AS THE world's networks in the trade of food grow ever more sophisticated and the damaging effects of this trade on our environment become clearer, it is increasingly difficult for thoughtful shoppers to make ethical choices. Take fish, for example. The Marine Stewardship Council has given Pacific cod its seal of approval because it comes from a sustainable stock. A British shopper, mindful of the state of endangered fish stocks, should, in theory, seek out the Pacific brand. The trouble is that Pacific cod is air-freighted to Britain and harmful carbon emissions will have been produced to get it here. How is the ethical shopper to decide between the twin evils of air pollution on the one hand and over-fishing on the other?

And then there is the dilemma faced by those who want to support local farmers, but who also feel a sense of duty towards producers in the developing world. Should the shopper choose green beans from the nearby farmers' market or green beans from Kenya available in the supermarket? The Kenyan beans may have accumulated hundreds of environmentally damaging "food miles", but to boycott produce from this part of the world would be to cut one of Africa's economic lifelines.

It is important, first of all, to acknowledge that much of the cost of transporting food huge distances by plane is hidden. For every calorie of carrot flown from South Africa to Europe, 66 calories are spent in fuel. The reason why imported vegetables are often still cheaper than local produce is because airline fuel goes untaxed. This may work out to the shopper's immediate advantage, but the emissions pumped out in each flight do grave damage to our environment. It is not sustainable.

The second thing to bear in mind is that there are often irrationalities in the way food retailers - particularly the larger supermarkets - function. Food that could come from local farmers, just as cheaply as imports, is often neglected. American raspberries, for example, are usually on sale in Britain at the height of our raspberry season.

The solution is not, as some have argued, to force supermarkets to stock only local produce. The free market has served Britain's shoppers fantastically well. The fact that today we have access to a range of products that would have been incredible to shoppers 50 years ago ought to be a source of pride. Nor is the solution to ban the import of products from the poor and distant countries of Africa. On the contrary, we should dismantle the Common Agricultural Policy and establish a fair trading system with those nations.

What is required is for supermarkets, shops and even restaurants to give us much more information about where the food they are selling has come from. Then shoppers can make up their own mind about what would be the most ethical purchase. The boom in demand for organic produce in recent years has shown that the public cares deeply about the environmental impact of the way our food is produced. The natural next step for the ethical consumer is to pay close attention to exactly what it takes to get the food "from plough to plate".