The idea that people can be healthy at any weight has gained credence in recent years, despite widespread evidence that obesity creates health risks. While the idea is attractive, it's also dangerous because it can lull people who need to lose weight now into a false sense of security.
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In a new book The Obesity Paradox: When Thinner Means Sicker and Heavier Means Healthier, for example, US cardiologist Carl Lavie argues that people with certain chronic diseases who are overweight, or even moderately obese, often live longer and fare better than normal weight people with the same ailments.
This may indeed be the case for a small proportion of people, but messages such as this are cause for concern because they can lead to complacency and delays in action against overweight by governments, health professionals and individuals alike.
Being fat and fit?
Lavie's idea is not new. An increasing number of reports show it's possible to have a body mass index (BMI) in the overweight (25 or more kilograms per height in metres squared) or obese (30 or more kilograms per height in metres squared) range and still be metabolically healthy. The latter is defined as the absence of certain risk factors for metabolic diseases typically associated with being overweight or obese, such as diabetes and cardiovascular disease.
While the proportion of obese people who are metabolically healthy varies depending on how obesity and health are defined, it's a much smaller proportion than those who are not metabolically healthy. And it's not possible to predict who will remain metabolically healthy despite excess weight gain.
What's worse, recent research suggests it's only a matter of time before obese people who are metabolically healthy start facing health issues. And whether or not a person with excess weight develops illness, sooner or later the mechanical effects of excess weight and the resultant gait abnormalities, combined with systemic inflammation, are likely to take their toll.
Overweight adults are more than twice as likely to develop knee osteoarthritis, and the risk increases with weight. Carrying excess weight also contributes to escalating difficulties in performing everyday activities, such as walking, getting out of a chair and climbing stairs.
Any delay in action is even more worrying because of emerging research in animals such as mice, rats and monkeys that suggests there may be a limited window of opportunity to do something about excess weight.
After a while on an excessive diet (several months in rodents; unknown in people), carrying excess weight may become "hard wired" into the parts of the brain that regulate appetite. It may then become almost impossible to lose weight.
Effects of processed food
Exposure to a processed, energy-dense diet that's high in fat, or high in fat and sugar (the default diet of modern societies), initially leads to physiological changes in animals and humans that tend to counteract weight gain.
These include a loss of appetite, but whether you actually lose weight in this phase depends on whether you pay attention to how you feel or eat when you're not hungry. Problems arise when signals for reducing food intake are ignored and people continue eating more than they need.
Chronic exposure to excess food in rodents leads to the breakdown of these compensatory responses. Changes in the brain similar to those seen in drug addiction also occur. Both changes are thought to contribute to a compulsive drive to overeat.
So, instead of the body fighting ongoing fat build-up, as is the case during the initial stages of kilojoule excess and weight gain, long-term excess and fat accretion lead to physiological changes that enable the body to put on weight more easily.
While we know this happens, we don't know why or how eating too much over time breaks down the body's natural defences against weight gain. And we don't know whether the effects of long-term overconsumption to promote a seemingly permanent state of obesity in rodents also occur in humans.
More importantly -- and more worryingly -- we don't yet know whether the detrimental effects of long-term excess can be reversed by switching to a healthier, lower-kilojoule diet.
Advantages of acting early
While there are gaps in the evidence for this idea, in light of emerging evidence from animals showing similarities with human brain pathways controlling body weight, it's probably safer to act now rather than wait.
That's why the increasingly widespread promotion of the idea of being healthy at any weight, which is potentially a recipe for complacency, is bad.
Governments should take urgent action to ensure that healthy diets are readily accessible to everyone, and that highly processed high-fat, high-sugar diets are difficult to access. We also need more research to find better ways to help people to lose excess weight.
Anyone carrying excess weight should do whatever it takes to rid themselves of it gradually. They should start as soon as possible -- while their body is still likely to be amenable to weight loss.
If you put off losing those excess kilos until later, it may be impossible to do it without bariatric surgery or other extreme measures that leave you feeling permanently hungry.
As a society we need to work together to nip excess weight in the bud -- the earlier the better, while it is still possible.
Amanda Salis is NHMRC Senior Research Fellow in the Boden Insitute of Obesity, Nutrition, Exercise & Eating Disorders at University of Sydney. She receives funding from the National Health & Medical Research Council (NHMRC) of Australia, in the form of research project grants and a Senior Research Fellowship. She is the author of The Don't Go Hungry Diet (Bantam, Australia and New Zealand, 2007) and Don't Go Hungry for Life (Bantam, Australia and New Zealand, 2011) and owns shares in a company (Zuman International Pty Ltd) that sells these books.