Nutrition (2 page)
The ideal is to strive for balance, so that you live to eat, as well as eat to live.
This book necessarily concentrates on the ‘eat to live’ side of the equation, exploring why you need macronutrients (carbohydrate, protein, fat, fibre) and micronutrients (vitamins, minerals, trace elements, phytochemicals) in your diet. It explains how food is digested and metabolized to obtain energy, and how it provides the building blocks for your body’s growth and repair. It looks at how much energy you need to fuel your basal metabolic rate – the amount of energy you burn at rest to maintain normal body functions – and physical activity levels, and even touches on weight maintenance and weight-loss diets. It offers a variety of detailed nutritional guidelines and shows you how optimal health can be achieved through the right nutritional balance.
The link between diet and health was suspected thousands of years ago. Hippocrates, a Greek physician born around 460
BC
, was famous for quotes such as: ‘Let food be your medicine, and medicine be your food.’ It was only in 1747, however, that James Lind conducted the first controlled experiment to show that a disease – scurvy – resulted from a dietary deficiency. Although Lind’s findings on scurvy were accepted at the time, it took forty years before the Admiralty ordered ships to receive a supply of lemon or lime juice, which helped eradicate scurvy from the Royal Navy. This is perhaps the first illustration of how slowly nutritional guidelines tend to change to keep up with published science.
BC
, was famous for quotes such as: ‘Let food be your medicine, and medicine be your food.’ It was only in 1747, however, that James Lind conducted the first controlled experiment to show that a disease – scurvy – resulted from a dietary deficiency. Although Lind’s findings on scurvy were accepted at the time, it took forty years before the Admiralty ordered ships to receive a supply of lemon or lime juice, which helped eradicate scurvy from the Royal Navy. This is perhaps the first illustration of how slowly nutritional guidelines tend to change to keep up with published science.
FINDING A CURE FOR SCURVY
On voyages of exploration during the sixteenth and seventeenth centuries, as many as 90 per cent of sailors died from scurvy. In 1747, naval surgeon James Lind discovered that oranges and lemons could cure scurvy in a controlled trial aboard his ship, HMS
Salisbury.
Salisbury.
In his publication
A Treatise of the Scurvy
(1753), he reported: ‘I took 12 patients in the scurvy … putrid gums, the spots and lassitude, with weakness of their knees. They … had one diet common to all … Two were ordered each a quart of cider a day. Two others took … elixir vitriol three times a day, using a gargle strongly acidulated with it for their mouths. Two others took two spoonfuls of vinegar three times a day, upon an empty stomach, having their gruels and other food well acidulated with it, as also the gargle for their mouth. Two of the worst patients … were put under a course of sea-water. Of this, they drank half a pint every day … Two others had each two oranges and one lemon given them every day. These they ate with greediness, upon an empty stomach. They continued but six days under this course, having consumed the quantity that could be spared. The two remaining patients took … an electuary recommended by an hospital-surgeon, made of garlic, mustard-seed, rad. Raphan, balsam of Peru and gum myrrh; using for common drink, barley-water well acidulated with tamarinds … The consequence was, that the most sudden and visible good effects were perceived from the use of the oranges and lemons; one of those who had taken them, being at the end of six days fit for duty. The other was the best recovered of any in his condition; and being now deemed pretty well, was appointed nurse to the rest of the sick.’
A Treatise of the Scurvy
(1753), he reported: ‘I took 12 patients in the scurvy … putrid gums, the spots and lassitude, with weakness of their knees. They … had one diet common to all … Two were ordered each a quart of cider a day. Two others took … elixir vitriol three times a day, using a gargle strongly acidulated with it for their mouths. Two others took two spoonfuls of vinegar three times a day, upon an empty stomach, having their gruels and other food well acidulated with it, as also the gargle for their mouth. Two of the worst patients … were put under a course of sea-water. Of this, they drank half a pint every day … Two others had each two oranges and one lemon given them every day. These they ate with greediness, upon an empty stomach. They continued but six days under this course, having consumed the quantity that could be spared. The two remaining patients took … an electuary recommended by an hospital-surgeon, made of garlic, mustard-seed, rad. Raphan, balsam of Peru and gum myrrh; using for common drink, barley-water well acidulated with tamarinds … The consequence was, that the most sudden and visible good effects were perceived from the use of the oranges and lemons; one of those who had taken them, being at the end of six days fit for duty. The other was the best recovered of any in his condition; and being now deemed pretty well, was appointed nurse to the rest of the sick.’
More recently, the link between diet and health – or lack of it – has become better understood, partly as a result of population-based observational studies. However, observational studies can only show an association – they do not provide definite proof. Correlations have been observed between the annual stork population in the Netherlands and the human birth rate, for example, but this does not prove that babies are delivered by storks. Interesting observations must therefore be followed up with trials specifically designed to test a particular hypothesis. These trials should, ideally, be:
• randomized (participants are randomly allocated to one or other groups)
• placebo-controlled (one group receives the active test treatment, while another group receives an inactive dummy treatment).
• double-blind (neither investigators nor participants know who is getting the true treatment, and who receives the dummy treatment)
Such double-blind, randomized, placebo-controlled trials are considered the gold standard for all academic research, including nutritional studies. The outcomes of those taking the active treatment are then compared with those taking a placebo and compared statistically. If the probability of the observed result occurring by chance alone is less than 5 per cent, the finding is considered statistically significant. This means that, in any one trial, there is still a chance, however small, that the ‘statistically significant’ result was due to chance alone. As a result, the concept of a meta-analysis has been developed, in which the findings from many similar studies are pooled together and the combined results compared statistically. This helps to balance out those findings, both positive and negative, that are due to chance and to give a more accurate assessment.
Another approach is to use a systematic review, in which a literature search identifies all high-quality research trials relating to a particular question to provide an exhaustive overview of the evidence base. These systematic reviews may also use statistical techniques such as meta-analysis to combine the results of the eligible studies. Where possible, the results of these larger analyses are included in this book to show the strength of evidence to support particular nutritional pearls of wisdom.
Changing your diet can have a massive impact on your health. To ensure that these changes and impacts are beneficial rather than detrimental, an abundance of scientific evidence is needed before nutrition scientists and governments are confident that new recommendations or guidelines are needed. This slow pace can seem frustrating at times. For example, in the 1990s, the recommendation for pregnant women to take folic acid supplements helped to cut the number of babies born with neural tube defects, such as spina bifida, by around 70 per cent. But the debate about whether or not to fortify flour with folic acid (to protect the babies of women who do not take supplements) is still continuing over twenty years later, as this would also increase intakes of folic acid for another sector of the population – the elderly – for whom folic acid supplementation could mask the red blood cell changes needed to help detect vitamin B12 deficiency. If left undiagnosed, a severe lack of B12 could lead to pernicious anaemia and a form of irreversible nerve damage known as subacute combined degeneration of the spinal cord.
This neatly illustrates why an exact understanding of nutrition is so important in order to do good while avoiding harm.
The food you eat provides energy, building blocks and vital nutrients – preferably locked up in delicious morsels. Before you can access these benefits, the food must be broken down into its simplest components and absorbed across your intestinal wall into your circulation.
Good nutrition depends on the proper functioning of your gastrointestinal tract, or gut, which forms a coiled tube around 9 metres in length. Complex food molecules are inserted at one end and worn down into simpler, soluble components through a combination of mechanical and chemical disruption – a process known as digestion. Valuable nutrients are then absorbed, while unwanted waste products are disposed of, usually in neat packages, at the other end.
The mouth
Digestion starts in your mouth. Food is mechanically disrupted by the biting, chewing and tearing action of your teeth. At the same time, saliva moistens food with powerful enzymes (molecules that trigger chemical reactions and speed them up); salivary amylase starts to dissolve the chemical bonds in starch, while salivary lipase starts digesting fats. A few other enzymes are also present, such as lysozyme and peroxidase to help kill bacteria – although as anyone who’s ever suffered from food poisoning will know, these aren’t always effective. Your tongue then rolls the moistened food into a ball and pushes it to the back of the mouth. This triggers the swallowing reflex (deglutition) and the bolus of food is pushed down into your stomach by a wave of muscular contraction.
FROM BLAND TO SWEET
You produce between 750 ml and 1,500 ml saliva per day. If you hold a starchy food such as bread in your mouth for any length of time, it will start to taste sweet as salivary amylase attacks starch molecules to release the sugar, maltose.
The stomach
Your stomach is a hollow, J-shaped muscular sac in the upper left-hand side of your abdomen, just beneath the diaphragm. Its inner surface is deeply folded, so it can ‘shrink’ when empty and expand after a large meal to hold as much as 2 litres of fluid at any one time. In fact, your stomach is the most elastic part of your body as it can stretch up to 50 times its empty size. The muscular walls of your stomach act like a concrete mixer, churning food and mixing it with gastric juices, of which you secrete around 3 litres per day.
THE ‘JUICIER’ THE BETTER
Stomach juices contain hydrochloric acid and two main enzymes: pepsin, which digests proteins into smaller protein chains called peptides, and gastric lipase, which breaks down dietary fats. Around 20 per cent of the lipase in your stomach comes from your saliva, which builds up in your stomach as you swallow between meals. To prevent the stomach from digesting itself, cells in the wall secrete mucin and bicarbonate, which line the stomach with a thick, alkaline, barrier gel. Stomach-lining cells are also shed and replaced every three to six days.
Depending on a meal’s composition, it takes from two to six hours for your stomach to convert food into a semi-digested, creamy slurry known as chyme. Wave-like contractions then start to push the stomach contents down towards the stomach’s exit, which is guarded by a powerful ring of muscle, the pyloric sphincter. This relaxes momentarily to allow small amounts of chyme to squirt into the first part of your small intestines, the duodenum. As more and more chyme passes through the pylorus and out of your stomach, your stomach gradually shrinks in size.
Small intestines
Your small intestines consist of three consecutive parts: the duodenum, jejunum and ileum. These form a highly coiled tube which, if fully stretched out, would measure 6 metres or more in length. High muscular tone in their walls constricts them down to half this length, however, so that they fit neatly into your abdominal cavity.
Your duodenum is a curved, C-shaped tube that encircles the head of the pancreas. It is around 25 cm long and secretes alkaline juices that neutralize the acidic chyme received from the stomach. This change in acid level is important both to avoid irritating the lining of your small intestines, and to help the next set of digestive enzymes work more efficiently. The duodenum also acts as the meeting place where chyme is introduced to the powerful actions of pancreatic juice and liver bile.
PANCREATIC JUICES
Secretions from the pancreas gland contain powerful enzymes that break down the chemical bonds in protein (trypsin, chymotrypsin, elastase), carbohydrates (amylase), fats (lipase, phospholipase) and nucleic acids (nucleases). Unlike herbivores, humans lack the enzymes needed to break down cellulose in plant walls and, instead, rely on the bacteria in the large bowel to help do this for us. Bile made in the liver is stored in the gall bladder. When food leaves the stomach, hormone signals cause the gall bladder to contract. Bile is then squirted down into the duodenum where it acts like a detergent, disrupting dietary fats to form small globules, which hastens their digestion by providing a greater surface area on which your digestive enzymes can get to work.
By the time food leaves your duodenum, complex molecules are almost fully digested and ready for absorption in the next part of the small intestines, your jejunum: long protein chains have been cleaved to release simpler units called peptides and amino acids; carbohydrates have disintegrated into sugars known as saccharides; triglyceride fats have split to release their glycerol backbone and fatty-acid tails.
The next part of your small intestines is where absorption occurs (although some substances such as alcohol and many drugs can be absorbed across the lining of the stomach or even the mouth). The small intestine below the duodenum consists of the jejunum, followed by the ileum. There is no distinct border between these two areas, and this division is somewhat arbitrary, with the jejunum consisting of around 40 per cent of the remaining small intestine, and the ileum 60 per cent. In general, the ileum is paler in colour than the jejunum, as its walls contain immune areas (Peyer’s patches) that help to protect against food-borne infections.
ENZYMES AND SUBSTRATES
An enzyme is a special protein that speeds up a chemical reaction, which, without the enzyme, either would not occur at all, or would occur too slowly to be of any use. The substance on which the enzyme acts is known as its ‘substrate’.
The jejunum and ileum secrete a fluid known as
succus entericus.
These juices contain enzymes that complete the digestive process in which your food is broken down into simpler chemicals ready for absorption into your body. Put succinctly:
succus entericus.
These juices contain enzymes that complete the digestive process in which your food is broken down into simpler chemicals ready for absorption into your body. Put succinctly:
• sucrase cleaves sucrose (table sugar, a disaccharide) to release glucose and fructose
• lactase breaks down lactose (milk sugar) to release glucose and galactose
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