By Alex Kirby
BBC News Online environment correspondent
World water supplies will not be enough for our descendants to enjoy the sort of diet the West eats now, experts say.
Livestock needs a lot of water
The World Water Week in Stockholm will be told the growth in demand for meat and dairy products is unsustainable.
Animals need much more water than grain to produce the same amount of food, and ending malnutrition and feeding even more mouths will take still more water.
Scientists say the world will have to change its consumption patterns to have any realistic hope of feeding itself.
Losing the race
The World Water Week conference is held annually in the Swedish capital, and is organised by the Stockholm International Water Institute, Siwi. This year's runs from 15 to 21 August.
Siwi says: "With about 840 million people undernourished or lacking a secure food supply today, and another two billion or more people... by 2025, feeding the world's growing population - and finding the water to grow the food - continues to be a basic and sizeable challenge."
A paper to be delivered during the conference, entitled Water: More Nutrition Per Drop, says: "For several decades, the increase in food production has outpaced population growth. Now much of the world is simply running out of water for more production... "
The World Health Organisation calls malnutrition "the silent emergency", and says it is a factor in at least half the 10.4 million child deaths which occur every year.
Anders Berntell, Siwi's executive director, told BBC News Online: "The basic problem is that food is the main global consumer of water, with irrigation taking 70% or more of all the water we use, apart from huge volumes of rainwater.
Grain goes far to feed the world
"The bottom line is that we've got to do something to reduce the amount of water we devote to growing food today.
Upturn in demand
"Animals fed on grain, and also those which rely on grazing, need far more water than grain crops.
"But in the developed world, and in parts of some developing countries, consumers are demanding more meat.
WATER AND FOOD
A kilogram of grain-fed beef needs at least 15 cubic metres of water
A kilo of lamb from a sheep fed on grass needs 10 cubic metres
A kilo of cereals needs from 0.4 to 3 cubic metres
"Of course people should have healthier diets and a higher intake of nutrients: we don't want to stop that.
Slow to dawn
"But it's going to be almost impossible to feed future generations the kind of diet we have now in western Europe and North America.
"Most of us don't appreciate, either politically or personally, the challenge of finding enough water to grow enough food, though in some countries it's a problem of everyday living.
Meat is a treat for the rich
"I think the world's future water supply is a problem that's an entire order of magnitude greater than we've begun to realise."
Mr Berntell said the rich would be able to buy their way out of trouble by importing "virtual water" - the water needed to grow the food they bought from abroad.
He said: "The transport of virtual water is huge. Australians were astonished to find that although their country is short of water, they're net exporters of water in the form of meat."