Science Articles

Lost your bottle?

Fri, 20th Apr 2007

Jemima Stockton

On everyone's lips

Milky TeaI'm drinking a cup of tea. With milk. According to a report in my newspaper today, research shows that the milk will stop the tea from boosting my cardiovascular function. I could be a little concerned that I'm blocking it with milk if I'd actually been aware of tea's health protecting property in the first place. But really, as I hadn't, I don't feel that my cardiovascular system is missing out. And, anyway, today's newspaper is tomorrow's recycling...

Over half of adults in the UK are either overweight or obese. It's no surprise that diet stories should pop up frequently in the news. And as milk is on the weekly shopping lists for many of us, this item grabs its fair share of the attention. Whether you love or loath the white stuff, it is unavoidable on your TV and in your paper. Is this happy news for the UK's dairy farmers? That depends on the latest study. Milk is sloshed around newsrooms these days; hailed and condemned for its health effects in equal measure. But, with the UK's two million or so dairy cows producing raw milk at a steady rate of around 14 billion litres per year, farmers and cows are evidently taking the highs and lows in their stride. Approximately half of raw milk is processed to produce liquid milk. Thus, it seems the product will be firmly lodged somewhere in the average household's fridge door, whatever the latest word. But why then, despite headlines such as “Milk causes serious illness for 7 million Britons”, “Insomnia, new milk is the cure” and “US mothers have flame retardant in their breast milk”, are we so unflappable and our faith in milk so unshakeable? In a world full of artificial colours, flavourings and preservatives, what could be more natural than reaching for a nice refreshing glass of clean white milk? The truth is that the most natural milk experience we ever have is during infancy.

Natural born milkers

In nature, female mammals produce milk for the sole purpose of nourishing and protecting their young. Such is the value of a mother's milk to her offspring that the World Health Organization (WHO) promotes breastfeeding as “an unequalled way of providing ideal food for the healthy growth and development” of babies. WHO guidelines state that infants should receive breast milk exclusively for the first six months of life, and alongside complementary foods- to provide the increasing quantities of required of nutrients- for two years or more thereafter. They don't specify when breastfeeding should stop but most babies are completely weaned by about two years of age. Still, this hasn't put off older humans from seeking out and drinking mammalian milk from other sources.

Suckling FoalDrinking the milk of another species is not unheard of in other mammals: there are reports of cats suckling puppies, dogs suckling kittens, and even of cats nursing hedgehoglets. However, consuming it post-weaning is a practise that is exclusive to humans. And this fact forms the basis of many anti-milk campaigners' argument that drinking cow's (or any other mammal's) milk is unnatural. But what I say is that the opposable thumbs we have evolved as primates are most definitely natural. And, as they are surely necessary for manual milk extraction, milking is naturally restricted to primates. OK, only to humans. But non-human primates are vegetarian and tend to be tree-dwellers so the opportunity to milk cows and the like hasn't arisen.

When did our thirst for milk first lead us to udders? And why did we think we could consume milk that wasn't from our own mothers, yet alone our own species? It seems we had a gut feeling that drinking animal milk might be a good idea long before science could provide answers.

A complete food

Milk is made up of water and macronutrients, calorie-providing substances that a mammal requires in large amounts for survival- carbohydrates, fats and proteins. Milk also contains minerals and vitamins, and certain friendly bacteria. Together these components constitute a complete food able to satisfy the nutritional and protective requirements of a growing and infection-prone- immunologically naïve- mammalian infant in the initial stages of life. Variation exists in the milk composition of different species. This can be attributed to differences in mammalian infants' nutritional needs because the composition of a mother's milk is suited to the specific needs of her infant. So what are these needs and how does milk cater for them?

Mostly water

Like cucumbers, animals contain more water than anything else. Most newborn mammals bodies are about seventy-five percent water; a bit less than cucumbers. And to support this body composition, their mothers' milk contains a correspondingly large volume of water. The amount of water is dependent on the amount of milk sugar the mother can make because the sugar draws water into the milk. The amount of milk sugar produced is controlled by the mother's diet, namely the carbohydrates.

Most UK dairy cows graze on fresh pasture during the summer months and on conserved grass, in the form of hay (dried) or silage (pickled), during the winter. It is the knowledge that more carbohydrates mean more milk sugar, more water and, thereby, a bigger milk yield that prompts most dairy farmers to supplement their cows' diets with concentrated feeds rich in carbohydrates and other nutrients. Supplements include foods such as maize, peas, rapeseed meal, sugar beet pulp and fishmeal. Munching on these energy-dense foods allow cows to produce much more milk than they would just chewing on low-calorie grass all day: cows fed supplements can make around ten times more than they would make naturally for their calves.

Milk sugar

Carbohydrates are the body's main source of energy. They are broken down into simple sugars which are used to provide energy for the growth, repair and activity of body cells. In most species the main carbohydrate in milk is lactose, found in no other substance in nature. Lactose is broken down by a substance, lactase, that is produced in the small intestine. Simple sugars can then be absorbed into the blood. Human milk contains 7% lactose whereas that of polar bears contains just 0.5%. This may explain why children crave chocolate whilst polar bear cubs dream of savoury snacks such as seals.

Lactose

One of the products of lactose digestion is lactic acid. When a human baby feeds from its mother, lactic acid-loving bacteria in the milk and on the skin around the nipple are ingested. These friendly bacteria use lactic acid to establish themselves in the baby's digestive tract, encouraging the growth of other good bacteria such as bifidobacteria. These bacteria create an acid environment to discourage the growth of harmful bacteria. However, bifidobacteria fail to thrive on the diet of proteins provided by the modified cow's milk used in formula feeds. Thus, a formula-fed baby is more prone to infection with harmful gut germs.

The healthy gut flora profile that is generated by a diet of breast milk is not only helpful in the prevention of gut infections. Studies have indicated that healthy bacteria are essential for gut development. When microbiologists compared the guts of mice raised in a sterile environment with those of mice raised in a non-sterile one, they found a striking difference. The guts of mice raised in a sterile environment, and therefore with no colonizing bacteria, showed stunted growth in the surrounding, nutrient-transporting blood vessel networks.

Killing the bugs

Most of the cow's milk that is consumed in adulthood is pasteurized. Pasteurization is the process wherein precisely controlled heat is used to reduce microbial populations. Intensive farming practices increase the likelihood that udders will become infected and, thus, milk contaminated with harmful bacteria. Cow mastitis, inflammation of the breast tissue, can be caused by a number of microorganisms harmful to humans such as Streptococcus agalactiae, Staphylococcus aureus, Mycobacterium tuberculosis and Escherichia coli. Huge numbers of the causative bacteria are passed into the milk from infected tissue.

Pasteurization removes both nasty and friendly bacteria, indiscriminately. So unlike those who drink pasteurized, raw milk drinkers don't miss out on the good bacteria. However, even the raw milk from less intensively farmed healthy cows can pose a potent health risk to consumers. Bugs that are not troubling to cows and that end up in milk do not always make welcome guests in human tummies. For example, Salmonella enterica and Listeria monocytogenes are commonly detectable in the digestive tracts of healthy dairy cattle. In humans salmonella can cause severe gastrointestinal disease. And in some, particularly those with less robust immune systems, the bacteria can enter the bloodstream, resulting in meningitis. Similarly, listeria can invade the bloodstream of vulnerable people to cause these potentially lethal illnesses. In pregnant women, it can cross the placenta from maternal blood to infect the foetus or infect the baby during the delivery process. Pregnant women are warned to avoid all unpasteurized dairy products such as soft cheeses.

Raw milk is held in high regard in many parts of the United States. But last year Californian state officials ordered San Diego retailers to remove raw milk from their shelves after two children became ill. The children tested positive for an E.coli strain that can cause haemolytic uraemic syndrome resulting in kidney failure.

Unfazed advocates of raw, unpasteurized milk suggest that its resident human-friendly lactic acid bacteria, absent in pasteurized milk, will bring their lactose-digesting lactase protein with them. Which could prove useful for those whose bodies cannot break down lactose on their own.

Lactose intolerance

The majority of the world's population is genetically programmed to loose the ability to break down lactose from around the age of two. By the age of about ten this ability is almost completely lost. Therefore, older children and adults have impaired tolerance to milk in their diet. Unabsorbed lactose remains in the gut where it attracts water. The majority of the lactose reaching the final part of the bowel cannot be broken down by resident friendly bacteria. The symptoms of undigested lactose can be tummy pain, farting and diarrhoea.

Lactose intolerance develops in most other species after infancy as well, presumably because there is no need to consume milk from then onwards. Interestingly, some human populations, most notably North Europeans, are largely lactose tolerant. The evolution of lactose tolerance amongst such populations may have occurred due to the survival advantage of people who could tolerate milk. This would suggest that milk played a vital role in the North European diet; perhaps because, in the absence of high levels of sunshine to stimulate Vitamin A production in the body, milk provided an excellent exogenous source. It would also imply that, historically, raw milk was free from the potentially deadly microbial contamination that necessitates many of today's processing methods. In Scotland, evidence of milk on Iron Age pottery shards has been unearthed by archaeologists, confirming that people were lapping up milk over three thousand years ago. In fact, researchers have estimated that the genetic change that provided lactose tolerance happened between 5000 and 10 000 years ago around the time that dairy farming started.

Biologists at University College London have recently showed that the genetic mutation that causes lactose tolerance in many groups of African herdsmen is distinct from the one that causes this condition in people of European descent. Thus, the conventional diagnostic genetic test for lactose tolerance may fail to identify some lactose tolerant peoples of non-European origin. The driving force for the evolution of lactose tolerance among the African groups- in a continent where sunshine is plentiful- may have been a need to survive for long periods in the absence of water and food. This need is still there: by definition, nomadic herdsmen move to where there is pasture for their livestock. This can mean spending weeks in remote areas without access to water and food other than that of the milk of their herds.

Other interesting genetic studies of lactose intolerance have led Italian scientists to hypothesise that cystic fibrosis (CF) may owe its prevalence to milk. CF is one of the most common lethal genetic diseases in people of European origin. Despite most sufferers succumbing before they have children, the faulty gene that gives rise to CF has remained in the population's pool of genes. The implication is that healthy individuals who have one copy of the faulty gene, as opposed to a lethal compliment of two, are even better off than those who have none at all. The researchers reckon that the faulty gene automatically comes with the added bonus of another one; the one that provides tolerance to lactose. People who could tolerate milk and, therefore, would not die of lactose-induced diarrhoea, would survive to pass on their good genetic information alongside their bad CF gene.

Of course, nowadays lactose intolerance is rarely fatal thanks to improved health care and sanitation, and access to a wide variety of nutrition sources. The extent of an individual's lactose intolerance is dependent on a number of environmental, genetic and physiological factors. Many lactose intolerant people can, in fact, consume a substantial amount of cow's milk in their daily diet without feeling any ill-effects. Although milk contains all the major and minor groups of nutrients essential to human health, many populations the world over live healthily on milk-free diets. If a lactose intolerant person in the UK opted out of milk-drinking they probably could do so without too much health bother. That said, there is marked variation in the dietary habits of different populations. Populations that do not depend on milk products for any part of their nutrition, compensate by consuming larger amounts of other foods to ensure a balanced diet. For example, in China, where most people are lactose intolerant and very little milk is consumed, vegetables are the main source of calcium.

The knowledge that they can survive on a milk-free diet may come as a relief, even for some lactose-tolerant people. Clinical and experimental research suggests that one of the breakdown products of lactose is toxic to certain ovarian cells, resulting in infertility. Researchers found that fertility rates in various countries correlated with the prevalence of adult lactose intolerance and per capita milk consumption. In populations with higher per capita milk consumption and more lactose digestion ability, fertility rates were lower at older ages and the decline in fertility with age was faster.

Drink milk for healthy bones...or not?

So milk might causes infertility then? Does this make you want to give up milk in order to conceive? If so, the Department of Health will urge you to take it up again as soon as you fall pregnant. Its pregnancy information booklet states: “Dairy foods ...are important as they contain calcium and other nutrients needed for your baby's development. Choose low-fat varieties...” This advice is sound in so much as important nutrients are present in milk, and low-fat won't clog up your arteries. But is consuming milk the best way to get hold of calcium?

Calcium is a mineral required for bone and teeth formation. In an infant undergoing the most rapid period of growth in its life, it is especially important. No wonder then that milk products have the highest concentration of absorbable calcium of any food. Calcium plays an essential role in blood clotting, nerve impulse transmission and maintaining a steady heart rate. So, whether you're a baby, a pregnant woman or neither, calcium is a must. Adequate calcium intake has long been associated with good bone health. Calcium is widely regarded to go hand in hand with milk but in some ways they go together more like chalk and cheese.

Aside from keeping us unfloppy and protecting the insides of our bodies, bones serve as a storage depot for calcium. If insufficient calcium is present in the diet, calcium levels in the blood are low and, as a result, non-bone-related calcium processes will be impaired. To remedy the situation, calcium is taken out of bone storage and sent to where it is needed.

The UK's Governments Committee on the Medical Aspects of Food and Nutrition Policy (COMA) recommends 525mg calcium per day for non-breast fed infants. For weaned toddlers the recommended daily intake (RDI) is 350mg. Thereafter, the RDIs increase until adolescence, when they peak at 800mg for girls and 1000mg for boys. Adults, including pregnant women are recommended to have a calcium intake of 700mg. (One 200ml glass of milk (skimmed, semi-skimmed or whole) will provide about 240mg calcium.) The government's Food Standards Agency (FSA) suggests that an adult's daily calcium requirements can be met through dairy products alone: “If you have one glass (200ml) of semi-skimmed milk, a small pot (150g) of low-fat fruit yoghurt and 40g hard cheese, that would provide roughly the amount of calcium an adult needs in one day.” In the spirit of super-sizing, the US government recommends even more; 1000mg for adults up to fifty years old, and 1200mg for the over-fifties.

The breakdown of protein from animal-derived products, such as milk, releases acid into the bloodstream. In order to neutralize the acid, calcium is leached out of bone. Therefore, the net effect of calcium intake through absorbable calcium-rich milk rich on bone health may be negative. One study of white women in the United States has found that women consuming greater amounts of calcium from dairy products had a greater risk of hip fracture. No increase in the risk was observed for the same level of calcium from non-dairy products. This suggests that some component of dairy products contributes to the elevated risk. However, in the same study, protein intake was not associated with hip fracture risk.

Like animal-derived proteins, plant proteins release acid into the blood when broken down. But plant proteins causes less calcium leaching. This may explain why populations consuming large quantities of dark green leafy vegetables, such as kale, but low amounts of milk, have lower rates of bone fractures. However, dietary factors such as per capita milk consumption, are unlikely to be the only ones that determine susceptibility to fractures.

Osteoporosis is a disease characterized by low bone mass and structural deterioration of bone tissue. There is great variation in the prevalence of osteoporosis, as indicated by fracture rates in older people, in different parts of the world. The WHO states that rates are many times higher in affluent developed countries than in sub-Saharan Africa and Asia. Malnutrition is significantly less common affluent countries, implicating a role for non-dietary factors in susceptibility to poor bone health.

In addition to calcium, milk contains all the other essential minerals and all the major vitamins. Vitamins A, D, E and K are fat-soluble so are found mainly in the fatty part of milk. Alongside calcium, Vitamin D plays a crucial role in bone health. Lack of Vitamin D can cause rickets, a condition in which bones are deformed. Although pasteurization reduces the Vitamin D content of milk, levels of this vitamin are not particularly high in raw milk anyway. Vitamin D can be produced in the body by exposure of skin to ultraviolet (UV) light. By definition, vitamins cannot be produced by the body. But Vitamin D is classed as one because many peoples of the world do not get exposed to sufficient UV light to maintain a healthy level of Vitamin D synthesis. Testament to this, routine Vitamin D fortification of cow's milk in the 1940s saw the incidence of rickets fall by 85%.

Milk is an excellent source of many of the B vitamins. These water-soluble vitamins play a variety of roles in helping the body's biological catalysts to metabolize carbohydrates, fats and proteins. Pasteurization can reduce the amount of these in the cow's milk we drink but only by 10% or less.

Fatty goodness...

CreamAs well as providing a home for fat-soluble nutrients, fats are valuable in many ways. Like carbohydrates, fats can be used as a fuel for the body. Although there is more energy available in fat, it is less easy for the body to harness that energy. Therefore, fat is stored in the body and only broken down when the body cannot get enough energy from other sources. Stored fat is deposited around the body's organs and under the skin to provide cushioning and insulation. Fat is also an essential part of all body cells. Brain cells are particularly fatty which explains why, excluding water, your is brain is about two thirds fat. Human and cow milk contain roughly 5% fat. Arctic and aquatic mammal milks tend to have much higher amounts of fat: colder environments demand lots of blubber. In fact, at 50%, grey seal milk contains as much fat as double cream (although to our tastes it would be a poor substitute on top of puddings as it contains considerably less sugar).

Dietary fat also contains substances called fatty acids. Some of these in milk are essential fatty acids (EFAs), so called because they are crucial for maintaining health and, unable to make them, the body relies on dietary sources. There are two families of EFAs, omega-3s and omega-6s. As an EFA can be converted to another type within its family, it is only necessary to consume one member of each of the two omega family. The diet of a mammal influences the fat composition of its milk. For example, dairy cows exclusively fed grass produce milk that contains equal amounts of omega-3 and omega-6 fatty acids. The milk of those whose diet is supplemented with grain contains more omega-6s and fewer omega-3s. Studies indicate that people who consume equal amounts of the omegas have a lower risk of cancer, cardiovascular disease, autoimmune disorders, allergies, obesity, diabetes, dementia, and various other mental disorders.

...And fatty badness

But you can have too much of a good thing. Milk contains saturated fat out of which the liver makes cholesterol. Although cholesterol is essential in the formation of the body's cell membranes and in the production of fat-digesting bile and Vitamin D, too much blood cholesterol is associated with cardiovascular disease. Cholesterol is a waxy substance that can stick to the internal walls of arteries and thereby increase blood pressure. Under high blood pressure, a cholesterol plaque on an arterial walls can become unstable and cause complete blockage of one of the heart's arteries. This can can deprive the heart's muscle of oxygen and in turn cause the muscle to die and the heart to stop.

Protein: the meat of milk

Proteins, the main components of the body's cells, are abundant in milk. Dietary proteins are needed for virtually all biological processes. Many of the proteins in milk are found no where else in nature, reflecting their specialized roles in infant nutrition. For example, alpha-lactalbumin is involved in the synthesis of lactose, essential for regulating the water content of the infant's feed. The breakdown of protein into its constituent amino acids is a slow process as proteins are complex molecules. Hence, proteins are a relatively long-lasting source of energy. Some amino acids can be synthesized from materials in the body but others, essential amino acids, must be consumed in the diet. All eight essential amino acids are present in milk.

The main proteins in milk are caseins of which most species' milks have three or four. (It is these naughty proteins that are believed to be directly responsible for stopping the healthy effect of my tea.) In milk, casein molecules group together to form a multi-molecular structure, a casein micelle. The water-hating (hydrophobic) portions of the casein molecules form the core of the micelle and, thereby, avoid direct contact with their watery surrounds.

So, milk is essentially a suspension of pretty solid casein micelles and an emulsion of liquid fat globules in water. The non-casein proteins are much more water-soluble. They are termed whey proteins. In cow's milk the major whey proteins are beta-lactoglobulin and alpha-lactalbumen. However, beta-lactoglobulin is not present in human milk. As it is not innately recognised by the human body, this protein may be the trigger for some people's allergic reaction to cow's milk.

There is great variation in the protein contents of different species' milks. At 1%, human milk has one of the lowest protein contents of all mammals. This low protein content is attributable to the relatively high- 7%- lactose content of human milk. The more lactose, the more water and, therefore, the lower the concentration of protein. This negative correlation is also very obvious in whale milk: it contains a whopping 14% protein but less than 1% lactose. In addition to interspecies variation in total protein levels in milk, there is variation in the types of proteins therein. For example, human milk contains more whey proteins than casein whereas that of cows contains more casein than whey proteins. Therefore, cows' milk forming the basis of human infant formula feeds must be extensively modified in order to resemble humans' more closely.

Caseins and the major whey proteins are made in the milk-producing tissue. Virtually all the other proteins, including antibodies that protect the infant against infection, are absorbed into the milk from the mother's blood. The milk the mother produces for a short period immediately after birth, the colostrum, contains a high concentration of these blood proteins. These provide an immediate boost to the infant's ability to fight infection.

Got milk? Get the hump...

Camel milk is rich in Vitamin C, unsaturated fat, B vitamins, iron and antibodies. These qualities have, in many parts of the world, helped it to achieve the super-food status that broccolli, blueberries and bananas have enjoyed on the shelves of supermarkets in the UK. For example, in India, Russia and Kazakhstan it is routinely prescribed to convalescing patients. In Africa, camel's milk is believed to provide valuable nutrition for AIDS patients. United Arab Emirates technologists are currently working on camel milk antibodies in an effort to engineer treatments to combat Alzheimer's disease, hepatitis C infection and certain cancers.

Researchers in India have found that a diet that includes camel's milk can reduce the amount of insulin required by patients to control their Type-1 diabetes. Insulin-like proteins in the milk can apparently supplement the therapeutic effect of the patients' medication.

Normal, organic or soya?

Although perhaps not a patch on camels', cow milk is readily available in this country and is evidently full of some pretty good stuff. And, even if it doesn't suit our bodies quite as well as a mother's home-grown variety suits her baby, in moderation, and in the absence of adequate nutrients from other sources, it can give us so much more than just a thick white moustache.

Before I started investigating milk, I was a fish-eating, second-hand leather sofa-owning, tea-with-milk-drinking semi-vegetarian. And I still am. But having stumbled across dozens of slightly extremist cow welfare and anti-milk websites during the course of my research , I have changed the milk I buy: I went from normal cow's to organic cow's to soybean's. I'm back on organic cow's now because I think cows make tastier milk than soybeans. I reckon it's hard work being a cow but, if you're an organic one life is, arguably*, considerably better.

My brief soya phase was met with numerous comments, the most discouraging of which was: “Soya milk? I read somewhere that it causes cancer...” Oh dear. Most probably. But doesn't just about everything kill us if we have too much? Or watch telly/ listen to the radio/ read a newspaper...

*The Soil Association, the UK's leading campaigning and certification organisation for organic food and farming, sets standards for organic dairy farmers:

“When housed, calves must have access to good quality straw, hay or silage and fresh clean water. It is recommended that calves should be kept outside in fields or in group housing with open fronted straw yards. The feeding of calves must be based on natural milk, preferably maternal milk for a minimum of three months. A calf may only be weaned when it is taking adequate solid food to cater for its full nutritional requirements. Calves cannot be weaned before three months of age. Tethering calves or the sale of calves under one month old to livestock markets is banned, as is live export of calves under Soil Association standards. Average yields in organic production are around a third less than in intensive production. Organic dairy cattle are therefore less likely to suffer from the health and welfare problems associated with very high yields. The Soil Association believes that the natural health and vitality of farm livestock is based on sound nutrition from before conception and throughout life. So organically grown feedstuffs form the basis of the diet. Organic cows are fed mainly on clover-rich grass and must be allowed to graze fresh forage throughout the grazing season. Organic dairy cows are fed a minimum of 60% forage and a maximum of 40% concentrates Non-organic dairy cows are fed a much higher percentage of concentrate, which leads to them producing more milk than they would naturally. Because of organic feed shortages and for a transitional period, the use of a limited amount of non-organic feedstuffs has been authorised. This is only permitted in those cases where the organic farmer is unable to obtain organic feed. The maximum amount of non-organic feed that can be given is 5% per year. We will be working with the other UK bodies to agree a decrease in these percentages in order to achieve 100% organic feed as soon as possible. If a farmer does need to use some non-organic feed, they can only use feed which has been approved by us for this purpose. They must also be able to justify the reasons why they had to use it, when they are inspected.”

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