TIME 4 FAT LOSS SERIES
This is the second in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.
PART 2 – How much fat should we have?
There is no simple answer to the question of how much fat should we have?
Although too much or too little body fat is considered unhealthy, there is no universally accepted definitive recommendation regarding the amount of body fat a person should have for optimal health. This is because it is likely to vary between individuals and is influenced by a number of factors, including race, gender and age. As a general guideline, the American College of Sports Medicine suggest that men should have a percentage of body fat of 10-22% while 20-32% is considered satisfactory for good health for women (1).
Why do women need more fat than men?
To answer that question, we need to look at where we store fat and what it does. Body fat is primarily stored in two sites referred to as essential fat and storage fat.
- Essential fat consists of the fat found in the marrow of bones, the heart, lungs, liver, spleen, kidneys, intestines, muscles and the tissues of the central nervous system, which is essential for normal physiological functioning. Women have sex specific essential fat found in their breasts, pelvic, thigh and buttock regions, which is important for hormonal functions and child bearing. Women have approximately four times the amount of essential fat than a man (12% body mass for women compared to 3% body mass for men) (2).
- Storage fat consists mainly of fat in adipose tissue. It includes the fat that protects the organs within the chest and abdomen from trauma, and the subcutaneous fat (fat beneath the skin). Men and women have similar proportions of storage fat (12% of body mass in men and 15% in women) (2).
Note: A certain amount of body fat is believed to be important for ensuring healthy menstrual function. It has been suggested that 17% body fat is the lower end necessary for the onset of menstruation with 22% needed to sustain a normal menstrual cycle. However, various studies have shown that female athletes may have body fat levels below the proposed critical level of 17% but still have normal menstrual cycles (2).
How can I find out if I’m too fat?
The most accurate way to find out how fat you are is to have your body composition assessed using specialised equipment, such as skinfold calipers. This will provide you with an indication of the relative proportions of fat and lean tissue you have, which is expressed as a percentage of your body weight.
If you don’t have access to such facilities, you can use a number of quick and easy methods that have been developed to estimate an individual’s level of health risk associated with their degree of fatness.
Body mass index (BMI) (Quetelet index)
A term you often hear with regard to body fat and health is body mass index. This is a mathematical formula used to express a person’s weight relative to their height.
It is calculated by dividing body weight in kilograms by height in metres squared (w/ht2) (2).
For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9.
BMI = 70 kg / (1.75 m2) = 70 / 3.06 = 22.9
BMI is used to classify people as underweight, overweight and/or obese.
As a quick guide you are considered to be:
- Underweight if you BMI is less than 18.5
- Normal weight if your BMI is 18.5-24.9
- Overweight if your BMI is 25.0-29.9
- Obese if your BMI is 30 or greater (3)
Note: Does not include pregnant women
How accurate is BMI?
BMI provides a better indication of an individual’s level of body fat than estimates based purely on height and weight because of the mathematical calculation that divides body mass by height squared (2). With exception of individuals with large amounts of muscle mass, it is well established that, those with a BMI of more than 30 have excess body fat (1).
Limitations of BMI
Although BMI is simple, quick and inexpensive, is the same for both sexes and for adults of all ages (4), there are a number of problems associated with its use:
- It tends to overestimate the fatness of people that are very muscular, such as body builders, and can underestimate fatness in people who have lost muscle mass, such as the elderly (5).
- It does not adequately reflect changes in fat and muscle mass that occur as a result of weight loss nor does it indicate where an individual’s fat is deposited (6).
- People who are less than five feet tall may have a high BMI that does not reflect their true body fat levels. This also applies to people suffering from excessive accumulation of fluid in the tissues of the body (5).
BMI and race
Until recently it was assumed that the relationship between BMI and level of fat an individual has is the same for all races, but this is not the case. (7). For example, Asians have a higher percentage of body fat than Caucasian people of the same age, sex and BMI, and so have a greater risk of obesity related illness. Therefore, the categories suggested for Asians are: less than 18.5 kg/m2 (underweight); 18.5–23 kg/m2 (normal); 23–27.5 kg/m2 (overweight) and 27.5 kg/m2 or higher (obesity) (7).
People of other ethnic groups, such as Afro-Caribbean, Native Americans, Aborigines and Polynesians, may be also be more sensitive to the health effects of excess fat and therefore may also require a lower BMI for good health (5).
Waist to height
Research has shown (8) that the ratio between our height and waist is a better predictor of health risk than BMI. This is because where we store fat on our body appears to have a greater influence on the health risks associated with obesity rather than our total amount of body fat (9).
Individuals who are apple shaped (android obesity), in which fat is mainly deposited on their trunk (abdominal fat) have a greater risk of suffering from obesity related conditions, such as type 2 diabetes, and cardiovascular, etc, compared to individuals with gynoid obesity (pear shape), in which the fat is deposited on the hips and thighs (2).
Ideally, we should aim to keep our waist measurement less than half that of our height (8). For example, a man 6ft (182.88 cm) tall should aim to keep his waist less than 36 inches (91.4 cm), while a 5ft 4in (162.56 cm) woman should keep hers under 32 inches (81.28 cm).
How to take your waist measurement
- Stand with your feet together and your abdomen relaxed
- Find the bottom of your ribs and the top of your hips
- Breathe out naturally
- Wrap a tape measure around your waist, midway between these points, without compressing the fat
- Ensure that the tape measure is horizontal
- Record the measurement to the nearest centimetre (10)
Note: Waist circumference can be less accurate in certain circumstances including pregnancy, when an individual is suffering from a condition that cause distension of the abdomen, for certain ethnic groups, and for children and young people.
Body composition and athletic performance
Your body composition may not only be dictated by your health needs but also the demands of a particular sport. For example, bodybuilding requires participants to maximise gains in muscle mass whilst minimising body fat levels.
In certain contact sports such as rugby, a higher body weight is generally seen as an advantage. On the other hand, for sports, such as marathon running, and gymnastics, a lower body weight and high power-to-weight ratio are extremely important. Therefore, low body fat and low body weight are necessary for optimal performance (11). Studies have shown some male marathon runners to have body fat levels as low as 3.3% (2).
The table below provides a number of examples of the levels of body fat associated with different sports.
Although many people in the media may attain a very low level of body fat for a particular event, rarely is such apparent leanness maintainable in the long-term, and is often aided by flattering lighting and even image manipulation. It’s easy to do a quick search on line to see what your favourite stars look like off screen, which is often very different to their onscreen appearance. So while you may wish to be ‘ripped’ and have a six pack, the most important thing is to maintain a level of body fat which is healthy for you.