Explainer: overweight, obese, BMI – what does it all mean?

Australians are getting fatter and there’s no dispute over how this increasing weight is affecting our health. Different methods of assessing body fat can give different interpretations of just how much excess weight a person is carrying, but all methods point in the same direction when applied over time.

The most common measure of body fat and associated health risks is body mass index (BMI). BMI was developed as a simple way to compare different groups of people, based on the correlation between height and weight as an indicator of excess body fat.

BMI is calculated by dividing weight in kilograms by the square of height in metres – a simple online calculator is available here. A healthy BMI for an adult is between 18.5 and 25 kg/m². Between 25 and 30 kg/m² is considered overweight and 30 kg/m² and above obese. At a population level, high rates of body fat above 30 kg/m² indicate an increased risk of death and disease.

BMI is the main measure used for international obesity guidelines and is recommended by the World Health Organization. But BMI isn’t perfect. People with the same body weight and height can have different proportions of body fat to lean muscle mass. Athletes with high muscle mass, for instance, can have a lower proportion of body fat than less muscular people, so a BMI calculation can put them into an overweight or obese category, even though their risk of obesity related disease is very low.

This is a frequently cited criticism of BMI, but it needs to be put into perspective. Such people are in the minority and a quick visual inspection will clearly show that it’s muscle, not fat, that such people are carrying the most of.

People from an Asian background tend to have more body fat on a leaner frame, so a lower BMI healthy-weight-range can be used. Conversely, people of a Pacific Islander background tend to have a higher proportion of lean muscle mass compared with fat, so a higher BMI healthy-weight-range is often recommended.

Alternatives to BMI
While BMI is a useful measure of overall health risks, it fails to take into account the distribution of fat throughout the body. For this reason, waist circumference was developed as a simpler and potentially more accurate measure of disease risk. Waist circumference is not only a gauge of body fat, but it specifically targets the most dangerous type of fat: visceral fat.

Visceral fat is found between the organs of the abdomen and contributes to belly fat. There’s a strong correlation between central obesity and cardiovascular disease, insulin resistance, type 2 diabetes, inflammatory diseases, high blood pressure and other obesity related diseases.

For men, the aim is to have a waist circumference below 94cm; for women it’s 80cm. Measures above 102cm for men and 88cm for women carry a very high risk of developing type 2 diabetes, high blood pressure, cardiovascular disease and even some forms of cancer. For people of an Asian background, slightly lower waist circumference goals apply: under 90cm for men and 80cm for women.

More recently, estimates of body fat percentage and health risks have looked at waist-to-hip ratio and even waist-to-height ratio. Both these measures take into account central fat stores so can give a better health risk estimate than BMI. For men, a waist-to-hip ratio below 0.9 and 0.8 for women correspond to a healthy weight BMI.

An advantage of using waist measures for body fat estimates is that it takes away the stigma of needing to step on the scales. It also allows for the use of cut-off values that avoid terms of overweight and obesity, and instead focus on the risk of metabolic disease.

Labelling a person as “obese” may not always be helpful in affecting positive behaviour changes, especially when a person already acknowledges that they’re carrying a bit of extra weight. “Unhealthy BMI”, “above the healthy weight range”, and “excess weight” can all carry the same message about the need to shed excess weight for better health and reduced risk of disease.

Another technique to measure body fat is by bioelectrical impedance. This method involves passing a small electrical current through the body, normally by a specialised set of scales that a person stands on. The scales measure water volume and, by the use of special algorithms, arrive at a body fat percentage estimate. The accuracy of such machines can vary dramatically, especially around the cheaper end of the price range.

By far the most accurate way to measure body fat is by magnetic resonance imaging (MRI), computed tomography (CT) or X-ray scanning, but such methods are not realistic for the public to use and belong firmly in the world of research.

So long as the limitations of a weight assessment method are understood, methods such as BMI and waist circumference are quick and simple validated ways to assess weight and disease risk that can be used by health professionals and the public alike.

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Had your 8 glass of water today? Time to bust this myth

If I had to nominate one pervasive myth in nutrition, it would be the popular advice that we should drink at least eight glasses of water a day. Of course our bodies need water, otherwise we would die from dehydration. But the amount needed is extremely variable and depends on a person’s body size, physical activity levels, climate and what types of food they are eating.

Water makes up about 60% of an adult’s body weight and is an essential nutrient, more important to life than any others. Water helps regulate body temperature, carries nutrients and waste products throughout the body, is involved in blood transport, and allows many metabolic reactions to occur. It also acts as a lubricant and cushion around joints, and forms the amniotic sac surrounding a foetus.

It is believed that the “eight glasses” myth was a US Recommended Dietary Allowance dating back to 1945. The guide said a suitable allowance of water for adults was 2.5 litres a day, but most of this water could be found in prepared foods. If that last, crucial part is ignored, the statement could be interpreted as clear instructions to drink eight glasses of water a day.

Even a comprehensive search of the scientific literature finds no evidence to support the eight-glasses-a-day advice. The clear reason that evidence for such prescriptive advice doesn’t exist is that a person can get all the water they need without consuming a single glass.

Drinks like soft drink, fruit juice, tea and coffee, milk, and foods like fruit, yoghurt, soups, and stews all have appreciable amounts of water that contribute to fluid intake.

Australian dietary recommendations also bust the eight-glass myth as the official Nutrient Reference Values state "there is no single level of water intake that would ensure adequate hydration and optimal health for the apparently healthy people in the population."

Don’t be concerned about seeing coffee listed as a fluid – the “coffee makes you dehydrated” mantra is another myth that needs to be busted. Drinks such as coffee, tea and cola do have a mild diuretic effect from the caffeine but the water loss caused by this is far less than the amount of fluid consumed in the drink in the first place. It’s only alcoholic drinks which have a dehydrating effect.

So how do you know if you are drinking enough water? Well. You can check this for yourself every few hours. If your urine is lightly coloured or clear, you’re drinking enough. If it’s dark, then you should drink more. How simple is that?

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Coffee powers endurance sports

Caffeine is a popular performance-enhancing supplement taken by both recreational and elite sportspeople. A recent clinical trial has found that getting a caffeine hit from regular coffee has been found to be just as effective on sports endurance as popping caffeine tablets.

The world of sports supplements is dominated by over-priced, over-hyped, and mostly ineffective pills and potions. There are though a handful of supplements that have a solid evidence base for their use and caffeine is one of those.

Caffeine acts as a neuromuscular stimulant and when used in the sports arena, acts to change perceptions of effort or fatigue, improve muscle fibre recruitment, and have a direct effect on cardiac muscle. Most of the research into caffeine has used the laboratory setting to measure time to exercise exhaustion on a bike or treadmill with the results from many studies pointing to a favourable benefit.

Caffeine was removed from the World Anti-Doping Prohibited List in 2004, partly because it is widely consumed in a normal diet, so it has been open season since then for anyone who wants to dose up on caffeine pills.

Caffeine doses of between 2 to 3 mg per kilogram body weight have been shown to be effective in a range of exercise protocols, with an average of around a 3% improvement in performance in time-to-exhaustion studies. For a 70kg person, two 100 mg No-Doz caffeine tablets would would give a caffeine dose of around 3 mg/kg body weight.

The research field of caffeine has mostly used it in a pure form, with very little research looking at ‘real world’ use by drinking coffee. For a typical person, two strong coffees should give the same caffeine dose as those used to show performance enhancing effects. Yet, not all research shows coffee being equivalent to pure caffeine, leading to speculation that substances in coffee may inhibit some of caffeine’s action.

I’ll have mine black
With limited evidence to date, researchers designed a new study to test the effectiveness of coffee versus caffeine in a laboratory time trial cycling experiment. Eight trained male triathletes attended the laboratory on four separate occasions for the time trial. Each session involved being randomly given either a drink containing caffeine, instant coffee, instant decaffeinated coffee, or a placebo. The amount of instant coffee each person drank was modified to provide the same dose of caffeine as that used in the caffeine drink.

The time trial efforts under the caffeine and regular instant coffee conditions were clearly superior, showing an approximate 5% quicker time compared to the placebo or decaffeinated coffee conditions. Greater power output was also seen under both the caffeine and regular coffee conditions.

The full research study is available online through the open access journal PLOS One.

What is all means
Coffee and caffeine are both able to improve exercise performance. The one point for budding athletes to be aware of, is that the amount of caffeine in coffee can be quite variable, meaning it is hard to know exactly what dose a person is having from their morning espresso.

For anyone contemplating experimenting with caffeine, it is best to get advice from the experts. See the AIS Sports Nutrition fact sheet on caffeine supplements.

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Lifestyle keys to preventing bowel cancer

Bowel cancer is one form of cancer where diet and lifestyle choices play a big part in determining a person’s risk of being diagnosed with it. Medical researchers have now put some hard numbers to how much this cancer can be prevented by following specific lifestyle recommendations.

Bowel cancer (also called colorectal or colon cancer) is the second biggest killer of Australians from cancer each year. Dietary factors alone are thought to explain almost half of bowel cancer risk, with physical activity (or lack of) around another quarter followed by genetics and family history. Processed meat, obesity (especially fat around the abdomen), smoking and alcohol are the diet and lifestyle factors that are considered to have the most evidence for being linked to colorectal cancer.

With such a large influence that diet and lifestyle has on a person’s risk of developing bowel cancer, combining these factors into a simple lifestyle assessment score could be an important public health initiative in identifying people at greatest risk of the disease.

Early detection of bowel cancer through screening has a dramatic impact on cutting mortality from the disease, so knowing who may benefit from earlier and more frequent screening would be valuable. Knowing what your individual risk assessment score was would also allow for targeted counselling on effective lifestyle changes.

Researchers from Denmark have evaluated a simple lifestyle risk assessment score for bowel cancer and estimated the number of cancer cases that could be linked to the risk score. The five factors included were physical activity, waist circumference, smoking, alcohol consumption and diet. Diet itself was an umbrella term which covered dietary fibre, fat, red and processed meat, and fruit and vegetable consumption.

The risk assessment was applied to an epidemiology model involving over 55,000 adult Danish men and women who were part of a long-term study looking at diet and cancer risk. The full study was published in the British Medical Journal.

After some complex statistical and mathematical modelling, there was a clear trend for a lower risk of bowel cancer for each lifestyle recommendation a person met. Adhering to guidelines for all five risk factors could have potentially prevented 23% of the cases of bowel cancer. Even if all the people in the study had of met just one recommendation, this could have markedly reduced the cases of colorectal cancer by 13%.

Recommendations to live by
So what where the healthy lifestyle guidelines used for each of the five risk factors?

• Not smoking at all
• Being physically active for at least 30 minutes per day
• Having 7 or less alcoholic drinks per week for women and 14 for men
• Having a waist circumference below 88 cm for women and 102 cm for men
• Eating more than 600 grams of fruits and vegetables per day
• Eating less than 500 grams of red and processed meat each week
• Having at least 25 grams of fibre per day
• Reducing fat to less than 30% of the energy in the diet. 

What is all means 
Most cases of bowel cancer are not inevitable and a person can significantly cut their risk by making changes to their diet and lifestyle today to reap rewards well into the future. The findings from this latest research tell an important message on where the focus on positive lifestyle changes should lie.

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It's true: hungry shoppers buy more food

A scientific study has finally given credence to what many people intuitively know: never go shopping on an empty stomach.

Fasting or food deprivation in a restrictive diet is well-known to increase both the quantity and quality of food a person eats when they break their fast or diet. It seems logically then that this would explain why advice is given to people trying to eat less to avoid shopping when hungry. Surprisingly though, the area of hunger and shoppers’ purchases hasn’t been well studied.

Do people really buy more when they are hungry and are they more likely to choose high-kilojoule foods compared to someone shopping on a full stomach? Researchers from Cornell University set out to study this serious gap in the dieting folklore mythology to see if there may be a kernel of truth to it.

Sixty eight adults took part in the study which was designed in the first part to be a lab experiment. Each person was asked to not eat for 5 hours before the late afternoon experiment kicked off. Just before the participants were let loose on the online simulated grocery store, half of the people were given some wheat biscuits to curb their well-developed hunger. The online store offered a variety of consumer staples as well as plenty of salty and sugary energy dense snacks.

People who hadn’t eaten any food in the afternoon were more likely to choose high-kilojoule foods when they were set loose in the simulated supermarket. Compare that against people who were given a snack before shopping: they bought less of the energy dense foods.

Interestingly, both groups of people bought an equal amount of the low-kilojoule food staples. So the hungry shoppers not only bought more food, but specifically more of the snack types.

Rather than just using a simulated supermarket, the researchers also looked at patterns in shopping purchases in real supermarkets. Shoppers were more likely to buy a greater amount of energy dense foods in the hours leading up to dinnertime of 4 to 7 pm compared to earlier in the day in the hours of 1 to 4 pm.

Details of the study can be found in JAMA Internal Medicine

What is all means
Hungry shoppers are more susceptible to buying energy dense foods which is not helped by the way supermarkets are cleverly designed to make us buy more at every turn. One way to fight back against this is to either shop soon after meals, or enter the supermarket after chowing down on a healthy snack.

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Pay attention if you want to lose weight

Attentive eating is a hot topic in nutrition research. A recent review of the research to date finds that mindful eating can be a powerful behaviour change in helping with weight loss.

Diets come and go, yet few offer any real long-term solution for weight loss and weight maintenance. An overall change in lifestyle and behaviour is fundamental to addressing decades of slow weight gain and failed dieting.

One very simple strategy to help with weight loss, so simple that it puts to shame many complex dietary recommendations, is to be more mindful when it comes to eating. Mindfullness can be described as learning to pay attention to the present moment experience and to let thoughts and feelings come and go without providing judgement.

Applying the principle of mindfullness to eating can be as simple as focussing on what is currently being eaten. Removing distractions such as television watching at meal times, savouring the taste of food as it is eaten slowly, and even a simple recall of what has been eaten recently all appear effective in changing future decisions about what and how much to eat.

Just how effective attentive eating can be has been the subject of a recent review published in the American Journal of Clinical Nutrition. From 24 studies examined, the findings were clearly in favour of attentive eating being able to change food consumption habits. Eating when distracted was seen to increase food intake not only at the meal time, but several meals after the distracted eating episode as well. In contrast, having a better memory of what was recently eaten reduces how much is eaten.

What is all means
Mindful eating free of mindless distractions combined with mindful recall of food previously eaten is an important part of any weight loss behaviour change targeted at eating less.

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