Inadequate Dietary Intake of Zinc

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In general, the risk of inadequate intake of dietary zinc within a population may be associated with the nature of the food supply, and its content and relative bioavailability of zinc. Animal source foods, in particular shellfish, small whole fish, beef, and organ meats such as liver and kidney, are rich sources of zinc. Furthermore, the zinc contained in animal source foods is more highly bioavailable than from plant source foods; the presence of certain amino acids (e.g., histidine, methioinine), or perhaps other unidentified factors, may facilitate the intestinal absorption of zinc from animal flesh foods. Plant source foods, such as most fruits and vegetables including green leaves, and starchy roots and tubers, have relatively low zinc content. While whole grains and legumes have moderate to high zinc content, these foods also contain large quantities of phytate (phytic acid or myo-inositol hexaphos-phate), the most potent identified dietary inhibitor of zinc absorption. The zinc and phytate content, and the phytate:zinc molar ratio of some foods are shown in Table 2.

Plants synthesize phytate, which occurs in highest concentration in seeds and to a lesser extent in vegetative plant parts. Phytate forms chelates with zinc and other minerals; as this compound is largely undigested and is not absorbed, it carries the chelated portion of dietary zinc out of the intestine, thus reducing the amount of zinc available for absorption. The phytate:zinc molar ratio of the diet can be used to estimate the bioavailability of zinc. Populations with a heavy dietary reliance on unrefined cereals or legumes, complemented with only small amounts of zinc-rich animal source foods, will have lower intakes of bioavailable zinc. Although milling cereal

Table 2 The content of zinc and phytate, and the phytate:zinc molar ratio in uncooked foods

Food

Zinc

Phytate

Phytate:zinc

(mg/100g)

(mg/100g)

molar ratio

Cereals

Corn

1.8

800

44

Pasta

0.7

282

40

Rice (milled)

1.1

352

32

Wheat or whole

2.9

845

29

wheat bread

White bread

0.9

30

3

Nuts and legumes

Lentils/mung beans

1.3

358

27

Peanuts

3.3

1760

53

Peas

2.9

1154

39

Red beans

2.9

1629

56

Roots and tubers

Cassava

0.3

54

18

Potato

0.3

81

27

Sweet potato

0.5

50

10

Vegetables

Cabbage

0.1

0

-

Green leaves

0.2

42

21

Onion

0.2

0

-

Tomato

0.1

6

6

Fruits

Banana

0.2

0

-

Coconut

1.1

324

29

Orange

0.1

0

-

Mango

0.0

20

-

Animal source foods

Beef

3.0

0

-

Chicken

1.3

0

-

Eggs

1.1

0

-

Fish

0.5

0

-

Milk

0.4

0

-

Pork

1.9

0

-

grains removes large amounts of phytate, it also removes large amounts of zinc. Thus, among populations with a heavy dietary reliance on refined cereals (e.g., rice) or starchy roots and tubers (e.g., potatoes, cassava) with small amounts of zinc-rich animal foods, the total intake of dietary zinc will be low. In either case, low food intakes due to food insecurity will exacerbate the risk of not meeting daily physiological requirements for absorbed zinc.

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