Dietary Recommendations for Increased Antioxidant Defense

Dietary recommendations that would result in increased antioxidant defense are not inconsistent with accepted recommendations for healthy eating. The recommendation to increase the consumption of plant-based foods and beverages is one that is widely perceived as health promoting, and the consistent and strong epidemiological links between high fruit and vegetable intake and the greater life expectancy seen in various groups worldwide whose diet is high in plant-based foods indicate that more emphasis should be given to this particular dietary recommendation. Vitamin C, vitamin E, various carotenoids, flavonoids, isoflavonoids, phenolic acids, organosul-fur compounds, folic acid, copper, zinc, and selenium are all important for antioxidant defense, and these are found in plant-based foods and beverages such as fruits, vegetables, nuts, seeds, teas, herbs, and wines. Dietary strategies for health promotion should be directed towards optimizing the consumption of these items.

It is recommended generally that at least five servings of fruits and vegetables are eaten each day. This recommendation is based on a wealth of epidemiological evidence that, overall, indicates that 30-40% of all cancers can be prevented by diet. However, it is estimated that most individuals in developed countries eat less than half this amount of fruits and vegetables, and intake by people in developing nations is often very low. Furthermore, the antioxidant contents (both of individual antioxidants and in total) of foods vary widely among different food items and even within the same food item, depending on storage, processing, and cooking method. In addition, the issues of bioavailability and distribution must be considered, and it is of interest to see where dietary antioxi-dants accumulate (Table 3). Vitamin C is absorbed well at low doses and is concentrated in nucleated cells and in the eyes, but relative absorption within the gastrointestinal tract decreases as dose ingested increases. Of the eight isomers of 'vitamin E,' a-tocopherol and 7-tocopherol are distributed around the body and are found in various sites, including skin and adipose tissue. Vitamin E protects lipid systems, such as membranes and lipopro-teins. While a-tocopherol is by far the predominant form in human lipophilic structures, there is limited information on the bioavailabilities and roles of the other isomers. Gastrointestinal absorption of cate-chins (a type of flavonoid found in high quantity in tea) is very low, and, although it has been shown that plasma antioxidant capacity increases after ingesting catechin-rich green tea, catechins appear to be excreted via the urine fairly rapidly. Some are likely to be taken up by membranes and cells, although this is not clear, but most of the flavo-noids ingested are likely to remain within the gastrointestinal tract. However, this does not necessarily mean that they have no role to play in antioxidant defense, as the unabsorbed antioxi-dants may provide local defense to the gut lining (Figure 7).

With regard to plasma and intracellular distributions of dietary antioxidants, if it is confirmed that

Table 3 Dietary antioxidants: source, bioavailability, and concentrations in human plasma

Dietary source

Bioavailability

Concentration

Comment

Ascorbic acid (vitamin C)

Fruits and vegetables,

100% at low doses

25-80 mmoll-1

Unstable at neutral

particularly

(<100 mg)

pH, concentrated in

strawberries, citrus,

decreasing to

cells and the eye

kiwi, Brussels sprouts,

<15% at >10g

and cauliflower

'Vitamin E' (in humans

Green leafy vegetables,

10-95%, but limited

15-40 mmoll-1

Major tocopherol in

mainly a-tocopherol)

e.g., spinach, nuts,

hepatic uptake of

(depending

diet is 7 form, but a

seeds, especially

absorbed

on vitamin

form is preferentially

wheatgerm,

tocopherol

supply and

taken up by human

vegetable oils,

lipid levels)

liver

especially sunflower

Carotenoids

Orange/red fruits and

Unclear, dose and

Very low

Lutein and zeaxanthin

(hundreds)

vegetables (carrot,

form dependent,

(<1 mmoll-1)

are concentrated in

tomato, apricot,

probably <15%

macula region of

melon, yam), green

the eye

leafy vegetables

Flavonoids (enormous

Berries, apples,

Most poorly absorbed,

No data for

Quercetin and

range of different

onions, tea, red

quercetin

most, likely

catechins may be

types)

wine, some herbs

absorption 20-50%,

<3 mmol l-1 in

most relevant to

(parsley, thyme),

catechins <2%,

total

humans health as

citrus fruits, grapes,

dependent on form

intake is relatively

cherries

and dose

high, there is some

absorption, possible gastrointestinal-tract protection by unabsorbed flavonoids absorption, possible gastrointestinal-tract protection by unabsorbed flavonoids increasing defense by dietary means is desirable, frequent small doses of antioxidant-rich food may be the most effective way to achieve this. Furthermore, ingestion of those foods with the highest anti-oxidant contents may be the most cost-effective strategy. For example, it has been estimated that around 100 mg of ascorbic acid (meeting the recently revised US RDI for vitamin C) is supplied by one orange, a few strawberries, one kiwi fruit, two slices of pineapple, or a handful of raw cauliflower or uncooked spinach leaves. Interestingly, apples, bananas, pears, and plums, which are probably the most commonly consumed fruits in Western countries, are very low in vitamin C. However, these, and other, fruits contain a significant amount of antioxidant power, which is conferred by a variety of other scavenging and chain-breaking antiox-idants (Figure 6).

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