For supper tonight, three-quarters of us will be eating mainly stodge - a pile of carbohydrates. One in 10 people on the planet will dine on a mash of cassava root (remember tapioca?), much of Africa will eat starchy porridges of plantain, yam, maize or other grains. Across the tropics, the evening meal will be based on boiled rice - the staple food of a third of all humanity. It's not very Atkins, but it is good, useful food: a solid belly filler, as anyone who's eaten African fufu or ugali will tell you.

Nutritionists say that we should get half our energy from carbs - in Europe that chiefly means potatoes, bread or pasta. The problem arises over the other half. All cultures put something on the stodge - in Scotland, say, you get curry sauce on your chips - and this is crucial. You need protein and fats to survive. But in most sub-Saharan countries and much of poorer America and Asia the protein and oils part of the diet is around 20 per cent - not the 40 or 50 per cent it ought to be. Even in the most meat-heavy supper in our survey - eaten by a street family in Port-au-Prince, Haiti - 14 people shared just 330 grams of goat. And so tonight 800 million people will go to bed chronically under-fed: 200 million of them are children.

So what happened to 'Feed The World'? The Band Aid anthem of 20 years ago has been overtaken by more sophisticated tunes. The calls from protesters at July's G8 meeting came in the language of the new radical geo-economists - for debt holidays and debt forgiveness; reforming the WTO and the IMF; for aid and trade.

But hunger didn't go away. If anything the problem is worse than it was in the Eighties, when Sir Bob woke up the world to the monstrous famines in Sudan and Ethiopia. Child mortality rates have dropped by a third in the last 20 years but, starkly, the world has failed to solve the hunger problem. Not just in the perennial famine areas but also in comparatively rich countries like Nigeria and India - both of which feature in our children's suppers survey - where close to 50 per cent of under-fives are so malnourished that their growth is stunted.

While we in the West worry about obesity in our kids, 200 million children across the world are suffering from chronic or acute malnutrition. That includes one third of all the children in sub-Saharan Africa - a figure that has hardly changed in 20 years. According to the UN agencies, 13 million children will die this year of easily preventable diseases - malaria, pneumonia, measles and diarrhoea - all of whose effects are exacerbated, or made lethal, by poor nutrition. While we have got better at emergency feeding - during wars or natural disasters - grand, cyclical famines like the current crisis in Niger still catch the aid world tragically unprepared. And the less dramatic but universal problem of habitual, corrosive under-nutrition - which annually kills more people than any headline-grabbing crisis - seems as insoluble as ever.

Take Cambodia, now a relatively stable country. It has swallowed billions in aid since the end of its wars 10 years ago, and successfully restarted its economy. It is the newest member of the club of free-trading nations, the World Trading Organisation. Yet 46 per cent of its under-fives are malnourished and stunted. Fewer children die there in infancy now, but the brains and bodies of nearly half its children will not develop properly and they will be prey to recurring illness for their shortened lives.

I met Ssuy Sittha, the mother of three of those children, in a dusty village in March, late in the dry season: a lean time for people all across the Mekong basin. Ssuy Sittha's cupboard - a plank under the banana-leaf roof of her stilt-hut - was pretty much bare. Beside her rice sack was a little jar of prahok - preserved, fermented rice paddy fish. This she mixed with a little raw garlic and some sugar and a pinch of MSG, which is cheaper than salt. There was one chilli left and - with the money from last season's rice crop already gone - no money to buy any more.

Ssuy Sittha's family waited patiently beside the charcoal fire as she boiled the evening's rice, but it was plain that supper was not going to meet anybody's requirements. When it was served, eight-year-old Wan Nut put away a lump of rice the size of his fist in less than a minute - he hardly paused for breath. Ssuy Sittha pushed over the lump of rice from her plate, adding a carefully measured spoonful of fish sauce.

This isn't bad nutrition - it's just nothing like enough and the balance is wrong. Two meals like this a day are meeting perhaps a third of the needs of a growing child like Wan Nut - which is why, at eight, he looked the size of my son at five. Ssuy Sittha - like most mothers we talked to round the world - knows what her children ought to get and a normal rural Cambodian diet is now quite close, in protein/carbs/fats balance, to what a doctor would recommend. But this is the best she can do. There hasn't been enough water to grow vegetables here since January; the rice crop was poor this year because of drought, and the rice price collapsed. Ssuy Sittha's husband left to seek work on the building sites of booming Phnom Penh a few months ago and hasn't been able to send much money back.

There would be little rice and even less protein for the family before the rains came again in May. So when I asked Ssuy Sittha what she thought of Wan Nut's supper she simply picked up one of his skinny wrists and showed it to me, smiling as Khmer people do in adversity. Ssuy Sittha was quite simply too poor to feed her children properly. In our survey, from Afghanistan to Zambia, we found parents struggling to get better food, more diverse food, to their children - saving up to buy a chicken, hoping to get some fruit from visiting relatives. The mothers know very well the consequences of poor nutrition and its way of making lethal what we would consider unthreatening childhood disease.

Outside Kabul, we met seven-year-old Rahila Haider, painfully small like her two younger brothers, but alive on a diet of rice and potatoes. This is sometimes livened up with some yoghurt or a little onion. What would she really like to eat? 'I like apples and also lamb very much,' she said. She did get an apple a while ago, but the only meat is the chicken her family manages to buy once or twice a month - a chicken costs a day of her father's wages on a building site. 'As a mother, I feel ashamed that I cannot give my children proper food,' said Taj Begum, Rahila's mother. 'It's lack of a good diet that has made them stunted - Rahila is nearly eight, but she hardly looks five years old. If there was money, I would have given them milk, fruits and meat,' she went on. 'But they have to be content eating just plain bread.'

Rahila had three more siblings - but they all died in infancy, taken by tetanus, diarrhoea and measles. These tragedies may have saved Rahila's life. 'The policy makers on famine and malnutrition ignore the family planning factor,' says Tim Dyson, professor of population studies at the LSE. 'There's enormous evidence to suggest that children in smaller families tend to be fed better, have better health, be educated better.'

Seven hundred children die in Afghanistan every day, according to Unicef, most of them from treatable illness. Of those that survive beyond five years old, 48 per cent are malnourished. This may mean many things: the children could suffer from anaemia because of lack of iron, or poor growth and night-blindness from lack of vitamin A. Lack of iodine, which causes mental retardation in children, is a common Cambodian problem. If children's protein intake was doubled in Cambodia, 20,000 lives would be saved each year according to the UN.

All the problems of the settled malnourished - as opposed to those displaced by war or disaster - can be traced back, quite simply, to poverty. But the causes of that are complex, and often quite new. Ssuy Sittha's rice crop from the little patch of paddy field behind her house might, in the past, have provided enough extra cash to help her family through the dry season. However the liberalised trade rules forced on Cambodia as part of the deal to let it join the WTO has meant opening its borders to rice imports. These have poured in from Vietnam, where cooperative farming produces much cheaper rice. The price of rice in Cambodia collapsed.

The impact of globalised trading systems can be huge for the settled poor. The path they walk between survival and hunger is narrow and the shock of the new can send them lurching towards disaster. From all over the world come the cries of the small farmers, their fragile worlds upset by economic forces far beyond their control. Hill tribespeople in Vietnam were pushed to rip up their fields in the mid-Nineties and grow a new high-earning cash crop - which was fine until the price of coffee dropped by two-thirds, leaving some four million of them destitute. There are the tomato growers in Ghana who made just enough to feed themselves and send their children to school - until the country, under pressure to join the world trading system, started accepting imports of cheap tinned tomatoes from the European Union. The poultry business in Senegal and other African countries has been destroyed by the dumping, subsidised by the EU, of cheap frozen chicken parts that no one in Europe wants.

The EU's Common Agricultural Policy (CAP) may indeed be one of the worst curses ever to descend on millions of rural poor in Africa, Asia and Latin America (three-quarters of the world's poorest people still live off the land). The CAP's complex system of subsidies and protections for Europe's bloated agricultural sector has created some famous monsters. There is the €2-a-day cow: the average EU bovine earns more in government handouts than the average African farmer gets through honest toil. Adding to the injury, that cow's excess milk is then dumped abroad - an operation the EU also subsidises. As a result, small dairy farmers in countries as far apart as Jamaica and India have been put out of business.

European sugar is another mind-boggling madness. According to Oxfam, subsidies and market restrictions aiding EU sugar (much of it produced by a few rich companies in East Anglia and Germany) has destroyed the livelihoods of thousands of smallholding sugar producers in southern Africa and in parts of Asia - in Mozambique the damage amounts to as much as a third of the value of the aid that the EU annually puts into the Aids-ravaged country. The WTO declared the EU's sugar support policies illegal in April this year, but no action has yet followed.

The Gleneagles G8 meeting agreed new spending on aid, but disappointed the activists (not to mention the developing world) by failing to name a date for an end to agricultural export subsidies and taxes on imports from the poor world. This was a big hole in Tony Blair's triumph. The extra aid is worth $48 billion by 2010, but according to the World Bank, a radical reform of the tariffs and subsidies - most of which hit farmers - would earn Africa alone $269 billion in a single year. The hold-up is due to a long-standing arm-wrestle between the farming lobbies in the US and the EU, neither of whom will budge on subsidy reform unless the other does - Peter Mandelson, the EU's trade commissioner, said recently that calls to do away with the Common Agricultural Policy are 'naive and misplaced'. But it's not American or European farmers who suffer - the collateral damage caused by this multi-billion dollar stand-off hits farmers in Africa, South America and Asia.

For them the promise from the G8 meet is aid, not trade. More handouts. No decent opportunity to make a living by selling produce to the rich world. But food aid can be disastrous. The arrival of sacks of foreign grain and rice in a famine-hit country has many effects beyond feeding the starving. For a start it can result in a drop in the market price of the same products - good news initially for the hungry, but potentially catastrophic over a few months. After the 1997 financial crash in East Asia, Japan and America shipped huge amounts of their own farmers' subsided rice into Indonesia, where the collapse of a government food subsidy scheme was causing shortages in rural areas.

But this glut of foreign rice destroyed the market for local producers, and put tens of thousands of farmers and distributors out of business. Indonesia, once a rice exporter, is now a rice importer - to the benefit of US and Japanese farmers. NGOs representing those forcibly-retired Indonesian rice farmers now suggest the move was deliberate - a Trojan horse sent in by two countries with a lot of subsidised rice to dispose of in order to force access to one of the largest rice markets in the world. It's a vicious business.

It would be wrong, though, to say that globalisation has been all bad for the hungry world. Some of the most spectacular changes for the good during the last few decades have come about in countries who opened up their economies to the outside world. China - home still to more malnourished children than any other country - saw a 49 per cent drop in underweight infants in the 1990s. Mexico, which signed up to the North American Free Trade Agreement (NAFTA) had a 46 per cent reduction. The link, though, is much debated. Nafta is also said to have put 250,000 maize farmers in Mexico out of work, their smallholdings no match for the industrial farms of the American Midwest.

Clearly, though, tackling poverty is the surest way to tackle habitual malnutrition. But how? Most of the middle-of-the-road campaigning organisations, like Oxfam, accept that it must be done by what the lobbyists call pro-poor trade: accepting the realities of globalisation but ensuring its impacts are cushioned for the vulnerable and poor. An example: cash crops are a way that the rural poor can hugely improve their incomes. But it's a risky strategy. 'Bio-diversity is the key to good nutrition,' says the UN's Food and Agricultural Organisation. If one crop fails you can use another. So when the World Bank gives loans to countries to move their farmers into producing crops for export, it must build in safeguards to ensure that farmers can still provide for their families' needs should the market go wrong - as it has in recent years with coffee, cocoa and tobacco.

Science is a bad guy in this debate at the moment, because of the corporate bullying of poor countries to accept genetic modification. But new seed types have made extraordinary changes to parts of Asia: the 'Green Revolution' brought crops to places people never believed could be fruitful. Many agronomists and even some NGO lobbyists now admit that bio-technology has to be allowed to play its part in providing for the world's future - rather as some Greens admit that we may have to tolerate nuclear power if we're to solve the problems around fossil fuels.

Currently, there is enough food on the planet - it's not like water, or oil, which are clearly running out. We grow enough wheat, rice and other grains to give everyone 3,500 calories a day (which is about 50 per cent more than an adult needs). We just don't distribute it properly (for a start, 30 per cent of the grain goes to feed animals). And we could grow more. An English farmer expects to get at least eight tonnes of wheat out of a hectare. An African farmer would be pleased to get one.

According to the radical campaign group Food First, there's enough food to give everyone 4.3lb a day - two and a half of grain, beans and nuts, a pound of fruits and vegetables and nearly another of meat, milk and eggs. The challenge is proper distribution. But the food is on the doorstep of most hungry people. Almost every stable country that has high malnutrition rates actually produces enough to feed its population. But its people don't have enough money to buy it. Or the country needs to sell the food to pay its debts.

But harder-headed analysts think this view - that the food mass is there, it's just the supply system that's doesn't work - impossibly idealistic. Tim Dyson, for one, discounts the view that a switch from meat-eating to vegetarianism is an answer. One of his areas of study is India, a largely vegetarian country where malnutrition levels remain very high, despite the nation's comparative wealth: 'Diversity [of food sources] is terribly significant. Indians consume virtually no meat, and there's ample evidence to suggest that children do less well as a result. Vegetarianism may be OK for kids in the very best conditions, but I can't see that it's a good thing for the poorest.'

Dyson is gloomy about the hunger of the future, and our attempts to alleviate it. 'I think it's unwise to state that the problem is just a matter of distribution. It's about production, and the world population, 6.1 billion now, is going to be as much as 9 billion. People have to recognise that some sections of the developing world will never be able to sustain their populations with their food production. China is a relative success story but look at Mali and Yemen and about 20 other countries where the predicted population growth is enormous. There are large sections of the global poor that are going to have real problems irrespective of the policies of the rich world. And there's no doubt that climate change will cause devastating famines.'

Hunger is still something we can tackle, claim the aid agencies. 'It's as simple as a mother in Bangladesh selling the bananas from the trees behind her home,' an Oxfam worker said to me. 'Her children need to eat them, because they badly need the potassium and the vitamin B6. But she has to pay the money-lender back for the cash she borrowed when her son was in hospital. Don't tell me that sorting out Bangladesh's macro problems - the debt, the trade issues, the bad aid and the corruption - won't help that family.' In a nutshell - or a banana leaf - we do have to make poverty history if we are ever to feed the world.

· If you would like to donate to Action Against Hunger, which fights poverty and malnutrition, call 020 73946300 or visit www.aahuk.org