A few miles from Rome are the ruins of the old port of Ostia. Bread prices were so stable in antiquity that they were chiselled on to the stone wall of the bakery.

Until this year, the idea of stable prices seemed equally indelible: powerful supermarkets, indecipherable commodities markets and a global patchwork of breathtakingly complex subsidies and tariffs seemed to work perfectly for consumers. Food was cheap and governments seemed to be providing the bread.

It was an illusion. With soaring food prices bringing riots to several countries, the world has reached a shocking realisation: it has suddenly rediscovered the astounding cost of a calorie.

Each calorie we eat conceals hundreds, if not thousands, more that we did not, but we have become used to paying very little for them. The global food system exists to disguise and protect us from this reality. It is only this year that the disguise has slipped.

The food crisis is at heart a political and economic beast and the conference must deal properly with the many confusing paradoxes thrown up in recent months.

Why does the World Trade Organisation force Japan to maintain an unexportable mountain of American rice? Why are Thai farmers taking up arms against the millers in what should be a golden age of rice profits? Does the record-high Russian wheat crop and the 80-year low in the size of Australia's sheep herds speak of irrevocable changes to the Earth's climate?

The conference may produce some workable temporary solutions to all of this — that the world is, for the time being, technically capable of feeding itself puts the onus of solving the crisis on diplomacy rather than agriculture.

But there are some very harsh long-term mathematics involved. Food economists talk in terms of calorie ratios — the total energy per calorie consumed by us that was originally required to produce it.

Into that equation go dozens of hidden energy expenditures, including the transport of seeds, the production and transport of fertiliser, the cost of moving a plough and a combine harvester through a field, and the cost of moving the food from the farm to our mouths.

Corn, for example, has a ratio of 12:1 — 12 calories are burnt for every calorie that reaches our stomachs. Meat, especially beef, conceals an even greater ratio — it takes about 8kg of feed to produce 1kg of beef on our plates. If the cow was corn-fed, that means a total calorie ratio of 96:1 for every bite of steak.

Certainly there are many factors involved in the alarming surge in food prices. Governments suddenly restricting rice exports have brought fundamental unease to a global trading scene where previously the problems were short-lived and harvest-driven.

Speculation has played a potent, but probably exaggerated, role. Biofuels have created strange new dynamics and the market has not taken well to them. Towering over all of this, however, is the $120 price tag on a barrel of crude oil: it is petrochemicals that ultimately provided most of the 96 calories that went into the single calorie of beef.

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