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 with three main symptoms.
• Hunger: when you start a weight-loss program, 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 10.30 in the morning. That's the famine reaction pushing you to eat more and 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 like 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 any more fat.
• Feeling cold and shivery, even in the middle of 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 incredibly powerful, and help explain why most people hit a plateau and gain back some or all of their lost weight shockingly quickly.
Health-care professionals, the weight-loss industry and members of the public alike 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 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 programs, 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 many 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 troublesome 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 those tiresome 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 currently investigating this strategy 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. It may even be a suitable weight management strategy for people who struggle with binge eating.
* Amanda Salis is a 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.
This article was originally published on The Conversation.
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