We have been brought up with a belief in the importance of eating our greens.

An intake of five portions of fruit and vegetables a day is said to improve our health and help in the fight against conditions such as heart disease and cancer.

But is it as simple as that?

As we face the festive period, when food intake traditionally rises, a study has found that if everyone in the UK ate their five-a-day and kept to recommended levels of salt and unhealthy fats, 33,000 deaths could be prevented or delayed every year.

The research, co-funded by the British Heart Foundation, highlights the difference that getting daily five-a-day portions could make to the nation’s health.

The study shows that recommended salt and fat intakes need to be drastically reduced to achieve similar health benefits.

But nutritionist Verner Wheelock, who runs Skipton-based food industry training centre Verner Wheelock Associates, says he’s sceptical.

“The big problem is we don’t know how important diet is in relation to other factors,” says Verner, referring to aspects of lifestyle such as quality of housing, smoking and alcohol consumption.

“Nobody has actually come up with the figures that they can stand by in relation to the impact diet has on long-term health and longevity.”

Verner also believes that reducing fat intake can actually increase obesity. He explains that by reducing fat you can increase the intake of carbohydrates, which increases the glucose in the blood and glucose turns to fat.

“We have actually reduced the fat content in our diet over the last 30 or 40 years, which covers the same time period when obesity has increased. If fat is so bad, why is it that we have got all these problems with obesity?” asks Verner.

Writing in the Journal of Epidemiology and Community Health, the Oxford University researchers behind the recent study base their findings on national data from 2005 to 2007 for the UK.

They used data on deaths from coronary heart disease, stroke and cancers, figures on the food and nutrients we consume, and analysis of previous studies into the contribution of diet to serious illness and premature death.

The information was fed into a model to calculate the number of lives that could be saved if UK dietary recommendations on the consumption of fats, salt, fibre and fruits and vegetables were all met.

The recommendations stipulate the daily consumption of five portions of fruits and vegetables, 18g of fibre, a third of total energy to be provided by fats, with saturated fat comprising ten per cent of this, and a maximum of 6g of salt.

Their calculations showed that the five-a-day recommendation for fruit and vegetables made the largest contribution, staving off more than 7,000 deaths a year from coronary heart disease and almost 5,000 from cancer, resulting in more than 15,000 preventable deaths a year.

The study says that almost 4,000 annual deaths could be prevented by sticking to the recommendations on dietary fibre, while those on fats and salt would save almost 7,000 and 7,500 respectively.

Researchers say that, when taken together, the figures add up to 33,000 lives a year.

Dr Peter Scarborough, from the University of Oxford’s BHF Health Promotion Research Group, which led the research, says: “Meeting dietary requirements would have a massive effect on the health of the nation. According to our model, the biggest impact would be eating more fruit and vegetables.

“But this doesn’t mean you should just stop at five – the more the better. In some European countries like Greece and Italy, they get to five a day easily. Adding fruit and veg into our daily diet is achievable for everyone.”

Earlier this month, however, a different study by Professor Tim Key of Oxford University’s Cancer Epidemiology Unit concluded that there was no convincing evidence that eating five portions of fruit and vegetables a day reduced the likelihood of developing cancer – though it did not look at coronary heart disease and stroke.

Victoria Taylor, British Heart Foundation senior dietician, says well-worn dietary messages shouldn’t be overlooked.

“They could have a huge impact on our health,” she says. “We need to keep promoting initiatives that will help people make healthier choices and improve their diet. Prevention really is the key to improving the nation’s heart health.”