INTRODUCTION – WHAT IS DUNG?

What's brown and sounds like a bell? Dung!

Monty Python's Flying Circus, December 1969

In its most familiar sense, either from our own personal first-hand experience, or from our close historical proximity to stock animals, dung is brown. This is the default colour of the scatological cartoon, or the plastic doggy poop bought in a joke shop. But anyone who has trodden the minefield of dog mess along an urban street knows that the droppings left behind by lazy and careless owners can be anything from pale yellow to red to black. And in widening the boundaries to cover all animal excrement, even this colour palette soon broadens out. Hyena dung is white, bird splashes are piebald, white and grey; reptile waste is anything from pale grey to inky blue-black (my pet garter snake Bella shunts out vaguely greenish goo twice a month); aphid honeydew is clear or, as its name suggests, slightly honey-coloured; caterpillar frass can be black, green or even turquoise verging on blue. Like many things in nature, colour is not a useful guide.

Instead, a better start can be made by trying to understand dung in terms of a simple schoolbook equation of its basic biochemistry:

Food - nutrition + waste = dung

Anyone who can remember back to biology lessons may recall vague snippets about salivary amylase, gastric acid or the pyloric sphincter. Whatever the complex chemistry going on in the intestinal tract, the process that takes food and makes excrement begins with digestion, and to fully appreciate exactly what dung is, it is as well to start with a brief look at this process.

Human digestion is fairly well studied, and since humans are omnivores, eating a huge range of different foodstuffs around the world, our understanding of how it all works is useful when looking at other animals, in particular those mammals with whose similarly brown faeces we are intimately familiar.

PAYING LIP SERVICE TO FOOD

It all starts with chewing, the obvious mechanical breaking down of big bits into smaller bits using the teeth. This makes swallowing a lot easier, but it also helps get the digestive juices working quickly on the ground-up mush, rather than having to cope with rough, tough chunks. Not all animals chew quite as politely as would-be humans at the dinner table, grinding the requisite 40 chomps before swallowing. Birds have no teeth, and apart from a cursory crunching, usually to stop it wriggling, they gollop down their food whole. Instead, they have a gizzard to do the chewing. This muscular portion of the upper stomach often contains grit or small stones and the rhythmic squeezing movement of the gizzard walls helps crush and pulverise the food into a more manageable substrate for digestion. Some reptiles and fish also have gizzards. Whatever the mechanisms for chewing, the result is a more-readily digestible raw material for the stomach – it's all about increasing the surface area of the food particles so that the chemical processes of digestion can get to work more easily.

In humans, chewing is not just about cutting and crushing; digestion of some foods actually begins in the mouth, with digestive enzymes in the saliva. Starch (the primary carbohydrate in foods such as bread, potatoes, pasta and the like) is attacked by salivary amylase to produce various sugars. A simple home experiment involves over-chewing a piece of bread (without swallowing), which becomes noticeably sweeter after only a couple of minutes. My old biology teacher Mr McCausland introduced me to that one during O-levels (the old name for GCSEs, for any younger readers), ostensibly to show us a practical demonstration of hydrolysing starch catalysis, but I suspect also to fill our incessantly noisy mouths with slightly stale loaf to shut us up for a bit. The mouth is a neutral or slightly alkaline environment, but the swallow takes boluses of chewed food down the oesophagus (gullet), into the highly acid stomach.

Although the hydrochloric acid in the stomach is strong enough to dissolve iron, its purpose is not just to attack the food, but rather to create the right chemical environment in which highly complex food-digesting enzymes can get to work. Serious protein digestion gets going now as these immensely complicated molecules are snipped into smaller units. The acid also kills most bacteria should any have been on the food when it was eaten. Eventually a sloppy 'chyme' is produced – this is the smelly pale-yellowish acrid liquid full of chopped carrots which is revealed if you are unfortunate enough to vomit a couple of hours after eating. Chyme is slowly released in gentle squirts through the pyloric sphincter, the muscle-ring one-way valve at the far end of the stomach, down into the small intestine.

The human small intestine is another world altogether. A convoluted 6 metre stretch of narrow, finger-thick tubing, it is the major site of digestion and food absorption. Several profound chemical changes start within the first few centimetres. Sodium bicarbonate is released by the pancreas, a large gland sitting under the stomach; this neutralises the stomach acid and creates a slightly alkaline background. The pancreas also releases alkali-controlled enzymes – important components are proteases and peptidases to continue the digestion of proteins, and more amylase to break down starches. A thick yellowish-brown liquor called bile is also dribbled into the intestine from the gall bladder. It helps break down clods of insoluble fat into an emulsion of microscopic globules. The bile also contains a yellow waste substance called bilirubin, which is made from haemoglobin (the red oxygen-carrying molecule in the blood) as damaged red blood cells are broken down and destroyed in the liver. As it traverses the digestive tract, bilirubin changes to another strongly coloured chemical called stercobilin, it is this dark brown pigment that gives mammalian dung its characteristic brown colour.

YOU ARE WHAT YOU EAT

Most of the bewilderingly sophisticated biochemical substances that make up living organisms, and therefore the food that we eat, are based upon long chains of repeating chemical units, like the beads on a series of pearl necklaces. The chains fold and twist, and are cross-linked like knitting to form everything from the proteins which build the bulk of muscles, and the meat that we eat, to starch (the energy source in foods such as spaghetti and doughnuts) and polyunsaturated fats (our consumption of which we try to reduce by choosing low-fat margarines). The enzymes from the pancreas, and others secreted by the small intestine itself, work on digesting our food like chemical scissors, trimming, then snipping off individual beads from these intricate long-chain necklaces. Truly vast substances containing many thousands (sometimes millions) of atoms are reduced to the basic tiny molecules of the individual units from which they are made up.

Proteins are reduced to their constituent amino acids, starches are reduced to simple sugars, and fats are reduced to short-chain oils. Each of these fundamental building block types is just a few atoms (maybe 10–100) in size – and small enough to pass through the semi-permeable membranes of the gut wall, to be whisked off around the body in the blood and lymphatic transport systems. To facilitate this removal of useful chemical nutrients from the chyme, the interior surface of the small intestine is wrinkled and minutely convoluted, covered all over with tiny finger-like extensions (villi) that give it the appearance under the microscope of a thick shag-pile carpet. This has the effect of hugely increasing the surface area across which nutrient absorption into the blood takes place. Choose your own incredible statistic here, the usual one is that the surface area of the human small intestine, if it were to be flattened out, is the size of a tennis court (260 square metres). If memory serves, that same Mr McCausland (in A-level biology now) had us calculating this based on a section of sheep intestine he'd got from his neighbourhood abattoir. After poring over the microscope and calibrating the graticule eyepiece measurement scale, we probably spent the entire lesson counting sheep villi and extrapolating up from microscopic cylindrical tendrils to so-many hundred square metres of laboratory floor carpet.

The final leg of the digestive conveyor belt is the large intestine, the colon, about a metre long and wrist-thick. Some last-minute nutrient absorption occurs here, but its most important function is to remove water from the digestion remains.

By now the semi-liquid chyme has become a stiff semi-solid. After, perhaps, several days of slow onward movement through the digestive tract, much of what the human body can use in the way of nutrients has been removed from the food. What remains is the key constituent of plant and vegetable foodstuff that we cannot digest – fibre (sometimes called roughage in older textbooks). Fibre is made up of undigestible chemical chains such as cellulose and lignin. These are the substances that give plants their incredible toughness and strength, and which in non-food species provide us with fibres for other uses – cotton jeans, linen sheets, wood-pulp paper.

Fibre was a major preoccupation for my parents' generation, and whereas I was fascinated by the numerous helpful vitamins thoughtfully provided and carefully listed on the packets by the manufacturers of sugary breakfast cereals, my Mum's shopping choices were more often influenced by the roughage content of the bran-based stodgier end of the edibility spectrum. It was during the 1950s and 1960s, when processed foods, cheaper meat and sliced white-pap bread started to appear on the supermarket shelves, that the connection between healthy food intake and healthy stool output was first used (albeit very tastefully and subtly) as a marketing tool. Being 'regular' was considered a feminine virtue and a sign of manly rectitude. There was, at this time, a growing realisation that the human taste for sweet, easily digestible titbits was replacing a more rounded diet of mixed fibrous fruit and vegetables, and that this was playing havoc with a digestive tract inherited from our long-distant ancestors, one more suited to the foraged nuts, fruit and roots on which proto-humans first fed.

Lack of fibre in the diet didn't just mean reduced faeces; this was not just a simple equation of less in one end, less out the other. It meant less regular throughput, intestinal stagnation, backing up of waste, and rectal compaction to the point of discomfort and risk to health. Constipation is a singularly human obsession, and we'll be straining to understand its implications regularly throughout this book.

By lucky happenstance, I was able to do some personal research into constipation early on in the writing of this book, when I was hospitalised with extreme abdominal pain and excruciating muscle cramps. Fearing it might be gall or kidney stones, hernia or diverticulitis, I trekked up to my local accident and emergency unit late one Saturday evening. I was poked and prodded, drained of numerous blood samples and eventually X-rayed, but the tests were negative and the diagrams showed it was, to give it the medical term, simply faecal loading. I was a bit bunged up in there. Several days of laxative oils and glycerine suppositories got things moving again, much to everyone's relief.

There is another key ingredient in the final quasi-excrement as it passes through the large intestine, and by the time it is ready to be ejected from the body, it makes up over half the dry weight of the faeces – bacteria. This is where our knowledge of human digestion starts to wear a bit thin. There may be 100 trillion bacteria in an average human intestine, that's 100 million million, or 1 with 14 zeros after it – a mind-boggling number, and more than ten times the number of body cells in the average human's entire body. There are thought to be 300–1,000 different bacterial species in there; it's difficult to quantify exactly, because they are difficult to identify and difficult to grow and study in laboratory cultures. What are they all doing?

Most people's idea of 'bacteria' may be of horrible germs that cause sickness, disease or death, but the gut flora, to give it its usual, slightly more passive, euphemistic name, is a perfectly normal, healthy, indeed necessary part of being a human. Unlike the bacteria that cause, say, tuberculosis, cholera or salmonella poisoning, these natural gut-dwelling microbes are not attacking or parasitising the human body, nor are they accidental inhabitants (sometimes called commensal, meaning non-harmful coexistence), they are better described as being mutualistic – host and occupier each benefiting from the presence of the other.

The bacteria benefit because they are supplied daily with a fresh input of partly digested food passing through the guts, on which they and their descendants can feed; and in return they further digest the remaining substances, their own very different enzymes snipping away at the slowly fermenting chemical dross that would otherwise be unavailable to our own bodies' somewhat limited digestive machinery. These last-minute digestive products are absorbed, along with some of the water, before the final bodily waste product is ready to be voided.

This usual, normal, natural process goes on in the human body every day, and for the most part we are completely oblivious of it, but when things change, we can get a fascinating insight into the digestive processes, and we can understand parallels in other animals.

RIGHTS AND WRONGS OF PASSAGE

Human stool is roughly 75% water. Anyone with a regular balanced diet should be familiar enough with their own bowel movements to know when things are 'fine down there', but when things go wrong, either way, we notice immediately. Water content in human faeces can actually range from about 50 to over 90%; this translates to an ease-of-passage range, from reluctant constipation to explosive diarrhoea. Helpfully, there is a simple medical scale, with pictures – the Bristol stool chart – which classifies this range into seven discrete categories by outward visual appearance. No messy weight and density tests are necessary.

At the one extreme, diarrhoea can be life-threatening; the International Centre for Diarrhoeal Disease Research in Dhaka, Bangladesh, was set up because, after malaria, diarrhoea is the world's single biggest killer of children under 5. The usual cause is viral or bacterial infection of the gut by inappropriate micro-organisms, and the body's response is to flush out the system as quickly and effectively as possible to get rid of the offending invaders. Shutting down or reversing the intestines' water-absorption pathways keeps the gut contents highly liquid, and the rhythmic waves of muscular contraction (peristalsis) that usually gently squeeze the digesting food through the digestive tract now force it out under pressure. Fast ejection (like vomiting) gets rid of the offending microbes and helps prevent dangerous, possibly life-threatening bacterial toxins building up; it may even stop these micro-organisms invading the interior of the body.

We all have our own diarrhoeal anecdotes, and in any other circumstances I'd keep mine diplomatically quiet. But since this is actually a book about my own exploration of excrement I cannot pass without at least commenting obliquely on the accident in the Temple of the Buddha's Tooth in Kandy, Sri Lanka, in 1992, where it was forcefully brought home to me that not all of the world's drinking water is safely potable. I was saved major embarrassment by the hasty hailing of a friendly tuk-tuk driver who quickly returned me to the safe and private confines of the family-run guest house in which we were lodging, where for several days I closeted myself away in my room and recuperated on thin vegetable gruel and bottled water.

Diarrhoea can also be caused by allergic reactions, physical or chemical damage to the gut linings, poisoning, alcohol abuse or age-related blood-vessel damage. As well as any underlying cause, the primary health risk, especially in the very young or the very old, is dehydration because of the body's continued and copious water loss. The usual medical response is treatment with rehydrating fluids (thin soup is ideal) carefully balanced to replace sugars and salts that also pass from the body during diarrhoeal attack.