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Evolution has a lot to answer for, writes Amanda Salis.
Almost everyone who has tried to lose weight has tasted the bitter pill of failure.
That feeling you get when, despite all your desires to be healthier, to fit into sassier clothes or to shimmy through life (and into aeroplane seats) with greater ease and comfort, you just can't stick with your diet and exercise plans for long enough to get there.
People who fail to lose weight frequently blame themselves, as does almost everyone around them.
Even many health professionals consider obesity to be an individual failing. But this attitude displays complete ignorance of human physiology and how it affects weight loss.
Fighting famines, not winning weight wars
In fact, most diets fail because the body activates a series of powerful physiological mechanisms - many originating from changes in the hypothalamus, which lies at the base of the brain - that help to protect us from losing too much weight too quickly.
These mechanisms, which I call the famine reaction, have been pivotal to our survival as a species because they prevent ongoing weight loss and promote weight regain.
In pre-agricultural times, which constitute the majority of human history, food supply was much more dependent on seasons and intermittent. Conserving energy ensured survival through the lean season.
But while the famine reaction undoubtedly helped the species survive recurring famines and hardships, it presents an enormous challenge in modern societies where the abundance of food (especially energy-dense food) means many people are now overweight or obese.
The famine reaction is a whole-of-body response to not getting enough food and has three main symptoms.
Hunger: When you start a weight-loss programme, you might not feel very hungry at all. But once you lose a certain amount of weight, you can eat your whole day's diet ration and still feel hungry ... and it's only 10.30am.
That's the famine reaction pushing you to eat more to help protect you from losing any more fat.
Lethargy: Keeping active can be a challenge at the best of times, but when you've lost weight and your famine reaction has been activated, you can feel as if you're dragging your whole body in mud just to get through the day.
You're being slowed down so you don't waste precious energy and lose more fat.
Feeling cold and shivery, even in summer: This is your famine reaction reducing your metabolic rate.
The result is that weight loss can come to a complete standstill (or plateau), even if you're still sticking rigorously to your diet and exercise plan.
These effects are powerful, and help explain why most people hit a plateau and re-gain some or all of their lost weight quickly.
Healthcare professionals, the weight-loss industry and the public assume the energy-conserving effects of the famine reaction are only felt by lean people after extensive or rapid weight loss (because they need to be protected from wasting away).
But this adaptation has been shown to happen even in overweight and obese people after the loss of as little as 6% to 12% of body weight, and even in cases when weight loss has resulted from moderate energy restriction, with or without physical activity.
Fighting the famine reaction
If you listen to standard advice from the majority of weight-loss programmes, you'll think the famine reaction and its secret weapon, hunger, will just go away with judicious use of willpower.
All you have to do in the meanwhile is ignore how hungry you're feeling, or try to quash your hunger by filling up with non-starchy vegetables, which are part of the ''free list'' in most diets because their low calorie and carbohydrate count means you can eat as much of them as you like.
But research shows the brain changes that cause the famine reaction don't just dissipate if you hang in for long enough and exercise more. So what can you do to avoid regaining the weight you've lost?One way is to work with your physiology, rather than against it.
This means eating more at certain times, notably when hunger raises its head.
Research shows eating more and undergoing a period of weight maintenance (rather than continuing efforts to lose weight) can deactivate aspects of the famine reaction.
In particular, it can block the reduction in metabolic rate that creates weight-loss plateaus.
This intermittent approach to weight loss - eating less overall but sometimes eating more - may improve the efficiency of weight loss.
It may also reduce the drive to eat large amounts of food when you've lost some weight, by taming the hunger pangs of the famine reaction.
My colleagues and I are now investigating this in a clinical trial. We still need to determine the optimum timing of intermittent energy restriction for different people.
But if you're on a diet and start to experience mounting, nagging hunger (a sign of the famine reaction at play), eat the types and amounts of (mostly healthy) foods that make you feel genuinely satisfied.
This may ultimately help weight loss more than the conventional advice to just keep going.
Amanda Salis is NHMRC senior research fellow at the Boden Institute of Obesity, Nutrition, Exercise and Eating Disorders at the University of Sydney.