Readers are quick to pick us writers up when we neglect any aspect of our task. Quite right, too.

Mr Howard's letter from Hertford in reply to my caveats about the new Flavor Saver tomato, which I deprecated (4 June), is well worth quoting. 'Mr B,' he wrote, 'should be looking into the vitamin C content of the designer tomato. Bred for a shelf-life which may stretch into weeks, and vitamin C being very unstable (indeed, it begins to deteriorate on picking), what is the purpose of it other than not rotting during Euro-transporting, hanging around in the shops where it will look attractive for as long as possible? . . . There is no British standard for (the) vitamin content of foods, and the long-life tomato is surely just the beginning.'

Amen, say I, for Mr Howard is right.

Mind you, vitamins play a narrow and particular role in our eating habits. A little will go a long way. They do not provide energy; they do not become a part of our body's structure. They are, in fact, narrowly specific and, to quote the immortal Harold McGee's On Food and Cooking, work on the 'metabolic processes that regulate growth, tissue replacement, and general cellular activity. They are unlikely to be panaceas . . . and . . . excessive doses can be downright dangerous'.

That does not mean, as scurvy has shown us, that we do not need them and that they are not best absorbed in their natural form. What it does mean is that if we have a balanced diet we need not spend our loot on supplements. Only a very few of us, those with specific medical conditions, need vitamins in excess of those we can obtain from eating properly.

Which brings me back to the tomato, and the proper treatment of vegetables and fruits in general, for there are conditions under which the vitamins natural to fruits and vegetables, and many other food products, are lost. Fat-soluble vitamins A, D, E and K are hardly affected by heat (in cooking).

This is not true of the water-soluble vitamins: the various B vitamins and especially vitamin C. B1 (Thiamine) exists in many foods but is often lost when grains such as rice are cooked in too much water. B2 (Riboflavin), found in meats, eggs and vegetables, is stable in heat but quickly degrades in light (bottled milk is not as good for you as milk in cartons). B12 deficiencies can occur among vegetarians.

However, as Mr Howard points out, vitamin C becomes unstable immediately on fruit being picked. It is further reduced by cutting, and exposure to air, heat, light, alkaline pH in water, traces of metal and so on. As Mr McGee puts it: 'Vitamin C foods should be eaten quickly, after as little processing as possible.'

What this means, in effect, is not that you should eat all your vitamin C sources raw, but that you should be aware that everything you do to any such vegetable or fruit, such as peeling or slicing will cause vitamin loss. If you chop or shred, you lose just about all the vitamin C. If you bake a potato, the enzyme that destroys the vitamin C has longer to act. Steaming or boiling in very little water is best, and not just for potatoes.

But the point Mr Howard raises is important, because most of our vitamin C-rich food is increasingly remote from its source and the Flavor Saver tomato, like our long-stored citrus fruits, has already lost much of its potency by the time it gets to the market.

And the longer it stays there, the more it loses.

I am far from being a food faddist, and I do not lose sleep at night contemplating vitamin deficiencies, for the fact is that what we know by taste and colour and smell is sufficient to preserve us from such deficiencies.

We all know, instinctively, that what is fresh is better. The tomato plucked straight from its vine is one thing; that which nestles in its little box, and has been nestling there for some weeks, is another.