The best way to shed extra weight and keep your health in check is to drastically cut back your calories a couple of days each week. That's the advice care of latest trendy, but controversial eating plan - the fast diet.
However, a New Zealand health expert says it is a radical way to loosen your belt and a "ticking timebomb for a mental breakdown."
BBC journalist, doctor and author of the best-selling The Official 5:2 Diet, Dr Michael Mosley has been studying health and the human body for 20 years.
However Dr Mosley hasn't always had his own health in check. A diabetes scare prompted him to find how he could repair his body without medication.
What worked for him, and was subsequently propelled into the public eye by the documentary Eat Fast, Live Long, was "intermittent fasting" - essentially cutting calorie intake to about a quarter of the usual level. For women, that means about 500 calories a day; for men, 600.
"What they know from all the studies in animals and about a dozen studies in humans is that a pattern of eating and not eating tends to lead to improved insulin sensitivity," Dr Mosley says.
"Insulin is important not just for diabetes but also because it's a fat-promoting hormone. High levels of insulin are associated with increases in cancer and dementia."
Dr Mosley cuts calories two days a week and says he will "absolutely" keep doing this for the foreseeable future. "Doing this seems to stress the body in a good way."
But New Zealand-registered dietitian and nutritionist David Shaw says it's unlikely intermittent fasting is necessary to improve lifespan and insulin sensitivity.
"With caloric restriction comes feelings of extreme hunger, tiredness, loss of concentration and agitation," he says.
"Then on normal days there's a temptation to binge. Managing intermittent days of starvation followed by carefree living is a major hurdle to overcome, and such habits will destroy your relationship with food.
"This is what the 5:2 diet boils down to - a radical and unhealthy start to any weight-loss programme and a longterm ticking timebomb for a mental breakdown."
However, Dr Mosley reckons it's easy to keep the temptation to binge at bay.
"Psychologically, because you're not on a diet seven days a week ... in a funny way you can resist temptation.
"I think most people don't binge because they realise it's stupid, that if you do you're not going to get the benefits."
Shaw says a far better approach to weight loss is "improving your ability to make the right food choices in different situations and to not let irrational thoughts dictate your decisions."
Dr Mosley is adamant this doesn't work.
"We know what the advice is. The advice is eat well and do more exercise. And we also know that's been falling on deaf ears for 20 years.
"The question is: how do you get them to eat well?"
Dr Mosley says eating "clean protein" like fish, chicken and veges on fasting days
encourages people to change their eating habits on the remaining days of the week.
"The problem with just telling people to eat well is that they don't do it, or they do it for a short period and they fall back into their bad habits."
You're more likely to stay on this diet than any other, Dr Mosley says. You will lose more fat and improve insulin levels, and the benefits can be retained with a 6:1 eating/fasting plan when you reach a healthy weight.
"Preferably you shouldn't count calories," he says. "But it's useful to be calorie-aware."
Exercise myths according to Dr Mosley:
• Exercise won't make you thinner:
And in some cases it won't even make you fitter. Dr Mosley says studies show that working out 40 minutes a day, five days a week for three months can help the heart and lung function of some people, but others won't see any aerobic improvements. He says a it all comes down to your genes.
"You can go down to the gym and exercise like crazy, but you will never build any muscle. You just don't have the genes that will enable you to do so.
"I don't think you can say that going for a run is good for everyone.
"You may just be dealt a bad hand of cards."
• Exercise makes you happy:
"It doesn't," Dr Mosley says. "If you like exercising then exercising makes you happy, if you don't like it then it doesn't."
The idea that working up a sweat will put give you a rush of endorphins and put pep in your step is rubbish.
This is based on "few studies on a few people," he says.
"Getting off your arse is more important that getting to the gym."
Being active is important for cutting blood glucose levels, "going for a run probably isn't."
• Eat Fast, Live Longer will show during the series What's Your Body Hiding?, on BBC Knowledge, Thursday, October 10.