KUALA LUMPUR (THE STAR/ASIA NEWS NETWORK) - Before continuing with the discussion about vegetarianism or other dietary choices, a couple of questions should be asked.

One is: Why do we eat? If that sounds a little silly and rhetorical, then try another question: Why do humans have the longest childhood of all known animals, requiring around 20 years before full maturity? No other creature needs such a long time to mature - even whales are considered mature between seven and 10 years of age.

The answers to both questions are related and complex - and they start with human breast milk.

A little story about human oligosaccharides

Around the end of the 19th century, researchers noted that human infants fed with their mother's milk had a higher survival rate compared with infants fed on cow's or goat's milk.

Then around the 1930s, a mysterious, indigestible compound called "gynolactose" was discovered in human mammary milk. Later, it turned out that gynolactose is actually a collection of more than 100 complex sugars called human milk oligosaccharides (HMO), and HMO is the third most common component in human milk after lactose and fat.

However, it was deeply puzzling as to why human mothers expend so much energy to produce oligosaccharides which are indigestible by their infants.

It was in 1954 that Austrian chemist Richard Johann Kuhn and American-Hungarian paediatrician Paul Gyorgy discovered that HMO is actually used to nourish gut microbes in infants, specifically an unusual strain with a digestive cluster made up of 30 genes called Bifidobacterium longum infantis, or B. infantis for short.

It appears that B. infantis converts HMO into short-chain fatty acids which actively feeds the gut cells of infants, promoting the production of adhesive proteins which help seal the gut from the bloodstream, thereby reducing microbial infections - and it also produces anti-inflammatory molecules for use by the infant's immune system.

There are other interesting and more fanciful theories about the benefits of HMO to human infants which are also not discounted - but the salient fact is that evolution has determined that human mothers produce HMO to protect and develop the guts of infants.

For modern humans, the brain is considered the most important organ for survival - humans actually have no particular attributes which are outstanding in the physical world, apart perhaps from the ability of our opposing thumbs to handle tools effectively.

HMO is a clear indication of the importance of the human gut - and the relationship between the intestines and the developing brains of young humans is a major factor why humans take so long to mature.

Young humans do grow at quite a fast rate, though not nearly as fast as most other animals, and a reason is that young humans have to balance the development of the brain with the physical development of the body.

Brains are extremely demanding in terms of energy for young humans, requiring around 40 per cent of the energy intake from food, compared with only about 20 per cent for adults - other primates use roughly only 8 per cent of their energy for their brains.

This is why young people tend to eat a lot, and not get fat, as the energy cost of growing and developing the synaptic connections in the brain is roughly double the energy of just using the brain.

At the same time, young humans have to develop other organs and skeletal structures as well - and therefore it is necessary to extract as much nutrients as possible from food via the intestines. These nutrients are not just calories, but also essential minerals and other compounds such as fats, vitamins and antioxidants.

As such, it is biologically very important for humans to maintain healthy intestines from birth - human guts are necessary to extract the huge amount of energy and nutrition needed over a long period of time - around 20 years - to fully develop the brain and the rest of the body.

The length of the intestinal system is also relatively longer in young humans, presumably to be more efficient at extracting nutrients.

And now, why we eat

The simpler question as to why we eat actually has two answers. The obvious one is that we need energy from food to breathe, think, move, maintain internal organs and disease defences, et cetera.

In short, we need food so that our bodies do not run short of fuel and die.

However, even after our bodies have matured after the initial 20 years or so, the fact is we are still growing - or rather, we are recycling and replenishing our bodies all the time, regardless of age.

For example, the human skin is constantly flaking off and renewing itself and a human adult in good health will attain a new layer of skin every 30 days or so. Adult tastebuds are renewed every two weeks, red blood cells every four months - even the liver is regenerated every year, and so on.

The only organs which do not regenerate are the central nervous system (including the brain), the lenses in the eyes and the ovaries of women.

All this rejuvenation again depends on the efficient extraction of energy and nutrients by our intestinal system, which was kick-started by HMO. At this point, it is important to note that as humans get older, the intestinal microbiota tends to change to include and accommodate many more types of bacterial and fungi.

The gut microbiota is an integral part of our digestive processes and simply would have adapted to our most common dietary habits.

After some years, the composition of human intestinal microbiota would normally have stabilised, though this can also often mean that it is likely to react to new, unexpected foods - having a stable gut flora attuned to only one environment is a plausible explanation for why some people get ill eating foreign food or certain new foods.

Unusual or alien foods (especially foreign spices, foreign additives, et cetera) may not be well-received by the gut bacteria or the gut itself and can cause reactions such as intestinal pains, constipation or diarrhoea.

And this sensitivity makes sense, especially if you consider that each human has around 30 trillion human cells symbiotically co-existing with a greater number (40 trillion) of bacteria of various strains - human health depends enormously on bacteria and ingested items which confound or damage our natural bacterial flora also damage our health.

Lactose, or usually a sign of trouble

Curiously, over 65 per cent of human adults are intolerant of lactose, even though lactose is the primary sugar in breast milk and dairy milk.

While human infants are capable of producing the enzyme lactase to digest lactose, the lactase-phlorizin hydrolase or LPH gene controlling the production of lactase peaks between the ages of two and 11 months and normally declines markedly after about the age of five.

There appears to be no evolutionary benefit in maintaining the LPH gene past a certain age as lactose is normally found only in human breast milk and lactase is not required once a baby has weaned.

It should be noted that dairy farming is a recent development in human evolution, dating from only around 10,000 years ago - it is also not prevalent around the world and therefore most humans have not adapted to digesting the lactose in cow's milk.

Apart from humans, no other species on Earth constantly consumes the milk of another species.

Saying that, the LPH gene in some humans has been found to continue into adulthood due to a genetic mutation identified mostly in communities historically involved with dairy farming.

But the fact remains that most people are incapable of drinking a glass of milk without suffering some discomfort. This discomfort arises due to lactose sitting around undigested in the gut, pulling in water and nutrients like a viscous, sticky sludge, causing diarrhoea, bloating, flatulence and possibly also vomiting.

It is also highly plausible that lactose intolerance can lead to other side effects outside of the intestinal tract, such as skin problems or respiratory issues, for example.

Studies have found that well over 90 per cent of Chinese and Japanese people are lactose intolerant, along with over 85 per cent of Asian Indians and over 75 per cent of Africans.

In these populations, one would suggest that a consumer warning would be appropriate for foods containing lactose, though that is seldom provided, at least not in the European Union (EU), where over 12 per cent of the population is also lactose intolerant.

In fact, most people are encouraged to ingest milk and dairy products on the basis that such products provide extra minerals and nutrients such as calcium and Vitamin D, which should help prevent bone diseases such as osteoporosis.

However, this advice may be flawed as people who drink a lot of milk also exhibit the highest incidence of bone fractures, especially in old age.

A Swedish study in 2014 covering 61,433 women and 45,339 men implied that higher mortality and higher bone fracture rates, particularly hip fractures in women, were linked to higher consumption of milk, though other factors were also likely involved.

However, regular outdoor exercise was linked to a reduced likelihood of osteoporosis.

Interestingly, many dairy products such as yogurts and cheeses are fermented with bacteria from the Lactobacillus and Bifidobacterium species, which can survive as part of the intestinal microbiota.

Although it has been claimed that these lactic acid bacteria can produce lactase in the gut, no convincing evidence has been found to support this - it is much more likely these strains of bacteria break down milk lactose into other compounds which are more digestible by humans.

Various strains of Lactobacillus and Bifidobacterium are commonly used in the production of dairy goods and this may be a reason why many people can tolerate dairy-based products rather well, even if they cannot tolerate milk itself.

So if you enjoy dairy products, then a suggestion might be to eat more aged cheeses and probiotic yogurts rather than milkshakes and cream pies.

This widespread intolerance of lactose by humans might easily be misinterpreted as an intolerance of meat, as meat is often presented with creamy sauces of dairy origin.

Also, many Western desserts after meals are often based on milk products and usually difficult to digest for many people - I personally now avoid dense creamy confections after meals as they make me sleepy and slightly nauseous.

Eating (some) meat may be bad

To be brutally honest, eating meat in some places is possibly pretty bad for health - this is due to the extensive use of growth-enhancing agents in some countries during the raising of livestock, or the subsequent treatments during meat processing, or the general poor quality of the meat itself.

Ingesting meat loaded with additives such as antibiotics, hormones, disinfectants and preservatives (such as nitrates and nitrites) can significantly damage human intestinal microbiota as well as other parts of the body.

As an example, the impact of animal growth hormones is considered so severe that the EU has banned, for years, hormone-treated beef from the United States.

Also, many people think that cooking meat kills all toxins and renders cooked meat safe, even bad meat. This is strictly not true - proper cooking can kill all food bacteria (and parasites), but some toxins produced by bacteria before cooking are often not affected by normal cooking processes.

Examples would be the botulism toxin produced by Clostridium botulinum, which requires 10 minutes of boiling before deactivation. The enterotoxins produced by Staphylococcus aureus can survive even longer periods of boiling and the toxins from Bacillus cereus seem impervious to any levels or duration of cooking heat.

Not only meat - plant agriculture is also problematic

But to be fair, it is not just meat that is potentially injurious to health - many plant products are treated and contaminated with pesticides and chemical preservatives as well.

There are hundreds of depressing statistics about the types and degrees of pesticide contamination of cereals, nuts and fruits - and that is just the breakfast items.

Furthermore, I am still personally uncomfortable as to why safety research on genetically modified organism (GMO) food seems to be always restricted to 90 days.

The extensive use of pesticides for both intensive and GMO farming is also causing huge ecological damage to world insect populations, particularly bees and other beneficial insects.

In Germany, a 2017 study of nature reserves found an alarming 75 per cent drop in the number of insects compared with just 27 years ago.

Without insects, it is possible that the global agricultural food chain can be severely disrupted, especially for the many fruits and crops that depend on insect pollination.

Part 4 looks into some awkward questions about our diets, especially for people who love eating meat, such as me.