EATING fruit and vegetables can double the effectiveness of a low-fat diet in reducing levels of harmful cholesterol, scientists have revealed.

The US-based research has demonstrated that while low-fat meals were healthier, they were not all equally healthy - making it more important to concentrate on what to eat, rather than what to avoid when aiming to lower cholesterol.

Meals rich in whole grains, beans and colourful fruits and vegetables, such as carrots, spinach and broccoli, produced the most dramatic results, scientists said.

Dr Christopher Gardner, from Stanford University Medical Centre in California, who carried out the study, said: “The effect of diet on lowering cholesterol has been really minimised and undermined by a lot of clinicians and researchers saying, ‘yes, it has an effect but it’s really trivial: it would be better to put you on drugs to control your cholesterol.”

Dr Gardner added: “But we think part of the reason was that we were not really giving diet a fair shake. We were so focused on the negative - just what to avoid - and not what to include.”

Such foods as spinach salad, egg, carrots or dark-green salads were a lot healthier than stir-fried lean beef with asparagus and low-fat chocolate chip biscuits or iceberg lettuce and frozen pizza - even when the amount of saturated fat and cholesterol in both was the same.

The study was based on two groups of 60 adults, aged 30 to 65.

One was given a conventional low-fat diet that focused solely on avoiding harmful saturated fat and cholesterol.

The second diet included the same proportions of fat and cholesterol, plus lots of plant-based foods. Those on the high-vegetable diet ate such foods as vegetable soups.

Both diets lowered total and low-density lipoprotein (LDL), or “bad” cholesterol, over a period of four weeks.

But, significantly, while the conventional diet produced, on average, a 4.6 per cent LDL decrease, the plant-based diet lowered LDL by 9.4 per cent.

No significant differences were found in levels of triglycerides, another harmful blood fat, or “good” high-density lipoprotein (HDL) cholesterol.

Dr Gardner, a vegetarian who specialises in nutrition and preventive medicine, said that a plant-based diet needn’t avoid meat, but must also incorporate a foundation of whole grains, vegetables, beans, nuts, seeds and fruits.

He recommended that people seeking to lower harmful cholesterol should “include more whole grains and vegetables and beans and colours - not iceberg lettuce, but red bell peppers and carrots and broccoli and red cabbage and the really colourful foods” in their diets.

“Those are all really low in saturated fat and cholesterol, and they’re really high in other nutrients and phytochemicals that are good for you.”

Dr Gardner said that the study has broken new ground - by proving that it is the plant-based diet which makes the difference.

Previous studies have shown that plant-based diets are effective in lowering cholesterol.

But because those who tend to eat this type of diet also consume less saturated fat than normal, it has been previously assumed that it is the latter which lowers the cholesterol levels.

Dr Gardner said that a plant-based diet combined with weight loss and exercise would achieve even more impressive cholesterol-lowering results.

Plus, he said, people can eat even less saturated fat and cholesterol than the study participants did - with better results.

The findings are published in today’s issue of the Annals of Internal Medicine.