The fruit Smoothie hit our supermarket shelves around five years ago, and in that time sales have soared by around 500 per cent.

So it seems more and more of us are taking advantage of the opportunity of swigging a significant share of our "five a day" in five or six convenient and seemingly nutritious gulps. So what is at the root of the current controversy and confusion over one of the food industry's
best sellers?

The problem here is that we are in real danger of turning one of nature's most pure and simple health foods into a processed product. Many smoothies have been stripped of all fibre, stuffed with sugar and preservatives and pasteurised to within an inch of whatever life is left in them. So what is the impact of this process to the fruit, and ultimately to the consumer?

Fruit is naturally high in sugar, but, as nature is clever, fruit also contains loads of fibre, both soluble and insoluble, in order to mute the effects of the sugars on blood sugar control.

The fibre in the fruit slows digestion and the rate at which glucose enters the blood stream, so avoiding the dangerous spikes in blood sugar which are commonly caused by today's diets high in refined foods. Regular blood sugar spikes can contribute to the conditions which cause weight gain, low energy and diabetes, so it is wise to try to keep blood sugar stable.

Fruit in its natural state does this job pretty well, but when we start messing around with it, removing the fibre and adding extra sugars in the form of concentrated fruit juices, you are no longer dealing with a product designed by nature.

The fibre in fruit, missing in many commercial smoothies, also brings other important health benefits. Fruit contains both soluble and insoluble fibre, both of which do different jobs.

Soluble fibre is responsible for the blood sugar stabilising effect, and also reduces the absorption of cholesterol. Insoluble fibre really looks after your bowel, feeding probiotic (good) bacteria, sweeping out toxins and fats, relieving constipation and may even protecting against bowel cancer.

Many of the other beneficial substances found in fruit, such as vitamins, minerals, phytochemicals and enzymes can only be obtained by eating the whole fruit.

Even so, it must be said that a smoothie, even of dubious quality (there is no legal definition of the word "smoothie") is going to be a far better option than Coke or Lilt, but consumers need to be aware that they are not always all they are cracked up to be.

That said, certain manufacturers stand head and shoulders above the rest, and the king of them all must be Innocent. Innocent smoothies not only contain the whole fruit, but have undergone minimal pasteurisation in order to keep as many of the beneficial nutrients in the fruit as possible active. Interestingly, due to recent lobbying from two of Britain's top smoothie manufacturers, Innocent and PJ's, the Government is considering allowing manufacturers of the highest quality smoothies to claim that they contribute two of the recommended five a day portions.

Fresh, natural fruit and vegetables will always be better for you than any processed product, but if you like smoothies, buy the best from a brand like Innocent.

Fruit juice is better for you than fizzy drinks, but will cause a rise in blood sugar, and should preferably be diluted with water or drunk with fibre rich foods.

Roz Witney is a Clinical Nutritionist at Neal's Yard in Leeds.