We love it. Food companies are refusing to cut it out for fear of losing customers. So just how dangerous can those white crystals really be?

Salt killed Christian Blewitt. The three-year-old boy was found unconscious in his bedroom and spent four days in a coma before doctors switched his life-support machine off. They had never seen anyone with so much sodium in their body. The question is how it got there.

The answer, according to a jury, was spoonful by sickening spoonful, forced into Christian's mouth by his foster parents. Ian and Angela Gay were found guilty of manslaughter, for making him eat up to four teaspoonfuls of salt as a punishment for being naughty at the dinner table.

The Gays deny it. They brought what they say is new evidence to the Court of Appeal last week, even as the Government and health campaigners were arguing about how much salt should be allowed in food. The Food Standards Agency announced new limits for a range of products, but they were far less strict than expected. The FSA had caved in to pressure from the food companies, it was claimed, and thousands of people would go on being ill and dying as a result of having too much salt in their diet.

Back in court, the appeal judges decided to reserve judgment for the time being. So the Gays are are still in prison this weekend, serving five-year prison sentences but protesting against the claim that they made Christian swallow up to 40g of salt - the amount contained in a litre of sea water - causing brain damage.

Michael Mansfield QC told the Court of Appeal there was no evidence Christian had been forced to eat the salt. The case was at the "frontiers of science", he said. And he called Dr Glyn Walters, a retired chemical pathologist, who believes there is a way the salt inside Christian could have built up naturally.

Normally, excess salt is excreted through the kidneys and the urine. But there is a very rare condition in which the brain malfunctions and the osmostats - which are like thermostats but regulate sodium concentration - reset themselves too high. Cases are extremely rare, said Dr Walters, but they do happen. The crown prosecutor argued against a retrial because he said there was nothing new in the evidence at all, as the science behind it had been known for years. The appeal judges have not said when a decision might be expected.

The case against salt as a killer appears more clear cut. Salt was in the tears that were wept for Christian, and the sweat on the brows of the doctors and nurses who tried to keep him alive. It is in our blood. We need sodium chloride to make chemical reactions in our bodies happen and our muscles work. And it makes chips taste great. But salt can also poison us. Christian died in extreme circumstances, but it is said that 26 million people in Britain overdose on salt every day, eating more than is healthy in the long term. Too much salt can cause high blood pressure, which increases the strain on the heart and makes a stroke more likely. More than 200,000 people die in this way every year.

Health campaigners want us all to cut down, and say there are dangerous amounts of salt in processed foods, from sausages and beans to biscuits and cereals. They saw the new FSA targets as a chance to bring down the average amount eaten by an adult every day from nine grams to six.

But the targets were a disappointment. The amount of salt in ketchup, for example, was to have been set at 1.8g per 100g, but after consultation with food manufacturers the FSA decided that such a low level "may not be possible for some traditional recipes" and settled for a limit of 2.4g. The limit on blue cheese was to have been 1.9g, but after hearing about "food and technical issues" the FSA decided it was not possible to set any target at all. The target for snacks such as prawn cocktail shells that children love was set at 3.4g - as salty as a tablespoonful of sea water, and more than a six-year-old should eat in a whole day.

Why does any of this matter? "Because if we lowered the average salt intake to six grams a day we would prevent at least 35,000 deaths a year," says Professor Graham McGregor. As a cardiovascular doctor working in the blood pressure unit at St George's Hospital, in south London, he sees patients every day who need to change their lifestyles and their diets to stay alive, and who have done terrible damage to themselves without knowing it.

Three-quarters of the salt we eat comes from processed food. "I am disappointed with the food industry," says Professor McGregor. "They put the salt in our food that is killing us, and they are refusing to take it out, for reasons that are clearly untrue."

The producers say salt adds taste, texture and long life to their food, is vital to modern manufacturing processes and cannot simply be taken out or reduced dramatically. They say they are doing what they can without going out of business, losing customers or tearing up the production lines: the salt in breakfast cereals has been reduced by a third since 1998, and the same level of reduction has been made in bread, soups, sauces and Kraft cheese spreads and snacks.

Professor McGregor is unimpressed. "The truth is that humans developed in a salt-poor environment, living on about 0.1g a day. There are still tribes living on that and they are very fit, they go chasing after each other's wives, thank you. There is no reason why we cannot live on far less salt."

Sodium chloride has been used as a food preservative since the first prehistoric person discovered that adding salt to meat would stop it rotting so quickly. In warm countries, salt appears naturally as springs, lakes and marshes dry up and leave crystals that can be harvested. Crystals can also be extracted by boiling sea water or leaving it to evaporate in pans. Or salt can be brought up from under the ground using pressurised water, as it is today in Cheshire. The county may as well be renamed Saltshire because it is here that most of Britain's table salt comes from, extracted from underground and formed 250 million years ago.

Salt is woven into the practice and symbolism of many religions, given as a blessing or used to keep away evil spirits - the tradition persists that if you throw salt over your left shoulder you will keep away the devil, or at least ward off bad luck.

Salt has also been a source of power. Roman and Chinese emperors controlled the trade in salt through their lands and raised taxes on it; as did the British Empire, fuelling protests that led to independence for India. We actually eat less salt now than we did under Queen Victoria, when around 18g a day was the average because of salted ham, bacon, other meats and fish. The invention of refrigerators threatened to make it obsolete as a preservative, but then processed food began to invade our kitchens. Salt gave longer life to everything from packet soups to tins of beans, and in many cases it also took the place of the original flavour. But now we are being told to eat far less of it (as well as to get more exercise, cut down on sugar and fat, and eat more fruit and vegetables). Some supermarkets, such as Sainsbury's, have agreed to a scheme labelling food red, amber or green depending on the high, medium or low levels of salt they contain. But others, including Tesco, say that is too confusing and refuse to take part.

Everyone seems to agree that we are eating more salt than is good for us as a nation. Everyone except David Steven. "Salt has been demonised," he says. "Salt is not bad, salt is essential to life. I compare it to water - we all need water to live, but some people have managed to drink themselves to death. Nobody suggests we lower water intakes."

Mr Steven is the operations and technical director at British Salt and speaks for the Salt Manufacturer's Association. "Some people need to eat more salt," he says. "The elderly, for example, if they are suffering from lower appetites; and people who exercise excessively and sweat salt away."

Campaigns to lower our salt intake are pointless and even dangerous, he believes. "We have never seen any evidence to suggest that tens of thousands of people will be saved. Where is the evidence, on the basis of health outcomes?"

Campaigners say there have been genetic studies, tests on animals and medical trials in which people were put on low-salt diets and their blood pressure fell. An outcomes trial would take 50 years, involve 100,000 people and put half of them in danger. The evidence against salt is already as strong as the evidence for a link between smoking and lung cancer, they say.

But Mr Steven does not believe them. "If it were as simple as saying that high salt levels are dangerous, then the country with the highest salt intake in the world - Japan - would have a low life expectancy. But they live longer than us." So for him, at least, the jury is out. But as Ian and Angela Gay wait to hear whether their conviction for poisoning a boy with salt will stand, the rest of us must decide for ourselves how much we really want to swallow.

How salt is mined, processed and served

1. Pressurised water pumped into a salt bed

Most British table salt comes from under Cheshire. A borehole is drilled and water pumped into it. Rock salt dissolves to create brine

2. Brine purified

The brine, which is 25 per cent salt, is sucked out and pumped to an overground purification plant. Calcium and magnesium are removed there

3. Kettles boiled

Steam boils the brine at 147C in the first of six giant steel kettles lined with anti-corrosive material. Water evaporates and salt crystals grow

4. Slurry made

The mix of brine and crystals thickens as it passes through five kettles, heated to successively lower temperatures

5. Slurry spun

After the last kettle the slurry settles as 70 per cent solids. These are spun in a centrifuge, separating the salt from the last of the moisture

6. E535 added

The salt is dried using a stream of hot air before the anti-caking agent E535 is added to prevent it forming into solid lumps

7. Driven away

Dry salt is packed into 25kg bags and taken off to the food industry, or stored in huge silos for use as water softener by laundries

8. On the shelf

Repackers at plants close to where the salt was brought out put it into new containers. Salt can go from the ground to the supermarket within a day

9. Shaken out

The average British adult consumes 9g every day, 75 per cent of it in processed food. The rest is added during cooking or at the table