memory impairment: new results of a randomized clinical trial. J Am Geriatr Soc 2000;48:1183-94.
38. Solomon PR, et al. Ginkgo for memory enhancement: a randomized controlled trial. JAMA 2002;288:835-40.
39. Uauy R, et al. Essential fatty acids in visual and brain development. Lipids 2001 Sep;36(9);885-95.
40. Morris MC, et al. Consumption of fish and N-3 fatty acids and the risk of incident Alzheimer?s disease. Arch Neurol 2003 Jul;60:940-6.
41. Crenson SL. Flip-side of a fish-heavy diet. Reported in San Jose Mercury News. October 20, 2002.
42. Hightower JM, et al. Mercury levels in high-end consumers of fish. Environ Health Perspect 2003 Apr;111(4).