Golden House-Sparrow Book Of The Week
by Stephen Davies and Alan Stewart
(ISDN = 0 330 28833 4)
Who Do You Want To Be Today?Food-faddists and naturopaths aside, most people find nutrition a rather dull subject. Certainly, compared to some of the hot-button topics featured on HedWeb, notably psychostimulants and state-of-the-art class-A drugs, Nutritional Medicine will appear worthy but unsexy. For if one eats a reasonably well-balanced diet, then the youthful human body typically seems pretty much to take care of itself. The insidious health-risks posed by such half-truths aren't always recognised.
For a start, foods are themselves akin in many ways to subtle drugs. The distinction between these two categories is by no means clear-cut. The blurring of boundaries is borne out by the recent baptism and rapid growth of so-called functional foods or nutraceuticals, "designer foods" that combine nutritional value with alleged disease-preventive and medicinal benefits. In fact, all the constituents of food are psychoactive. We just can't practically distinguish their differential contribution to the psyche. In choosing to eat this food or that, one literally determines who and what one is going to become, a choice arguably a matter of some importance.
In the human mind/brain, the half-life of a typical protein is perhaps ten days. Thus a large part of one's deepest thoughts, feelings and memories is made up of the dietary ingredients of one's previous few dozen meals. So whereas sceptical philosophers have sometimes held that there is no way to penetrate the veil of perception and grasp the nature of the Kantian thing-in-itself, perhaps the best way to come to know about the noumenal world's intrinsic properties is to eat and self-incorporate (bits of) it. The role of edible knowledge, whether in nutritional or psychopharmacologic guise, has yet to gain epistemic legitimacy in academia.
Possibly if one's conception of personal identity is purely functionalist, and one sees oneself as just some type of abstract pattern only contingently implemented in a particular substrate, then one's choice of body and brain parts shouldn't matter. These parts are all interchangeable with type-identical constituents elsewhere. Yet even then, as Davis and Stewart's book amply makes clear, the particular foods and nutrients one chooses to consume do in practice exert a host of subtle functional effects on mind/brain and body. These effects will be masked, but they won't be nullified, by a hectic life-style.
Davies' and Stewart's basic thesis is that many chronic and acute diseases are caused or exacerbated by nutritional imbalance or sub-clinical deficiency. The first section of Nutritional Medicine examines the role of vitamins, minerals and other dietary factors in health and disease. The second and third sections explore how a wide range of medical conditions can be tackled by using specific diets and nutritional supplements.
Eating sensibly is important because the best way to lead a long and healthy life is to eat as little as is prudently possible. The anti-ageing effects of being systematically under- but not mal-nourished have been demonstrated in all species tested so far. Balanced low-calorie diets tend to boost immune function, preserve memory and reduce blood pressure. Unless Homo sapiens is inexplicably unique, caloric-restriction will give its practitioners the chance to extend life-expectancy by up to 40%; and to retain youthful vitality for longer. The low-calorie, high-nutrient diet in Okinawa, for instance, probably explains why its incidence of centenarians is forty-fold higher than on the major Japanese islands.
Even so, the damage that food does to us still isn't fully understood. What's clear is that burning food to produce energy in the mitochondria, our intracellular powerhouses, produces toxic free radicals. Mitochondria are found in every cell of the body. Free radicals act as electron-grabbing vandals. The damage they cause to the ubiquitous mitochondria also ages the body as a whole. Exercise, appetite suppressors and vitamin E mop up toxic free radicals before they can do too much harm.
Unfortunately, chronically under-eating can leave one feeling chilly, thin, hungry, and grumpy. Achieving maximum human lifespan entails getting the nutritional and caloric balance exactly right. Mice whose caloric intake is restricted by up to 50 percent will live up to 50% longer. If their caloric intake is restricted by 60 percent, they starve to death.
Possibly a long-term, low-dose regimen of SSRIs would alleviate the malaise commonly felt on a severe regimen of sub-lethal, genuinely life-extending caloric restriction. However, SSRIs have pitfalls of their own. Perhaps the serotonin precursor 5-HTP can offer a useful "natural" alternative to pharmaceuticals. But it's unpatentable; so well-controlled long-term clinical trials are lacking.
Alternatively, potent sirtuin stimulators may soon be synthesised that mimic the life-extending effects of caloric restriction without the need for heroic asceticism. Eventually, we may all have the option of feeling slim and abundantly well-fed - even on a "starvation" diet. This prospect is probably still a decade or more away. But the putative calorie restriction mimetic resveratrol may extend lifespan without leaving the user chonically hungry.
Nutritional Medicine is firmly, and at times too uncritically, in the Nature-Knows-Best camp. Nonetheless the authors are both highly-trained doctors. They recognise that in many serious medical conditions, optimal diet can play only an auxiliary role beside hi-tech orthodoxy. Auxiliary or otherwise, it's still appalling that hospital food is frequently sub-standard or at best mediocre. And it's a disgrace that in their many years of allotted medical education, trainee doctors are expected to race through Nutrition in perhaps a week. General practitioners are taught to recognise well-defined vitamin deficiency diseases - which are rare in the West. Yet they'll most often miss a rash of subtler and sub-syndromal conditions linked to poor diet. Moreover a convergence of laboratory and epidemiological evidence suggests that some 40% of cancers are linked to dietary choices. Our unhealthy eating habits are sometimes caused by poverty, sometimes by agribusiness- and ad-industry-promoted ignorance, and usually both. Here, however, the focus will just be on the role of food and food supplements in the sub-field of nutritional psychiatry.
The links between food, mood and cognition are subtle and complex. They are also subject to numerous and often uncontrolled variables which easily disguise any differential effects and their importance. For instance, the long-term role of the essential (linoleic and linolenic) fatty acids in emotional, intellectual and physical health is vital. Yet appreciation of their value is clouded by the popular notion that fat is bad for you. Mild-to-moderate functional deficits of omega-3 fatty acids are common in the modern era. Such deficits can contribute to a variety of distressing ailments such as atopic eczema and bronchial disorders. Essential fatty-acid deficiency can also cause depression; worsen attention deficit hyperactivity disorder (ADHD); and increase the risk of dementia. There is a case for more TV ads promoting the sexiness of sunflower-seeds, and less glamorising sugar-coated junk food.
The risks of fatty-acid deficiency start early. During infancy, a lack of docosahexaenoic acid (DHA), the omega-3 long-chain polyunsaturated fatty-acid found in breast milk, increases the later risk of developing schizophrenia among the genetically susceptible. DHA ensures the fluidity of neuronal membranes. It is thus essential for the transmission of nerve-signals in the brain. DHA-deficiency can cause clinical depression and coronary heart-disease in later life. It can be biosynthesised in the body from alpha-linolenic acid derived from dietary sources such as flaxseed oil. Scandalously, and in defiance of World Health Organization recommendations, infant-formulas in many parts of the world still fail to include it. The major shift over the last hundred years in the ratio of n-6 (arachidonic acid, linoleic acid) to n-3 (docosahexaenoic acid, linolenic acid) fatty acids in the typical Western diet may have played a role in the increasing global incidence of depressive disorders. Our dietary intake has shifted from a ratio of perhaps 1:1 on the African savannah to a ratio of somewhere between 10:1 and 25:1 in North America today.
The role of several key amino acid constituents of proteins is also vital to mental health. They serve as the precursors to the monoamine neurotransmitters which help mediate mood and emotion. In effect, different precursor amino-acids compete to get into the brain. The amino acid composition of the blood is in large part a reflection of one's last meal. So it might be supposed that one could boost or diminish the function of different neurotransmitters in the brain by dietary means: either by eating a diet whose amino-acid composition was artificially skewed in favour of foods containing unusually high levels of particular amino acids, or by trying "precursor loading" of specific free-form amino acids and their vitamin cofactors.
To a limited extent this is true. The brain, however, like the rest of the body, has a set of complex homeostatic mechanisms. Their effect is to buffer the body from variations in the composition of food. Hence precursor loading with l-tyrosine and l-phenylalanine, or choosing a diet abnormally rich in these two amino-acids, won't on its own predictably and consistently raise the corresponding downstream levels of dopamine and noradrenaline in the brain to any great extent. This is because tyrosine hydroxylase, a key enzyme in the metabolic pathway which converts the amino acids into neurotransmitters, is normally saturated. It's usually also the rate-limiting step in the production process. Cofactors that are involved in the synthesis of these neurochemical transmitters can affect neurotransmission [e.g. Vitamin B6, zinc and magnesium] They are cofactors in the enzyme aromatic acid decarboxylase, which converts dihydroxy-phenylalanine to dopamine and 5-hydroxytryptophan (5-HTP) to serotonin. Thus a deficiency in vitamin B6 can result in a decrease of a particular neurotransmitter.
There are nonetheless distinct advantages in choosing a diet high in the scarce essential amino acid l-tryptophan. Across the industrialised world, there is an inverse correlation between dietary l-tryptophan intake and the suicide rate. Increasing one's intake is feasible either by taking supplements of the free-form amino acid or eating tryptophan-rich foods such as pecan nuts and bananas. This is useful because tryptophan is the precursor and rate-limiting step in the production of the strategically important neurotransmitter serotonin. One recent Canadian study suggested the rate of serotonin synthesis was on average 52% higher in the men compared with the women. This may account in part for the higher incidence of unipolar depression in women than men. The functions of serotonin's 15 presently identified receptor sub-types and their metabolic cascades shouldn't really be compressed into a sentence. But, simplifying grossly, serotonergic pathways mediate emotional resilience, relaxation, sociability and a sense of things being normal and "all right." Low levels of serotonin metabolites, on the other hand, are associated with irritability, anger, depression and violence towards oneself and others. So whereas eating an artificially tryptophan-poor diet, or consuming a free-form amino-acid-mix minus tryptophan on an empty stomach, can quite rapidly cause depression, a tryptophan-rich diet is likely to be good for mood and mental stability.
In fact, matters are inevitably more complicated. It's simplistic to think that more serotonin must somehow be indiscriminately "good" for us. Selective activation of the serotonin 5-HT2c receptors, for instance, is subjectively quite unpleasant. Hence selective 5-HT2c antagonists like agomelatine are effective mood-brighteners. Moreover not everyone reacts to food in the same way. "Biochemical individuality" shapes our responses to nutrient intake as well as different drugs. Paradoxically, the most immediately effective way to raise neural serotonin levels is simply to eat, not a protein snack, but a high carbohydrate meal. The insulin released lets more tryptophan cross the blood-brain barrier by sweeping competing amino-acids out of the bloodstream.
On a much grander scale it may be conjectured, albeit unprovably one way or the other, that the often pacifistic and fatalistic religious traditions of the Indian subcontinent are linked to their high-carbohydrate but tryptophan-rich staple diet of rice and legumes. Conversely, it might be expected that tribes or civilisations typified by high-protein red-meat-eaters, many other things being equal, should as stereotype suggests be temperamentally more aggressive to each other as well. For a high-protein diet will tend to diminish serotonin function at the expense of norepinephrine and dopamine. These are two catecholamine neurotransmitters equally implicated in mood - but not of a socially empathetic flavour. Such speculations do not lend themselves to controlled study.
There is a sense, however, in which nutrition is, or rather ought to be, a dull subject. For after one has got one's diet and life-style just about right, then one has reached a genetically constrained plateau of well-being - or alternatively become trapped in a local minimum of ill-being - beyond which there's nowhere else to go; or at least nowhere to go via what are conventionally regarded as natural means. From then on, only the psychopharmacology, genetic therapy and then nano-level hedonic engineering as sketched in The Hedonistic Imperative are going to make life significantly better.
The great majority of people haven't attained even their miserly genetic emotional ceiling. So Nutritional Medicine is a valuable manual. For most us, "You are what you eat" is a tag so denatured by overuse that we rarely think through its implications. This food's-the-best-medicine sort of tract is probably just the right kind of corrective.
There is, however, a more sombre perspective to be taken on any book about food. For the story behind our last meal is frequently a sordid catalogue of cruelty. By our choice of purchases, we pay anonymous others to commit frightful crimes of violence, subjugation and terror on our behalf. This is emotive language. Yet it still fails to evoke the full horror of factory farming and slaughterhouse killing. These cruelties are inflicted on our inoffensive fellow creatures simply because we like the taste of their flesh. Indeed we practise what our post-Darwinian descendants may view as systematic animal-abuse in the guise of dietary choice. We do so even though neurochemical, genetic and behavioural evidence suggests our victims are functionally equivalent to human babies and toddlers in their intellectual capacities and, crucially, their capacity to suffer. Set against the enormity of such a holocaust, it can be hard to treat the bodily fine-tuning of Nutritional Medicine as anything but a frivolous sideshow. The rattle of the death camps is too loud; or rather it ought to be. Perhaps one day the uncensored dreadfulness of what's going on can be piped via the Net into our living rooms to remind us of what's really happening. Later this century, perhaps superhealthy, supertasty vat-food will supersede the non-human animal holocaust. In the meantime, most of us just don't want to know.
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