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Last Updated: Tuesday, 29 August 2006, 07:05 GMT 08:05 UK
Getting the best out of your food
By Rebecca Morelle
Health reporter, BBC News

Spinach and juice (BBC)
A glass of orange juice along with spinach boosts iron intake

A balanced diet is the key to a happier, healthier life, so the mantra goes.

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?

Food scientists, working in an area called bioavailability, are trying to answer this question in a bid to discover how people can get the very best out of what they eat.

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.

"Bioavailabity 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."

Pop-Eye's favourite

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 such as spinach.

However, this vegetable contains the mineral in a form that is not so readily absorbed - non-haem iron.

Tomato on tin (Science Photo Library/Cristina Pedrazzini)
Tinned tomatoes are a better source of antioxidants

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).

"You can manipulate to some extent the gastric and small intestinal chemistry by what you eat and combine."

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.

Once you can manipulate how nutrients are delivered into the body, it can help you to tailor a product that can deliver certain attributes
Richard Faulks

"But if you boil or can them the antioxidant potential goes up five or six-fold."

"This happens because the lycopene in the raw tomato has been transformed to trans-lycopene in the cooked version, and trans-lycopene is much more readily absorbed."

Likewise, says Dr Collins, cooking carrots makes the beta-carotene, another form of antioxidant, more available because you break down the cell wall with the cooking process.


A nutrient's relationship with water and oil can also alter bioavailability.

Mr Faulks said: "To absorb fat-soluble nutrients, you have to get them out of the cellular structure, and then they have to be transferred into lipophilic - or fat-loving - carriers in the gut to be absorbed."

Lutein is one such nutrient. Present in spinach and other green vegetables such as kale, broccoli and peas, some evidence suggests it can protect or slow age-related macular degeneration, a leading cause of blindness.

Olive oil
Dressing some raw vegetables with oil helps nutrient absorption

"Lutein is actually absorbed more efficiently if it is eaten with a little bit of fat, the oil helps it to hop onto to the fatty acids in the gut and to be absorbed," said Dr Steer.

Mr Faulks believes bioavailability is a growing area, and something that has the potential to impact on the health-food industry: "Once you begin to understand and can manipulate how nutrients are delivered into the body, it can help you to tailor a product that can deliver certain attributes."

However, says Dr Steer, it is a complex field, especially because different nutrients interact in different ways.

"For a long time breakfast cereals have been fortified with added iron. But they also contain phytates - a form of energy store - which are present in whole-grain cereals, and these actually inhibit the absorption of iron.

"And calcium in dairy products can also inhibit iron absorption. So you have your cereal with iron and phytates, but then you pour on your milk that will also contribute to inhibition of the absorption of iron. It can get very complicated."

Jigsaw puzzle

According to researchers, there are pros and cons of combining food in this way.

By cooking tomatoes to gain antioxidants, vitamin C is degraded. By adding fats to green vegetables, your cholesterol intake is boosted.

And, experts add, more is not always best. High intake of some vitamins and minerals has been associated with risks.

But, says Dr Steer, the more we understand about how the body absorbs nutrients, the better the dietary recommendations we can make.

"It is all about understanding how these nutrients are absorbed in the body, and how much of them we need to reduce or even prevent certain diseases while avoiding any adverse symptoms.

"Understanding bioavailability it is really part of the jigsaw of understanding optimum nutrition."

Food Nutrient Benefit Why it works
Spinach and orange juice Iron Helps prevent anaemia, energy loss, infections Juice turns iron into more easily absorbed form
Canned or cooked tomatoes Lycopene Antioxidant thought to halt cell damage Lycopene turned into more readily absorbed form
Cooked rather than raw carrots Beta-carotene Antioxidant Cooking breaks down cell wall allowing easier absorption
Leafy vegetables dressed in oil Lutein May prevent eye disease Lutein is fat-soluble, so oil aids absorption

Veg 'prevents artery hardening'
18 Jun 06 |  Health
Vitamins 'do not stop infections'
04 Aug 05 |  Scotland


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