by Rebecca Morelle
Health reporter, BBC News, 29 August 2006
Experts advise us to eat more fruit and veg; boost protein and fibre intake; make sure we get the optimum levels of vitamins and minerals.
But what actually happens to these nutrients once they are inside the body?
Toni Steer, a nutritionist at the Medical Research Council Human Nutrition Research Centre in Cambridgeshire, said: "The idea that we absorb everything we eat just isn't true.
"While you may have a certain amount of a nutrient within a food, what is actually absorbed may be less. "Bioavailability means how much of that nutrient within a food is usefully absorbed."
But, she says, bioavailability is not set in stone, and researchers are working to find ways of manipulating the levels of nutrients that can be absorbed by the body.
"If people are meeting dietary requirements, all of the nutrients they need are probably being absorbed."
"But for people who suffer vitamin or mineral deficiencies, or for those in developing countries where nutrition is poor, research into bioavailability can be very useful."
Take iron - a lot of people are just not getting enough of it, and too little can lead to anaemia and increased susceptibility to infections.
But how we get our iron can impact on the amount we absorb, says Richard Faulks, a senior researcher at the Institute of Food Research in Norwich.
While red meat contains the type of iron - haem-iron - that is most readily absorbed by the body, vegetarians are pointed towards iron-rich foods like as spinach. However, this vegetable contains the mineral in a form that is not so readily absorbed - non-haem iron.
But, explains Mr Faulks, a glass of orange juice alongside your plate of spinach can make all the difference. "Vitamin C in orange juice changes the iron to its non-oxidised state (haem iron) - which is much more readily absorbed than the oxidised iron (non-haem iron).
Conversely, explains Dr Steer, tea and coffee contain compounds called phenols that inhibit iron absorption - so they shouldn't be consumed alongside iron-rich foods.
Raw vs. cooked
Whether your food is raw or cooked can also make a difference.
Tomatoes contain lycopene, a form of antioxidant. Antioxidants have been hailed for their ability to neutralize free radicals, which are linked to ageing, stroke and heart disease.
"If you have fresh tomatoes, they have a total antioxidant potential of about 80," explains Dr Catherine Collins, a dietician at St George's Healthcare NHS Trust.