Zinc

Role of zinc

• Acts as a co-factor for over 70 important enzymes

• Involved in the action of hormones

• Crucial in structure and function of biomembranes

Requirements

For breastfed infants after 3-4 months is <1.0mg

Absorption ranges between 10% and 40%

With net absorption of 20% estimated dietary requirement for weanlings would be about 2-5mg/day

Zn in the diet

• Found in both animal and veg sources

• Zn from animal sources are better absorbed

• Zn in plant foods is bound by phytates

• Abundant in meat, liver, eggs, dairy products, some sea food and vegetables

• Whole grains are good sources but refined are not

Zn deficiency

• Public health problem

• Associated with complications of pregnancy and birth outcomes

• Impaired immune function

• Increased duration and severity of diarrhoea

• Growth retardation

• More prevalent in less developed countries

In long standing deficiency

• Retardation of growth• Dwarfism• Reduction in brain size of offspring• Poor mental development• Decrease tolerance to stress• Membranes are susceptible to peroxidation,

allergens and infections• Impairment of iron transport• Primary hypogonadism• Impaired testicular function

Zinc deficiency leads to

• In children

Retardation of growth

Dwarfism

• In pregnancy

Reduction in brain size of offspring

Poor mental development

Decreased tolerance to stress

(SZn - >550μg/l in 1st & > 700 μg/l in 3rd)

Clinical significance

• More marked in infants and young children: cessation of growth & development, severe failure to thrive if untreated

• Characteristic feature is acro-orificial dermatitis• Mucus membranes are affected: stomatitis and gingivitis

• Diarrhoea• Anorexia• Adverse effects in brain function: depressed mood

• Frequent bacterial and fungal infections: defects in immune systems

• Mild deficiency – likely to commence at the time of weaning

• Deficiency detrimentally affect children’s mental development (cognition) and behaviour

With dietary Zn supplementation

• Growth limiting and mild Zn deficiency can be corrected

• Declining growth percentiles can be partially reversed at an early stage

• During 4-9 months a significant linear growth & wt. gain

• Low birth weight children with 6 months of supp perform higher scores on the locomotion subscale of Griffiths test

On behavior & cognition

• Deficiency affects children’s mental development and behavior

• Low birth weight children have shown higher scores on the locomotion subscale with supp.

• In school children, benefits shown on a wide scale with 10 months of supp.

Factors determine the prevalence

• Quality of the diet

• Incidence of infection

• Physiological state

Factors that increase the risk of Zn deficiency

• Gender – males are more prone (increased requirements due to higher muscle mass)

• Periods of rapid growth• Physiological state – pregnancy• Diets inadequate in Zn – plant based (low)• Low energy intakes – results in low Zn intake• Diarrhoea & infections – increased losses

(common in malnourished children)

Diet & availability of Zn

• Phytic acid in cereals and legumes – potent inhibitor

• Fiber reduces availability

• Reduced phytate content in fermented foods

• Low protein and high iron in meals affects bioavailability

• Low meat intake causes deficiency

• Phytate-Zn molar ratio

Level of phytic acid relative to total Zn can predict Zn deficiency.

Ratio >15 – high risk of Zn def

These diets are based on unrefined

If fermented, the phytate content is reduced, the microbial phytase enzymes break down the phytic acid into smaller cpds, that can not interfere with Zn absorption.

• Competitive interactions between

Zn and Cu & Zn and Fe

• Breast milk is a good source with 41% absorption, Zn is bound to citrate

Zn in BM is 2.3mg at early lactation

1.0mg by 3 months

1.0 – 0.5 by 7 months

• Cow milk inhibits, bound to casein, as such, absorption is 28%