Diseases of Civilization by fjzhangxiaoquan

VIEWS: 2 PAGES: 16

									                                  Diseases of Civilization

•   Although our evolutionary heritage has saddled us with some significant health problems, in
    the long run it has served us well…
•   Our cultural evolution has saved us from the fate of our distant ancestors

•   No longer locked into lives that were short and brutal, toiling dusk „till dawn to survive
•   Modern medicine, despite its many problems, has added years to our lives, reduced child
    mortality to very low levels

•   At the same time, we‟ve created many new environments in which pathogens flourish
•   We‟ve provided many new opportunities for pathogens to spread
•   We‟ve adopted new cultural lifestyles which affect our bodies in ways evolution did not
    prepare it for

•   Many pathogens have greatly benefited from human civilization
    > Fleas, ticks, and lice that sought refuge in our furs and fabrics
    > Rats and mice that fed in our fields and storage bins

•   But cultural evolution has given us new weapons in the eternal war against microbes
•   We are no longer helpless in the face of epidemic disease
•   An army of experts is on hand to deal with emergent and re-emergent diseases

•   But many of our remaining health problems may be our own doing
•   Who will protect us from ourselves?
•   These new diseases spring from the lifestyles allowed by modern society
•   They are diseases of civilization…

•   Consider the endless hours of repetitive motion required by farm labor throughout society‟s
    history
•   Now mostly replaced by repetitive motion at the mouse and keyboard
•   Arthritis, carpal-tunnel syndrome, are diseases of civilization

•   Analysis of ancient bones shows that arthritis became much more common when agriculture
    replaced hunting and gathering
•   In ancient nomadic communities, arthritis was mainly confined to the very elderly
•   No longer the case today…

•   Diseases of civilization include:
    >   Cancer
    >   Rickets
    >   Dental caries (tooth decay)
    >   Obesity
    >   Drug addiction
    >   High blood pressure
    >   Heart disease

•   The transition from tribes of nomadic hunter-gatherers to settled agricultural and urban
    communities changed the human race forever

•   One of the most significant changes in our cultural evolution is the change in our ancestral
    diet
•   This change in diet created many nutritional problems, including tooth decay

•   Our dental fillings are a mute testimony to our transition to an agricultural society, in the
    Neolithic
•   Dental caries is caused by bacteria, which cling to the teeth, secrete acid

•   Studies of ancient remains shows significant tooth wear, but little decay
•   Tooth decay is still rare today in cultures like the Inuit who don‟t eat a lot of grain
•   Cooked grains, especially wheat, are very sticky, cling to the tooth surface

•   Sticky film on our teeth is a perfect habitat for tooth decay bacteria
•   Combination of wheat, sugar, and milk is like a bacterial smorgasbord…
•   Abrasion of raw food (ancient diet) wears teeth down more quickly, but also scrapes off the
    bacterial buildup, prevents decay

•   Modern agriculture is the source of many of our chronic health problems
•   Consider that classic New World combo, corn and beans…
•   Corn (maize) and beans formed the staple diet of many New World cultures, still true today in
    many regions

•   Nutrionally sound, from the point of view of proteins
•   Need to get or make all of the 20 amino acids our bodies use to make proteins
•   Corn and beans between them have all 20 amino acids, in addition to carbohydrates and fats
•   Southern US natives switched to a corn/beans diet ~1,500 years ago
•   Made nutrition easier, with reliable crops, healthy protein diet
•   But not as healthy as the diet of their hunter-gatherer ancestors

•   Ancestral skeletons of southern natives are more heavy set, fewer nutritional problems evident
    in their bones
•   Corn and beans is great for proteins and carbohydrates, but lacks some essential vitamins and
    minerals

•   Milk is another nearly perfect food, with milk sugar (lactose), fats, and plenty of high-quality
    protein
•   Designed by evolution to nurture the young of mammals (Latin word “mamma” means breast)

•   Digestion of milk sugar (lactose) requires a special enzyme, protein called lactase
•   Lactase breaks lactose down into two simpler sugars which babies can digest
•   After weaning, the lactase gene is switched off in all mammals, body can no longer process
    milk sugar

•   Human babies are born able to digest lactose
•   Presumably, our ancestors were like other mammals, and lost that ability after weaning

•   Before the days of soy milk (~ 30 yrs. ago), babies who couldn‟t digest mother‟s milk were
    dead babies
•   Strong selective pressure for being able to digest milk sugar before weaning

•   No selective pressure to retain that ability after weaning (mother‟s milk was the only source)
•   But many human races, unlike other mammals, remain lactose-tolerant after weaning

•   In lactose-intolerant people, milk sugar is not digested, provides rich food for tummy bacteria,
    get severe digestive problems
•   F.J. Simoons (1978) looked at 197 populations from around the world, and found a very
    interesting pattern

•   Lactose-tolerant populations were concentrated in particular ethnic groups
•   They were mainly northern and western European races, and their New World and Australian
    descendants
•   Included also were a few African, Mideastern tribes
•   Durham (1991) chose 60 populations to represent the full spectrum of human agriculture:
     > hunter-gatherers
     > crop farmers
     > dairy farmers
     > mixed dairy/non-dairy ancestry

•   Clearly shows the relationship between type of subsistence and lactose tolerance
•   Dairy tribes tend to be very lactose tolerant, hunter-gatherers and non-dairy tribes tend to be
    lactose-intolerant

•   African-Americans are mostly lactose-malabsorbers (= lactose intolerant)
•   Populations considered to be “lactose malabsorbers” are those where 70% or more have
    trouble digesting lactose

•   73% of African-Americans are lactose-malabsorbers to some degree, vs. 16% of the white
    population (Lockwood and Caldwell 1995)
•   African-Americans are mostly descended from tribes that did not raise cattle for milk

•   No one knows when dairy farming began
•   Maybe 5-8,000 years ago…
•   Did the regular availability of animal milk as food select for the ability to digest it after
    infancy?
•   Perhaps, perhaps not…

•   There is a widespread cultural solution to lactose-intolerance
•   Many dairy cultures use milk in the form of cheese and yogurt

•   Cheese and yogurt are OK for lactose-intolerant people
•   In the fermentation of milk, bacteria change lactose to lactic acid, or back into simpler sugars

•   Use of cheese and yogurt is widespread, but has not always selected for lactose intolerance
•   Some cheese/yogurt cultures are lactose intolerant, some are not
•   And the percentage of lactose tolerant dairy farmers, though logical, is high (over 90%)
•   Durham reasoned that these inconsistencies suggested a deeper pattern
•   There must be an additional source of evolutionary pressure selecting in favor of lactose
    tolerance
•   And therein lies a tangled tale…

•   Durham went back to his original study, this time looked at where lactose-tolerant groups
    lived
•   Found a strong correlation with latitude - lactose tolerant groups were more northern
•   Why did latitude correlate so strongly with lactose tolerance?

•   Before we can solve this puzzle, we must first consider another curious way in which cultural
    evolution has caused disease
•   It began hundreds of thousands of years ago, when we first migrated north, out of Africa…

•   Cultural evolution allowed our species to establish itself in parts of the world where more
    primitive man could not survive - high latitudes, high altitudes
•   These places are mostly cold and dark…

•   Ability to make fire, build shelters, skin animals, enabled us to move to higher latitudes
•   Many people today live in very cold climates…

•   Living in colder, darker climates has caused some interesting and unusual health problems
•   Our distant African ancestors lived mostly outside, in the open, lots of sunlight
•   Dark skin pigment is an adaptation to such high-UV equatorial environments

•   Risk of sunburn, skin cancer is reduced by high levels of the protein melanin in the skin (same
    protein gives hair/eye colors)
•   The level of melanin in the skin is an evolutionary trade off, a compromise…
•   Humans also need an important substance called Vitamin D

•   There‟s no shortage of Vitamin D in Africa, created by sunlight striking cells in the skin
•   As long as your skin isn‟t too dark, you can both synthesize Vitamin D, and be protected from
    excess UV
•   When we moved north, we changed the rules…
•   Now we were living in environments that were cloudier, lower intensity sunlight, so we had
    less of a UV problem
•   But less sunlight also meant less Vitamin D

•   To make matters worse, we stayed indoors a lot more
•   And when we did venture out, we bundled up against the cold, not much exposed skin

•   Even in more temperate climates, sun bathing is a relatively recent thing
•   Our ancestors didn‟t have much spare time to indulge in it…

•   Lack of Vitamin D causes the nutritional deficiency disease known as rickets
•   Used to be called the English disease, it was so common in British populations
•   Bones become soft, weak, abnormal bone development, young victims esp. get characteristic
    bow-legged stance

•   Victims bones are so soft as they grow, they bend outward under the child‟s weight
•   Vitamin D is important for good health
     > Helps calcium transport from the small intestine to the blood
     > Controls calcium deposition in growing bones
     > Helps maintain adult bone structure

•   Natural selection favored lighter skin pigmentation in climates where sun exposure was
    reduced
     > Stopped favoring darker pigmentation, no need for heavy sunblock
     > Started actively selecting for lighter skinned individuals who could absorb enough sunlight
       to get plenty of Vitamin D

•   Skin pigmentation represents an evolutionary balance between two conflicting selective
    pressures
     > Darker skin provides better UV protection, but restricts Vitamin D synthesis
     > Lighter skin allows more synthesis of Vitamin D but doesn‟t provide as much protection
       from UV

•   But not all fair-skinned races suffer from chronic Vitamin D deficiency
•   Norwegian and Japanese don‟t have this problem
•   Their fish diet is very high in Vitamin D

•   Ricketts was a major problem in the US until the 1930‟s, especially high among young
    African-Americans
•   Government decreed that dairy farmers must add Vitamin D to milk
•   Reduced incidence of rickets…
•   Still a problem in the US
•   Which brings us back to Durham‟s problem, why does latitude correlate so strongly with
    lactose tolerance?

•   Mothers synthesize the lactose in breast milk from two simple sugars, glucose and galactose
•   Babies must use lactase to break the lactose down again before they can use it…
•   Why not just give the kid straight glucose??

•   Lactose also helps the transport of calcium from the intestine to the blood - just like our old
    friend Vitamin D
•   In effect, lactose can substitute for Vitamin D in the diet in building strong bones (Durham
    1991)

•   Northern cultures cover themselves up, dietary lactose helps to compensate for that (in
    addition to being great baby fuel)
•   So northern latitude dairy farmers could compensate for their reduced exposure to sunlight by
    drinking milk after weaning

•   There were two big exceptions to Durham‟s correlations
•   Native Inuits and some equatorial east African dairy tribes are lactose intolerant

•   Inuits eat a diet high in fish, so they get lots of Vitamin D
•   They can be lactose-intolerant and still survive bundled up at high latitudes, little exposure to
    the sun, without getting rickets

•   Some North-African/Arabian dairy tribes are also lactose-intolerant
•   Their infants get a lot of intense sunlight, no selective pressure to tolerate lactose after
    weaning, plenty of Vitamin D from the sun
•   Cheese or yogurt for food, no need for liquid milk after weaning

•   So selection for lactose tolerance may be primarily determined by the ability to get natural
    Vitamin D
•   The ability to get Vitamin D closely correlates with latitude

•   African-Americans aren‟t the only ones with a lactose problem, over 90% of Asian-
    Americans are lactose intolerant
•   Does that meanb the Vitamin D added to milk to prevent rickets does not help these
    populations, which can‟t drink milk?
•   Yes and no – many substitutes, like soy milk, lactase pills or liquid can help
•   Can also “grow out of it”
•   Lactose intolerance is rare in babies under the age of two – why??
•   Lactase gene still switched on!

•   Usually kicks in around age 5-7, can get worse or better
•   Taking small amounts of milk etc. can build up a tolerance, often fades away on its own

•   Even breast milk is not perfect solution
•   Not enough Vitamin D for those with darker skin pigmentation
•   Increasing rate of rickets now reported in African-American mothers who breastfeed

•   Natural selection often works in such subtle and unexpected ways
•   Sometimes the problems it creates are not so subtle
•   The biggest problems agriculture has created for human health have something in common -
    too much of a good thing

•   Sheer quantity and variety of food available to most humans today is without parallel in
    human history
•   For better or worse, we can pick and choose as much or as little of any food we want
•   Often the choices we make seem to be very maladaptive…

•   We load up on sugars, fats, salts, etc…
•   Bring on those cheetos…
•   Unfortunately, our bodies did not evolve to handle such high levels of nutrition
•   Today's staples were yesterday‟s luxuries

•   All of the items forming the bulk of our modern diet were in short supply on the African plain
•   Meat, fat, sugar, salt were rare treats for our ancient ancestors

•   There‟s absolutely nothing wrong with sugar, or salt, or fat, or meat…we‟d be hard pressed to
    survive without them
•   Healthy natural diets often contain high amounts of all these things
•   Getting these things on the plains of Africa meant the difference between life and death
•   Big selective pressure in early human evolution to get as much of these items as possible
•   That‟s why they taste so darned good - the body‟s little reward for a job well done…

•   We still want as much of these things as we can get
•   But it‟s no longer such a great idea, now that they‟re readily available in infinite supply
•   Creates several big health problems…

•   Consider the effects of excess dietary fat
•   Fat is high-energy food, calorie rich
•   Good way to store extra calories in the body, for a rainy day
•   Normal cycle in the wild is feast and famine, fatten up to get through winter

•   Now we have a uniformly high food supply all year long
•   And at the same time, our general activity has dropped to very low levels
•   It‟s no longer adaptive to be lazy…

•   Once upon a time, long, long ago and far, far away, being lazy was a good idea
•   Our ancestors needed to conserve their energy for hard times, or bursts of power or speed to
    escape predators
•   Now we are lazy all the time - not such a great idea any more

•   Very few urban jobs require substantial physical activity - mailman, garbage man,
    construction…
•   Most are sedentary - retail sales, secretaries, clerical work, computer work…
•   Combination of low activity levels and high-fat diet is a prescription for obesity

•   Excessive dietary fat can cause or contribute to
     > Heart attack
     > Stroke
     > Arterial blockage
     > Diabetes
     > Cancer risk

•   Getting excess dietary fat is almost inevitable, even in a “normal” American diet
•   Wild game is ~15% fat, pampered cattle have much more fat (~20% or more)
•   Average hunter-gatherer got ~20% fat in diet
•   Average American diet gets 40% of its calories from fat!
•   Couple this high-fat diet with physical inactivity, and you‟re living the American Dream!

• Teo thirds of the American population are overweight or obese
    American male = 191 lbs (25 lbs more than in 1960)
    >
    American female = 164 lbs (20 lbs more than in 1960)
    >
• Roughly 39 million Americans are obese, 30 pounds or more above their optimal weight
• LA #5 among states for obesity

•   Obesity contributes to 300,000 deaths a year
•   Obesity costs the American public a cool $147 billion a year in health and related costs
•   Also costs ~ one billion extra gallons of gasoline to carry our big butts around!

•   Jan.‟03 lawsuit vs. McDonalds thrown out
•   Way too much of a good thing…

•   Spring 2004 - WHO guidelines launched an international effort to control obesity
     > Reduced sugar, salt, fat in processed food
     > Tighter control of food commercials aimed at children
     > Better nutrition labeling
     > Better nutrition education

•   There are many similar examples of how our basic American diet is at odds with our African
    evolutionary heritage
•   Pima Indians of Arizona are poor, have frequent famines
•   Food assistance in modern times has created a big problem…

•   Pima have exceptionally high levels of obesity and diabetes
•   Their bodies evolved to handle a greatly restricted diet
•   Japanese immigrants have a similar problem

•   Japanese traditional diet is high in fish, low in fat
•   Japanese-Americans have over twice the incidence of heart attacks as native Japanese, and
    much higher levels of serum cholesterol

•   The classic “food pyramid” has been recently reformulated, to encourage better dietary
    choices

•   But a recent discovery may render it somewhat academic…
•   Another curious heritage of civilization is drug abuse
•   No doubt that it is a serious health problem, but why did it evolve?
•   Many hypotheses, no clear answers…

•   Man is not the only animal that gets stoned
•   Birds often get roaring drunk if they eat berries that have fermented in the sun
•   Do they intentionally seek out such bushes?
•   Only anecdotal evidence…birds are pretty smart!

•   Opportunities for drugs in the wild was pretty chancy before the rise of agriculture
•   Evidence of ancient use of plants in healing (aspirin from willow bark, ephedrine from
    gnetophytes)
•   Many tribes incorporated mind-altering substances in religious rituals

•   One such psychoactive drug was derived from Amanita mushrooms (fly agaric)

•   Used for centuries by tribes in Siberia to induce visions
•   Viking berserkers used Amanita to pump themselves up for plunder

•   Another psychoactive drug used in several cultures came from Psilocybe mushrooms
•   Source of psilocybin

•   Sacred mushrooms were used for thousands of years in Mexico, American southwest, Central
    and South America
•   Thought to reveal the future

•   First recorded binge is Noah‟s drunken behavior on the Ark
•   Earliest archaeological evidence of alcohol use dates to 6400 BC
•   By 2225 BC, alcohol was so widespread that Hammurabi‟s Code had to lay down laws for
    liquor stores and taverns

•   Earliest access to alcohol for nomadic tribes would have been fermented fruit
•   First alcoholic beverage was mead - fermented honey dissolved in water
•   Top alcohol content for fermented beverages is ~12%

•   Fermented beverages became much more common with the rise of agriculture
•   So many grains, so little time…
•   Arabs developed distillation in the 9th Century, could now get very high alcohol %
•   Ironically, one of the only group of people today who do not abuse alcohol are Arabs!

•   Before distillation, alcohol was probably limited to small batches of weak beverages, brewed
    at home
•   Distillation made the process much more efficient, even a small still could produce large
    amounts of potent brew

•   Natural tobacco is also harder to abuse
•   Raw leaf is very harsh, hard to smoke enough for addiction
•   Processed tobacco is smoother, allows much higher intake level

•   Same is true for many recreational drugs
•   Cocaine now refined into crack, opium into heroin etc…
•   Civilization has refined many natural drugs, suddenly making them much more powerful,
    easy to abuse

•   Drug use is usually seen as maladaptive
•   Many hypotheses have been proposed to suggest it might have evolutionary roots
•   If nothing else, considering the problem from an evolutionary viewpoint might suggest a
    solution

•   Moral perspective has not worked…
•   Problem once again is ready availability of substances that used to be relatively rare in our
    environment
•   The same applies to our consumption of excess dietary salt

•   Chronic high blood pressure is a major modern health problem
•   One of the primary contributing factors is excess dietary salt
•   Larger body size, stress, also contribute

•   Our average body size is larger than it used to be - blood pressure is adjusted to a lower range
    of body sizes
•   Brain not quite caught up with regulating blood pressure in larger bodies
•   Stress of modern life also adds to high blood pressure

•   High blood pressure is a common symptom of many chronic degenerative diseases (CDD‟s)
•   CDD‟s may be caused by multiple factors
•   May also be a genetic predisposition to CDD‟s
•   It‟s easy to dismiss CDD‟s as part of the price for growing old - Alzheimer‟s disease, heart
    disease, etc…
•   Hard to study them from an evolutionary perspective, because they may be multifactorial

•   Monogenic diseases - expressed regardless of environmental conditions (due to pathogen
    only)
•   Polygenic diseases - multifactorial, show significant effects due to environmental causes
•   Diseases of civilization are polygenic

•   Life style factors are important in the onset and progress of CDD‟s
•   The discord between our genes and lifestyle factors like diet, exercise, exposure to toxins
    from smoking, drinking etc. all promote CDD‟s

•   Genes that selected for survival in young primate foragers were favored, bad effects didn‟t
    kick in until extreme old age, which most animals never reached in the wild
•   The basic problem today is that we live much longer than we used to

•   We live longer now than ever before in human history
•   Much of that increase in life span is recent
•   CDD‟s could be due to evolutionary lag time
•   Increased life span is so recent, evolution hasn‟t had time to adapt to it

•   So CDD‟s could result from
     > Evolutionary lag time
     > Delayed expression - declining effects of natural selection with increased age

•   Medical establishment usually attributes CDD‟s to one or more environmental risk factors
•   Risk factors for heart disease include
     > Smoking
     > High fat diet
     > High blood pressure
     > Family medical history


•   Statistically, each of these risk factors can be used to predict potential victims of heart disease
•   But ultimately, they explain nothing…
•   Risk factors are not a causal explanation
•   Risk factors are merely guilt by association
•   Risk factors may be involved, but they may not tell the whole story…

•   Quainn‟s Dictionary of Medicine (1884) lists these risk factors as causes of TB
     > Family predisposition
     > Syphilis
     > Mental depression
     > Bad ventilation
     > Climate

•   This standard text was published a mere two years after Robert Koch identified Myobacterium
    tuberculosis as the cause of TB
•   Discovery of the germ is mentioned, briefly discussed along with some anecdotal evidence,
    mostly supported in England
•   Hindsight is always 20/20….

•   Will some future student quietly chuckle over our list of risk factors associated with heart
    disease?
•   Paul Ewald thinks it‟s a distinct possibility
•   In fact, he‟s staked his reputation on it…

•   Ewald has recently suggested that many CDD‟s may turn out to be caused by infectious
    agents
•   His book Plague Time: The New Germ Theory of Disease (2000) makes a compelling case for
    this radical hypothesis

•   Not a popular idea in the medical establishment, or among his colleagues
•   Ewald says he‟s writing for 10-20 years from now - wait and see…
•   Society may not be ready to accept such radical ideas

•   Ewald proposes that heart disease, cancer, Alzheimer‟s and other CDD‟s may be caused or
    aggravated by unknown pathogens
•   There are no lack of candidates…

•   We have only managed to culture about 1% of the world‟s bacterial species in the lab
•   Consider the recently isolated Chlamydia pneumoniae, which may hold the key to heart
    disease

•   Chlamydia pneumoniae was first isolated from the eyes of a Japanese child
•   It turns out to be very common, may cause 20% of all pneumonia cases
•   Finland has an exceptionally high level of heart disease for some unknown reason
•   Study found antibodies against Chlamydia compounds in 70% of Finland heart-attack victims
•   Not conclusive by itself, by middle age 40% of global population has the same antibodies

•   Coincidence or cause and effect?
•   Hard to say…C. pneumoniae follows a 6-7 year infection cycle, doesn‟t correspond to the
    pattern of heart attacks
•   Perhaps heart attacks are associated with chronic infections of C. pneumoniae (Saikku 1993)

•   This hypothesis created an uproar, and is still highly controversial
•   C. pneumoniae was found in the arteries of coronary bypass patients in 1992 - coincidental?
•   About 40 studies published since then have touched on this link, ~ 75% supported it

•   C. pneumoniae would explain why inflammation is always associated with atherosclerosis -
    maybe the bacterium causes the inflammation that starts it
•   C. pneumoniae is attracted to lipids, might explain why fat accumulates in damaged arteries

•   C. pneumoniae oxidizes fats, might explain association between atherosclerosis and oxidized
    fats in the arteries
•   C. pneumoniae develops extremely well in the damaged lungs of smokers, might explain why
    smoking is a risk factor in heart disease

•   C. pneumoniae might even explain how second-hand smoke might cause atherosclerosis in
    others - bacteria travels via aerosol in the coughs of smokers
•   C. pneumoniae could even explain why daily aspirin reduces the risk of heart attacks - it
    reduces the inflammation caused by the bacterium

•   It remains to be seen whether CDD‟s are caused by infectious pathogens, or are merely one
    more legacy of our biological evolution, or one more health problem aggravated by our
    cultural evolution

•   It has been a mere 10,000 years since we forsook the role of hunter-gatherer for the staid and
    settled world of agriculture and urbanization
•   In some ways, our bodies still live on the great plains of Africa…
•   Our daily environment has changed radically in the scant millennia of our human civilizations
•   In evolutionary terms, our bodies have not always kept pace with many of these
    environmental changes

•   We must cope with these new environments as best we can, in bodies designed over millions
    of years for a vanished way of life

								
To top