If you are in the habit of poring obsessively over food and drink labels, you may have spotted the words 'Contains a source of phenylalanine' following the lengthy list of additives. You might even have wondered what phenylalanine is and whether you should be worrying about it. Those words are starting to appear in a surprisingly wide range of products - anything from yogurt to chewing gum.

A few moments Googling and you might discover that 'phenylalanine' is one of the eight amino acids that we must obtain from our diet. Amino acids are the building blocks of all proteins and are essential for our health, so why the need to flag one of them up on a can of cola?

In 1965, an American chemist called James Schlatter was working on a new drug to treat ulcers. He had managed to join two different amino acids together - phenylalanine and aspartic acid. After absent-mindedly licking his fingers in the lab, he found that this mixture tasted particularly sweet. Dr Schlatter had discovered Aspartame and the commercial potential for this very sweet, low-calorie product was quickly exploited. By the early 1980s aspartame was passed as safe in America and it is now sold around the world under brand names such as NutraSweet, Equal and Canderel. Aspartame is also used as an ingredient in more than 6000 different processed foods and drink and total sales exceeded $1 billion last year.

For a few people, the fact that phenylalanine is unexpectedly found in a soft drink is vital information. These people have a disease called phenylketonuria, or PKU, which affects about 1 in 10,000 people. If PKU is left untreated in babies it leads to mental retardation within a few years. PKU is one of the diseases that doctors look for when they perform the 'heel-prick' test on all newborn babies.

So can the rest of us simply ignore the warning about phenylalanine? If you search for the word Aspartame, you soon uncover a storm of controversy on the Internet. The manufacturers, and the US and European authorities, have declared it safe and there are many weighty studies to support that decision, but there are many people who violently disagree. The allegations against aspartame are numerous and varied - it is blamed for anything from causing migraines to cancer to impotence. None of these claims have been proved beyond doubt but the controversy continues, and so does the research.

The EU Scientific Committee on Food released a report in 2002 stating that after a full review of recent research studies they found "no evidence" that they should change their previous ruling that aspartame was safe. Another large study that ran from 1999 to 2002 at King's College in London looked into the claims that aspartame caused brain tumours, but they found no evidence to support the claim.

At the end of 2005 the aspartame debate ignited again when a group of Italian scientists published their results in the respected European Journal of Oncology. They fed aspartame to rats over a long period and concluded that aspartame was the cause of some types of cancer in those rats.

When we digest aspartame, it breaks down into the two amino acids, but also tiny amounts of methanol and formaldehyde. Both methanol and formaldehyde are known poisons, but the tiny doses were previously considered safe. The Italian scientists suggest that we must look again at what is considered a safe level of aspartame in our diets, especially for children.

A few years ago, the evidence in favour of aspartame was overwhelming, despite the vocal opposition and many individual claims against it. Recent research is beginning to turn the tide. There is no doubt that artificial sweeteners are useful in a calorie-controlled diet and have been a huge benefit to diabetics, but it is worth finding out more about the long-term health risks of all food additives and then it is up to you to make your own decision about potential risks.