Diet debate: Low-fat or high-fat - does it matter?

Is fat the great evil of our time responsible for seducing us into an early, extra-wide, grave with its delicious succulence?

Or is it as misunderstood as it is mouth-watering? And in need of a welcome return to our plates?

As the campaign against sugar has ratcheted up over the past year or two, there have been growing voices trying to redeem fat.

For decades it has been labelled public enemy number one and a "low-fat" food label is used to convince us that what we're buying is healthy.

The problem is low-fat can mean vegetables or just clever marketing for "we took out all the fat and then pumped it full of sugar".

So there I was having a moment in the supermarket - a tub of low-fat yoghurt in one hand and a full-fat one in other - pondering which was actually better for me.

If I had a third hand, it would have been scratching my head. And I'm not alone.

"When there's a huge wall of yoghurt, even I find it paralysing," said Susan Jebb, a nutrition professor at the University of Oxford.

When you take the fat out of products, particularly dry ones like cake or biscuits, then something has to replace it.

"It tends to be sugar - the calories in digestives and low-fat digestives are almost the same," Prof Jebb continued.

"Lots of yoghurts are rammed with sugar, that is the thing that annoys me about yoghurt."

There is a simple answer with yoghurt - a few brands are both low in fat and sugar, although I need to chuck in a bit of fruit to make it palatable.

But what about the case that we should be eating more fat?

Some have argued that the message about cutting all fats when discussing bad saturated fats from processed foods was oversimplified.

While others have made the case that favouring carbohydrates in our diet - particularly refined carbs like white bread and pasta, is playing havoc with our hormones to increase the risk of type 2 diabetes and making us pile on the pounds.

More from our Diet Debate series:

Read: Is breakfast a waste of time?

Watch: How healthy is your breakfast?

Fatty joy?

We do all need fat in our diet - it contains essential fatty acids and is important for absorbing fat-soluble vitamins such as A, D and E.

The question has always been: "How much fat should we eat?" And the mantra has been low-fat, high-carb.

The World Health Organization advises that between 30% and 35% of our calories should come from fat arguing there is "no probable or convincing evidence" that the total amount of fat in our diet is altering the risk of cancer or cardiovascular disease.

So when it comes to the total amount of fat (and there is a separate argument when we come to consider different types of fat) it's really a question of how it affects our waistlines.

And fat is certainly calorific.

A gram of fat is worth around nine calories - twice the amount as carbohydrate or protein at four calories per gram.

Too much fat, like too much of anything, will make you put on weight and it is incredibly easy to overeat calorie dense foods.

So it appears to be an easy target for people trying to lose weight.

Appetising?

"There is very good evidence that if you cut down on total fat it causes a small reduction in weight, but it's not big," said Dr Lee Hooper from the University of East Anglia in the UK.

She conducted a large review of 32 separate trials, involving around 54,000 people.

It showed people who shifted between 5% and 10% of their calories away from fats lost around 2kg during the studies.

However, she is not convinced the weight-loss is actually down to fat but more a result of people thinking more about what they eat and avoiding burgers, ready meals and other processed foods.

"I suspect they'd do exactly the same thing if they targeted sugar," she concluded.

So how do diets compare when we target carbs?

Doctors at the Harvard School of Public Health in the United States reviewed 53 weight loss trials involving 68,128 people.

The results, published in the Lancet medical journal, showed that both low-carb and low-fat approaches led to decent weight-loss.

But those eating relatively more fat actually lost marginally more weight.

Some fats are relatively healthy, but still calorie dense

Dr Deirdre Tobias, who led that study, told me: "If you're trying to reduce your calories and you take out the fat then you get a lot of bang for your buck, but that strategy clearly doesn't play out.

"Fat has been villainised because there's a mentality that 'fat makes you fat'. I think our evidence pretty much puts a nail in that coffin."

She is not saying that carbs are the villain instead, but that the best diet is the one you can actually stick to - some people would find it pretty easy to give up on white bread and pasta while others would find it impossible.

But she did warn that focusing on simply avoiding fat risked missing out on known beneficial foods - such as nuts, oily fish and olive oil - or convincing yourself that a low-fat muffin is healthy.

Cutting carbohydrates rather than the fat has also shown some benefit in patients with type-2 diabetes, at least for a short while.

When refined carbs are digested they rapidly lead to a spike in blood sugar levels and in turn of the hormone insulin. People with type 2 have difficulty controlling their blood sugar levels so preventing the spike could help in theory.

Although studies show the advantage of cutting carbs was not sustained in the long-term.

Rethink required?

In the UK the total amount of fat being eaten is broadly in line with recommendations, but with slightly more saturated fat than advised.

Dr Hooper concluded: "I would be saying we don't need to be cutting down on fat, but we do need to think of the type of fat."

That's an issue we'll consider on Wednesday when we ask: "Is butter back?"

But clearly there is never going to be health advice to just pour cream down our throats and polish off all the pies and biscuits we can.

Even drowning a salad in olive oil could lead to weight gain.

Going overboard on fat, just as having too much sugar or refined carbohydrate, is a bad thing. Sugar is just stealing the headlines at the moment.

"The reality is that nutrition comes and goes in waves, we've had a fat wave and we're for sure in a sugar frenzy," says Prof Jebb.

She says she worries "enormously" when people reduce all the nation's health problems to being "all about fat or all about sugar".

We need to think about both.

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