Nutrition (7 page)
High-density lipoprotein (HDL) cholesterol forms larger, heavier particles that are too big to seep into artery walls. For every 1 per cent rise in your blood level of HDL-cholesterol, your risk of cardiovascular disease falls by as much as 2 per cent. This is due to reversed cholesterol transport in which HDL sweeps excess LDL-cholesterol away from your circulation and carries it back to your liver for processing.
CHOLESTEROL BALANCE
Your blood cholesterol level is a balance between the amount of cholesterol released into your circulation by your liver, and the amount removed from your circulation by body cells. When a cell needs cholesterol it sends LDL-receptors to its surface to ‘catch’ passing LDL particles. Most (70 per cent) of the cholesterol removed from your circulation is taken back up by liver cells and usually suppresses their need to produce new cholesterol. Some people inherit genes that are less sensitive to this suppression so that their liver produces more and more cholesterol, even though their levels are adequate. Others have faulty LDL-receptors that are less efficient at trapping and absorbing circulating cholesterol, so LDL-particles remain in the circulation for longer than the normal average of two and a half days.
The optimal level for cholesterol is not clear-cut but in general you want:
• a total cholesterol of less than 5 mmol/l (200 mg/dl)
• an LDL-cholesterol of less than 3 mmol/l (100 mg/dl)
• an HDL-cholesterol greater than 1 mmol/l (40 mg/dl) for men, or 1.2 mmol/l (50 mg/dl) for women.
If you have other risk factors for cardiovascular disease (e.g. smoking, high blood pressure, diabetes) the recommended total cholesterol level is even lower (below 4 mmol/l) with an LDL-cholesterol of less than 2 mmol/l. Because this is difficult to achieve via diet, it usually means taking a statin drug.
Low-cholesterol diet
Dietary changes can lower your total and ‘bad’ LDL-cholesterol level while raising your ‘good’ HDL-cholesterol. Traditional advice is to lower your intake of cholesterol-rich foods (especially liver and caviar) and to cut back on saturated fats, as these are what your liver uses to make cholesterol.
Although eggs used to be frowned on, studies show that, in most people, eating eggs has minimal impact on LDL-cholesterol levels while providing health benefits in the form of antioxidants, omega-3 fatty acids and important vitamins and minerals. In fact, research involving over 100,000 men and women shows that eating one egg a day does not increase the risk of coronary heart disease or stroke – even if your cholesterol level is raised.
You may also be advised to eat more porridge oats, as many studies now show that oats, oatmeal and other oat-based products, such as porridge, can reduce ‘bad’ LDL-cholesterol. Why? Because oats contain fibre, which acts like a sponge to bind cholesterol and slow its absorption, and because they contain substances (betaglucans) that act on the liver to reduce your own natural cholesterol production. Similarly, snacking on a handful of almonds, walnuts or macadamia nuts per day has been shown to lower LDL-cholesterol and increase HDL-cholesterol.
STEROLS AND STANOLS
Just as animals make cholesterol, plants produce similar chemicals called sterols and stanols. These have a similar structure to human cholesterol and can block the receptors that absorb cholesterol in your small intestines without being absorbed themselves. As a result, people who eat the most plant sterols have the lowest cholesterol levels. They occur naturally in small quantities in vegetable oils, nuts, seeds, grain products, fruit and vegetables, but for optimum cholesterol-lowering benefits, you need to consume at least 2 g per day. The average diet provides less than 500 mg plant sterols/stanols daily, so foods fortified with sterols and stanols (spreads, yogurts) or sterol supplements are needed to boost intakes if you have a raised cholesterol level. Using these products can lower your LDL-cholesterol by 10 per cent within as little as 3 weeks.
Unlike plants, you cannot synthesize energy using the power of sunlight, so all the energy produced in your body must ultimately derive from the macronutrients in your diet – the proteins, carbohydrates and fatty acids.
Kilocalories and kilojoules
Energy is the dynamic force that fuels all the biological processes of life. You need energy for everything you do – from physical activities such as walking, running and playing sports, to thinking and storing memories. Energy is especially important for growth and reproduction.
The chemical energy in food can be measured in units known as calories. This is also called the standard calorie (cal) spelt with a small ‘c’. The unit you are probably more familiar with, and which is used when discussing slimming diets, is the kilocalorie or kcal. This is also known as the Calorie which, by convention, is spelt with a big ‘C’. One kilocalorie (kcal) is equivalent to 1,000 calories.
Because of the confusion caused between calories with a small ‘c’ and Calories with a big ‘C’, scientists now tend to use a more modern, SI unit called the joule.
Around half of your daily energy expenditure is used up through general physical activity. The other half of your energy expenditure is used up by basic metabolic functions, such as maintaining your heartbeat, respiration, digestion, body temperature and cell division. This is known as your basal metabolic rate (BMR).
ENERGY DEFINITIONS
One calorie is defined as the amount of heat energy needed to raise the temperature of 1 g of water by 1 ºC from 15 ºC to 16 ºC. One joule is defined as the energy expended (work done) when applying a force of 1 Newton through a distance of 1 metre. Put simply, 1 joule is essentially the amount of energy needed to lift a small apple 1 metre straight up into the air.
1 kilocalorie (kcal) = 1,000 calories
1 kilojoule (kJ) = 1,000 joules
1 kcal = 4.2 kJ
1,000kj = 1 Megajoule (MJ)
Basal metabolic rate
Your BMR is tightly defined as the energy you expend just lying in bed, at complete physical and mental rest, twelve to fourteen hours after last eating, in an ambient temperature of 26 ºC to 30 ºC. The three main cell activities that contribute to your basal metabolic rate are:
• synthesis of chemicals in cells: proteins, fats, glucose, urea, neurotransmitters (40 per cent BMR)
• active transport of salts and proteins across cell membranes, especially the sodium-potassium pump, which forces sodium out of your cells by swapping it for potassium which is forced inside your cells; this maintains the electrical charge across your cell membranes which is vital for life (38 per cent BMR)
• involuntary muscular activity: breathing, heartbeat, gut peristalsis (22 per cent BMR).
Your basal metabolic rate varies depending on your age, gender, muscle bulk (lean body-mass percentage), nutritional status and genetic inheritance, which dictates the efficiency of metabolic reactions. The type of food you eat also plays a role as energy is used up, and heat produced, during its metabolism. This effect is known as dietary-induced thermogenesis and accounts for 10 per cent or more of the energy provided by foods.
It is possible to estimate your average BMR, based on your weight, age and gender, using mathematical formulae known as the Schofield equations (
Table 7
).
Table 7
).
Age (years) | Males | Females |
0–3 | BMR = 60.9W –54 | BMR = 61.0W –51 |
3–10 | BMR = 22.7W + 495 | BMR = 22.5W + 499 |
10–17 | BMR = 17.5W + 651 | BMR = 12.2W + 749 |
18–29 | BMR = 15.3W + 679 | BMR = 14.7W + 496 |
30–59 | BMR = 11.6W + 879 | BMR = 8.7W + 829 |
Over 60 | BMR = 13.5W + 487 | BMR = 10.5W + 596 |
BMR = basal metabolic rate; W = body weight in kilograms
For example:
A 50-year-old male weighing 70 kg has a BMR of (11.6 × 70) + 879 = 1,691 kcal/day.
A 40-year-old woman weighing 60 kg has a BMR of (8.7 × 60) + 829 = 1,351 kcal/day.
Additional energy is also needed to fuel physical activity, depending on your weight, the type of activity and its duration.
Calculating total energy requirements
The most accurate way to measure your total energy expenditure is to put you in an insulated chamber and directly measure the heat loss from your body. This is known as direct calorimetry.
Another way, known as indirect calorimetry, predicts your energy expenditure by measuring the amount of oxygen you consume, the amount of carbon dioxide (a waste gas) you exhale, and the amount of nitrogen you excrete. Together, these measurements show how much protein, carbohydrate and fat your body has burned as a fuel.
These are complex, time-consuming processes, for which volunteers are thin on the ground – few people are happy to spend a day locked in a chamber or having their body wastes closely measured and scrutinized. This being the case, scientists usually estimate energy requirements by multiplying your basal metabolic rate (BMR, predicted from the Schofield equations shown in
Table 7
) by a factor known as your physical activity level (PAL). The PAL takes into account all the activities you do during the day. Your total energy requirement is calculated by multiplying your BMR by your PAL.
Table 7
) by a factor known as your physical activity level (PAL). The PAL takes into account all the activities you do during the day. Your total energy requirement is calculated by multiplying your BMR by your PAL.
Energy requirement = BMR × PAL
For a sedentary person who sits around all day (your average couch potato), PAL is about 1.2.
For a lightly active person (such as an office worker stuck at a desk all day), PAL works out at 1.4.
A person who is moderately active during both work and leisure activities (a sales representative, perhaps) will have a PAL of 1.6 (females) or 1.7 (males).
An individual with high levels of physical activity during both work and leisure time (a sporty type who is always on the go) has a PAL of 1.8 (females) or 1.9 (males).
Individuals with a vigorously active lifestyle, such as an elite athlete, may have a PAL of 2 to 2.4.
To work out your daily energy needs, multiply your BMR by your PAL.
In the examples above, the 50-year-old male weighing 70 kg had a BMR of 1,691 kcal/day. If he drives to and from work, has a desk job and rarely exercises in his spare time, his PAL is 1.2. His estimated total energy requirement per day is therefore: BMR (1,691) x PAL (1.2), and he needs around 2,029 kcal per day.
The 40-year-old woman who weighs 60 kg and has a BMR of 1,351 kcal/day is very active, however. She jogs to and from work, is on her feet all day and plays sport most evenings and weekends, giving her a PAL of 1.8. Her estimated total energy requirement per day is therefore: BMR (1,351) x PAL (1.8), and means she needs to eat around 2,432 kcal per day.
These types of calculations suggest that the averagely active adult male needs around 2,605 kcal (10.9 MJ) per day, while the average adult female needs around 2,079 kcal (8.7 MJ). The values vary according to age, as shown in
Table 8
.
Table 8
.
Age | Males (kcal) | Females (kcal) |
0–2 months * | 574 | 502 |
3–4 months * | 598 | 550 |
5–6 months * | 622 | 574 |
7–12 months * | 718 | 646 |
1–3 years | 765, 1,004, 1,171 | 717, 932, 1,076 |
4–6 years | 1,386, 1,482, 1,577 | 1,291, 1,362, 1,482 |
7–10 years | 1,649, 1,745, 1,840, 2,032 | 1,530, 1,625, 1,721, 1,936 |
11–14 years | 2,127, 2,247, 2,414, 2,629 | 2,032, 2,103, 2,223, 2,342 |
15–18 years | 2,820, 2,964, 3,083, 3,155 | 2,390, 2,414, 2,462, 2,462 |
19–24 years | 2,772 | 2,175 |
25–34 years | 2,749 | 2,175 |
35–44 years | 2,629 | 2,103 |
45–54 years | 2,581 | 2,103 |
55–64 years | 2,581 | 2,079 |
65–74 years | 2,342 | 1,912 |
Over 75 years | 2,294 | 1,840 |
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