A FEW years ago, I was driving with some young Muslims in their car through the streets of south London. It was early evening during Ramadan. After checking the time, they pulled over to the side of the road. One produced a box of dates, and another a bottle of water. It was time for the ritual ending of the day's fast.

These young men had been without food or drink for more than 15 hours. In that time they had done a full day's work. They had watched others eating, but they themselves had kept strictly to the rules of the Islamic month of abstinence. I remember thinking how half-hearted the Christian fast of Lent had become by contrast.

For Muslims, fasting is the fourth "pillar" of their faith. It is obligatory, and also spiritually advantageous, in that it enables believers to "guard against evil", and to "develop God-consciousness". For Christians, gluttony is condemned, but fasting, although highly recommended, is not compulsory.

At one time, food and the rhythm of the Christian year were intertwined. Fasts were marked by spiritual solemnity, and frugal living; feast days were both spiritual celebrations and times of good eating. Shrove Tuesday was the last day of unrestricted eating before the Lenten fast. Christians bade farewell to meat: carne vale. The day became known as Fat Tuesday - "Mardi Gras". There was merriment before sobriety; but the Church taught that it was also a day for confession, so that the faithful could enter suitably "shriven" - penitent, and absolved - into the period of fasting.

Once, superfluities and corresponding shortages of food made following the church calendar a practical necessity. Today, in the Western world, we have overcome the problems associated with seasonal gluts of food followed by shortages, so that, for most of us (cosmetic considerations apart), fasting is now an exclusively spiritual discipline.

Some people seek special spiritual merit in the asceticism of extreme abstinence. There are stories of mystics who have shunned food for years, and saints who have reportedly survived on nothing but the communion host.

The side-effects of starvation are both physical and mental. Extreme fasting may produce hallucinations, or, in a religious context, visions. Christ prepared for his ministry through fasting, during which he experienced the three temptations in the form of vivid apparitions.

Although Jesus rebuked the tempter with the words "man cannot live by bread alone," it should be noted that he never said "man can survive without food." While fasting can be a practice done in imitation of Christ, he made sure that the crowds that came to listen to him did not go hungry.

For the believer, what is eaten, and how it is consumed, becomes integrated into the religious life. In Judaism, for instance, religious identity is expressed and emphasised through food. And, if feasting creates fellowship, so, in its own way, does fasting. "There is nothing like being in a Muslim country during Ramadan. . . no ritual, no sense of solidarity to rival it. . . knowing that everyone is abstaining," a Muslim acquaintance once told me.

Fasting tests personal resolve; it heightens appreciation of food, and reminds the abstainer that food is ultimately a gift from God and not to be taken for granted. Abba Poemen, a fifth-century Desert Father, taught that fasting led to a righteous fear of God. "How can we gain the fear of God when we have bellies full of cheese and jars of salted fish?" he asked.

Spiritually, it can be argued that we are not simply what we eat: we are also shaped by how we eat. If we eat with gratitude, if we share food, and we eat in fellowship with others, then our souls are nourished as well as our bodies. These principles, of course, lie at the heart of the eucharist.

Arguably, the secularisation of the West can be observed in our eating patterns. Eating is increasingly done in isolation. Many families no longer have a dining table.

In medieval times, Lenten practice grew out of necessity. The legacy of the Lenten fast lingers in the idea of "giving something up" for Lent; but Lent also offers a time to reflect on our lifestyles, and to review in depth what we eat, how we eat, and how we share the food that is available.

In reviewing our diet, we are led on to examine ourselves, and our relationships with others. St John Chrysostom said: "Let the mouth also fast from disgraceful speech. For what does it profit if we abstain from fish and fowl and yet bite and devour the brothers and sisters?"

Ted Harrison is a former BBC Religious Affairs correspondent.