Each year there’s some new diet promising remarkable results with little effort; think Atkins, Dukan, and Southbeach. What these diets have in common is they’re almost impossible to follow and the weight creeps back on as rapidly as it was shed. So here’s the new diet on the block: intermittent fasting. What’s behind the craze and is there any scientific basis to it?

Intermittent fasting has soared in popularity since the Horizon documentary ‘Eat, Fast and Live Longer’ aired last year. The press had a field day, promising an end to arduous diets with the tempting notion you can eat what you want as long as you eat less than 500 calories (600 for men) twice a week. Apart from losing weight, the diet also claims to lower risk of diabetes, heart disease, cancer and Alzheimers’ – essentially it claims to slow the ageing process.

How does it work?

There are many variations to the diet, but the main principle is the same. The theory is that fasting lowers levels of insulin-like growth factor (IGF-1) and improves lipid (fat) levels. High levels of IGF-1 stimulate rapid new cell growth, rather than repairing existing cells. This speeds up the ageing process and susceptibility to age-related diseases, including cancer. Lowering levels switches on a number of repair mechanisms in the body.

There’s a growing body of remarkable animal evidence that fasting and calorie restriction can be good for your long term health, longevity (and weight). It’s no new thing – our ancestors would have had restricted food on a regular basis. Fasting has been shown to turn on a whole load of genes with all sorts of health benefits – when our bodies are constantly dealing with food, there’s no spare time to clean up cells and repair damaged tissue.

But why the weight loss? Well apart from reduction in calories, the diet helps make cells more sensitive to insulin, so we produce less – this lowers fat storage and increases the burning of fat. Indeed lowering insulin is the mechanism behind the Atkins diet, but the problem with this method is the high animal protein levels (especially from red meat and dairy) actually increase levels of insulin-like growth factor (IGF-1).

So what’s the problem?

The problem isn’t the fasting, it’s the notion you can eat what you like on ‘normal’ days, with all the indulgence that encourages. The critical point to remember is calorie restriction works by lowering insulin, so to be effective you should also take care to keep your blood sugar balanced on ‘normal’ days although you don’t have to be so strict on calories. This means combining carbs with protein, ensuring the carbs you do eat are slow-releasing (brown rice, pasta, wholegrains) and avoiding all refined (‘white’) carbs and processed foods.

So the weight loss is to do with insulin and the potential effects on longevity to do with lowering IGF-1. But fasting is not as simple as it sounds – have you tried to get through the day on 500 calories? It’s not that easy. And if your ‘normal’ days are unhealthy, you’re unlikely to reap any benefits. There’s simply not yet enough human evidence to know if it really works and the long term effects. Common sense warns me to be suspicious of any diet that advocates gorging a few times a week.

If fasting is not for you, you can control insulin levels by keeping your blood sugar levels stable and you can keep your IGF-1 down by minimising meat and dairy products – milk contains an abundance of IGF-1 (to make calves grow) and is known to contribute to the risk of cancer. Eating a more plant-based diet helps keep IGF-1 levels down.

A word of caution, fasting is not suitable for everyone – pregnant women or diabetics on medication should not undertake this kind of regime.

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