Nutrition (4 page)
Dietary proteins originating from animals (meat, fish, eggs and dairy products) contain significant quantities of all the essential amino acids. Vegetable sources of protein (rice, beans, nuts, seeds) each contain some, but not all, of the essential amino acids.
Within a vegetarian diet, eating a variety of plant products is important to ensure a balanced intake of essential amino acids.
Within a vegetarian diet, eating a variety of plant products is important to ensure a balanced intake of essential amino acids.
When your diet is rich in protein, excess amino acids cannot be stored in their original form. Instead, excess protein that is not needed for immediate growth or repair of body tissues is used directly as a fuel for energy, converted into glucose or, if energy is plentiful, converted into energy stores for later use.
ENERGY STORES
Starchy glycogen, which is stored in your liver and muscle cells, is an excellent energy source as it contains lots of chains of glucose that are readily accessible when needed. In contrast, fatty acids within your adipose (fat) cells are stored as triglycerides – three chains of fatty acids each bound to a molecule of glycerol – that take longer to break down and burn as a fuel.
As each of the twenty-one amino acids vary in structure, the body uses twenty-one different metabolic pathways to process them and release substances that your body can use to produce energy. When following a balanced diet, these pathways normally account for between 10 and 15 per cent of the energy your body produces. When you follow a high-protein diet, however, the amount of energy derived from dietary proteins increases, and your liver produces more of the enzymes needed to process them as an efficient energy source.
Dietary carbohydrates
Carbohydrates are usually the main energy source within your diet. As your body can make all the various carbohydrates it needs from other sources, no dietary carbohydrates are deemed ‘essential’ in the same way as some amino acids and the essential fatty acids are. But as glucose is the only type of fuel that brain cells can use, it is vital that your body continues to maintain a steady blood glucose level – if dietary carbohydrates are in short supply, your liver must make glucose from other substances, especially protein.
The simplest forms of dietary carbohydrate are single sugars, or monosaccharides, such as glucose (grape sugar, also known as dextrose), galactose (a milk sugar) and fructose (a fruit sugar). Two single sugars can join together to form a double sugar (disaccharide) such as ordinary table sugar (sucrose), lactose (a milk sugar) and maltose (a sugar found in cereals). Sugar molecules can also form chains. Those formed from three to ten sugars are known as oligosaccharides. Those containing a greater number of conjoined sugars, such as starch, are called polysaccharides.
The types of carbohydrate in your diet are shown in
Table 1
.
Table 1
.
Types of carbohydrate | Examples |
Monosaccharides | glucose (grape sugar) |
galactose (a milk sugar) | |
fructose (fruit sugar) | |
deoxyribose (a sugar used to make your DNA) | |
Disaccharides | sucrose (glucose + fructose) |
lactose (glucose + galactose) | |
maltose (glucose + glucose) | |
Oligosaccharides | fructo-oligosaccharides (short chains of fructose) |
galacto-oligosaccharides (short chains of galactose) | |
Polysaccharides | starch, glycogen, inulin, cellulose |
Simple sugars (monosaccharides) pass from your intestines into your bloodstream unchanged. Because disaccharides are made up of two sugar molecules joined together, they must first be separated into their individual monosaccharides by digestive enzymes (salivary and pancreatic amylase) before they are absorbed. The digestion of starch takes even longer, but releases a steady stream of simpler sugars into the circulation, which helps to maintain an even blood-glucose level. Many plant oligosaccharides and polysaccharides, however, cannot be broken down as our bodies lack the enzymes needed to digest them. Instead these form the bulk of dietary fibre – a crucial dietary component which is explored later.
Monosaccharides pass through the gut wall into your circulation and travel directly to your liver via the hepatic portal vein. Most are taken up by your liver cells, as one of their most important jobs is to maintain blood sugar levels by releasing a constant supply of glucose. When dietary glucose is plentiful, excess is stored as glycogen, a starchy emergency fuel, or converted into fat for long-term energy storage. When dietary glucose is in short supply, the liver can also make new glucose from fructose, glycerol, lactic acid and certain amino acids – but not from fatty acids. In fact, the conversion of milk and meat proteins to glucose is so efficient that your liver can produce 50 g of glucose from 100 g of protein.
Glucose is an important fuel for all your body cells, but different cells absorb it with different degrees of efficiency. Liver and brain cells contain special proteins that act like pores to allow glucose free entry into these cells. However, muscle and adipose (fat) cells contain a different type of glucose receptor known as Glut-4 glucose transporters, which only allow glucose to enter the cell if the hormone insulin is present. This is because Glut-4 transporters are stored
inside
the cells and only come to the surface to provide a glucose entry channel when insulin is present.
When insulin levels fall, the Glut-4 transporter proteins move back into the centre of the cell, closing the channel so that glucose can no longer enter. This mechanism is thought to ensure that some glucose remains in your circulation at all times, for use by your brain cells, for which glucose is such a vital fuel that they can absorb it without the need for insulin.
inside
the cells and only come to the surface to provide a glucose entry channel when insulin is present.
When insulin levels fall, the Glut-4 transporter proteins move back into the centre of the cell, closing the channel so that glucose can no longer enter. This mechanism is thought to ensure that some glucose remains in your circulation at all times, for use by your brain cells, for which glucose is such a vital fuel that they can absorb it without the need for insulin.
Insulin is made in your pancreas, a gland that lies just beneath the stomach. The pancreas contains millions of clusters of specialized cells (the Islets of Langerhans) that secrete a variety of digestive enzymes and hormones. The scientists who first discovered these named the types of cell according to the first letters of the Greek alphabet, and it is now known that it is the beta cells that secrete insulin in response to a rise in your blood glucose concentrations. This increase in insulin production occurs within minutes of glucose levels rising after a meal, and provides the key to start letting glucose into muscle and fat cells. Once inside muscle cells, glucose is burned to produce energy, and excess is converted into glycogen. Within fat (adipose) cells, excess glucose is converted into triglyceride fats for storage, as we will discuss shortly.
As glucose moves out of the circulation, and blood glucose levels fall, the beta cells stop producing insulin. Glut-4 transporter proteins move back into the centre of muscle and fat cells so glucose can no longer enter. If blood glucose levels fall too low, another type of pancreatic cell – this time the alpha cells – secrete another hormone, glucagon, which has the opposite effect to insulin. Glucagon causes the liver to break down its glycogen stores to release glucose back into the circulation. In this way, circulating blood glucose levels are normally kept within a tight range. Different units of measurement are used in different parts of the world, so a normal glucose range is either 3.9 to 5.6 mmol/l (millimoles per litre) in the UK, or 70 to 100 mg/dl (milligrams per decilitre) in the US.
Both the type and amount of carbohydrate in your diet has a major impact on your blood glucose levels and your secretion of insulin and glucagon. Glucose causes a rapid rise in blood glucose levels, while other simple sugars (monosaccharides), such as fructose and galactose, have a lesser effect as it takes time for the liver to convert them into glucose. Starch releases a steady stream of glucose to produce a lower, but more sustained rise in blood glucose levels.
Glycaemic Index
The way in which different carbohydrates affect your blood glucose levels can be measured and quantified. This concept, known as the Glycaemic Index (GI), rates how eating 50 g of digestible carbohydrate from different foods affects your blood glucose levels, compared with eating the same amount of glucose. The effect of glucose is given an arbitrary GI value of 100, so a food that raises blood glucose levels by half as much is assigned a GI value of 50.
Foods with a high GI (70 or above) have a rapid effect on your blood glucose levels (see solid line on the graph above). Foods with a medium GI (56 to 69) have a more sustained effect on your blood glucose levels (see dashed line on the graph above), while foods with a low GI (less than 55, see dotted line on the graph above) contain few carbohydrates, or carbohydrates that are metabolized slowly and have only a minor effect on your blood glucose levels.
Although the Glycaemic Index is a useful concept to help you select a healthy diet, it is based on eating whatever quantity of food contains 50 g of digestible carbohydrate. In the case of parsnips, for example, a steep rise in blood glucose levels would only occur after eating more than most people could manage, and you don’t need to worry about including them in your diet. Similarly, an average serving of white pasta contains considerably more than 50 g digestible carbohydrate and has a larger impact on your blood glucose levels than you might expect from its GI alone.
Hence another, more realistic system has been developed called the Glycaemic Load (GL). This provides more useful information, as it takes into account the amount of carbohydrate present in a typical portion.
GLYCAEMIC LOAD (GL)
The GL of a food is calculated by multiplying that food’s GI value by the amount of carbohydrate measured in grams found in a typical serving, then dividing the result by 100 as a percentage.
Foods classed as having a high GL (20 or more) release energy quickly, within minutes. Those with a low GL (10 or less) are digested, absorbed and processed to release glucose more slowly, over several hours. Those with a medium GL (11 to 19) have a moderate effect on your blood glucose levels.
Some useful GL values are given in
Table 2
.
Table 2
.
After eating a carbohydrate-rich meal, the rate at which blood glucose levels rise to trigger an insulin response depends on the balance of monosaccharides, disaccharides and complex carbohydrates you have eaten. Monosaccharides cause a rapid rise, disaccharides a moderate rise and complex carbohydrates a lesser but more prolonged response. It also depends on the amount of fibre and fat present in the meal, as these help to slow the digestion and absorption of carbohydrates. For example, eating bread and butter produces a slower rise in blood glucose levels than eating a slice of bread alone.
Foods with a high GL | Foods with a medium GL | Foods with a low GL |
White rice, boiled | Cornflakes | Muesli |
Condensed milk | Wholemeal spaghetti, boiled | Sweetcorn, boiled |
Raisins | Dried, tenderized figs | Dried apricots |
Baked potato, without skin | Brown rice, steamed | Wholemeal rye bread |
White spaghetti, boiled | New potatoes, boiled | Most fresh fruit and vegetables |
Banana, ripe | Cooked beans | |
Unsweetened fruit juices | Parsnips, boiled | |
Honey | Carrots | |
Oat porridge | ||
Sweet potato | ||
White bread |
GI and GL values vary slightly from different sources. A large searchable database is available, free, at a University of Sydney website:
www.glycemicindex.com
www.glycemicindex.com
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