Iodine in ruminant nutriotion- Mohammad Behroozlak

Iodine in ruminants nutritionBy: M. Behroozlak

Jan. 2014

Introduction

The relationship between iodine nutrition and goitre only emerged in the l9th century.

In 1927, a putative hormone, containing 65% iodine by weight, was isolated from thyroid tissue and called ‘thyroxine’.

Treatment and prevention of goitre with thyroid extracts or iodized salt became routine. Extensive ‘goitrous’ areas were eventually discovered on every continent (Fig. 12.1), often associated with an environmental deficiency of iodine, and these can still cause problems in livestock.

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Dispersion of goitre in world

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Relation between iodine and goitre

The link between geochemical iodine status and incidence of goitre is not straightforward , being complicated by the presence of organic, goitrogenic factors in foods.

Subsequent discoveries of the adverse effects of iodine deficiency on brain development, of implied effects on embryonic viability and of selenium deprivation on iodine metabolism.

This meant that the terms ‘iodine deficiency’ and ‘goitre’ were not synonymous, and ‘iodine deficiency disorder’ (IDD) replaced ‘goitre’ for describing the varied consequences of iodine deprivation.

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1993

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Minimum iodine requirements of livestocks

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Function of Iodine

ORD

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Regulation of thyroid hormones activity

TPO

ID

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Function of thyroid hormones

Thyroid hormones have a thermoregulatory role, increasing cellular respiration and energy production, and have widespread effects on intermediary metabolism, growth, muscle function, immune defence and circulation.

These are particularly important in facilitating the change from the fetal to the free-living stage.

The seasonality of reproduction in ewes is related to seasonal changes in thyroid activity.

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Function of thyroid hormones

As a determinant of metabolic rate, T3 interacts with hormones such as insulin, growth hormone and corticosterone, and regulatory proteins of exocrine origin (leptin).

For example, leptin production by adipose tissue is induced by thyrotrophic hormones and controls appetite; thyroidectomy raises plasma leptin in the ovine fetus.

In the long term, life is unsustainable without a thyroid gland

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Metabolism of Iodine

Whereas phosphorus is recycled via saliva in ruminants, iodine is recycled via secretion into the abomasum.

Unheated soybeans contain a heatlabile factor that can induce goitre by impairing the intestinal recycling of iodine.

Absorbed iodine is transported in the bloodstream loosely bound to plasma proteins.

Excess dietary iodine is excreted predominantly via urine as iodide, but in lactating animals significant amounts can be secreted in milk (Fig. 12.3).

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Relationship between intake and milk secretion of iodine

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Activation of T4

T4 is activated by two selenium dependent deiodinases: ID1, which can be inhibited by propylthiouracil (PTU), and ID2, which is relatively insensitive to PTU.

In early lactation in cows, there is a fall in ID2 activity in the liver and a rise in mammary tissue, accompanied by decreases in plasma T4 and T3 concentrations.

In newborn mammals, serum T4 and T3 concentrations rise immediately after birth following the ingestion of colostrum rich in T4 and T3.

This probably reflects the importance of T4 and T3 to survival in the neonate.

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Sources of Iodine

Soils high in iodine such as boulder clays and alluvial soils generally produce plants richer in the element than iodine-low soils.

However, correlations between soil iodine and plant iodine are often poor because plant species and strains differ widely in their ability to absorb and retain soil iodine.

Proximity to the sea has a major influence on plant iodine, as illustrated in Fig.12.4.

Forages

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Relation of sea & plant iodine

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Sources of Iodine

Cereals and oilseed meals are poor sources of iodine.

Fish meals are exceedingly rich in iodine; animal protein sources are generally intermediate, but can be increased by iodine supplementation of the source animals.

Seaweed may contain as much 4–6 g I kg−1 DM, and its inclusion in the rations of cows and hens greatly increases the milk and egg iodine.

Other foodstuffs

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Sources of Iodine

The iodine content of drinking water largely reflects the iodine content of surrounding rocks and soils and correlates with iodine concentrations in local plants of an area.

Sea water is rich in iodine, containing > 50 μg I l−1, and deposition of marine iodine influences drinking water iodine.

Water

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Sources of Iodine

Iodine concentrations of milk are extremely variable in all species due to the ease with which iodine crosses the mammary barrier.

Winter milk contains over twice as much iodine as summer milk.

Milk iodine is also influenced by stage of lactation: colostrum is two to three times richer in iodine than main milk and concentrations decline towards the end of lactation.

Milk products such as dried skimmed milk, buttermilk and whey usually contain less iodine.

Milk & milk products

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Sources of Goitrogens

Plants (Brassica, white clover, cassava) can contain one or more of about 100 different glucosinolates, along with β-glucosidases such as myrosinase, that release HCN after structural damage to plant tissue, during harvesting or processing.

HCN is converted to thiocyanate (SCN) or isothiocyanate and the process can continue during digestion, through the activity of myrosinases.

Thyroperoxidase inhibitors

The effects of CG goitrogens are attenuated by iodine supplementation.

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Sources of Goitrogens

The synthetic goitrogen PTU can induce IDD by inhibiting ORD1 in mammals, and inhibition can therefore take place outside the thyroid.

Rapeseed meal (RSM) can contain 44 mmol progoitrin kg−1 DM as its major goitrogenic component.

Iodine supplementation does not counter the effects of ORD1 inhibitors, and short-term exposure to them may cause goitre which is not an IDD.

Deiodinase inhibitors

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Iodine Requirements

The minimum ‘iodine requirement’ depends on the criterion of adequacy: requirements for growth are not necessarily those for reproduction and lactation, or for maintenance of thyroid structure and circulating levels of T4.

Furthermore, critical levels for all species are likely to vary with environmental temperature, rate and stage of production and the efficiency of energy utilization.

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Ruminants

The mean requirements are 0.11 mg kg−1 DM in summer and 0.54 mg kg−1 DM in winter for sheep, with a similar winter value for cattle (0.52 mg kg−1 DM).

A significant reduction in thyroid weight in peri-natal calves (from 22.9 to 15.8 g) when the pregnant cow’s iodine intake from a grass silage diet.

Herbage iodine levels of 0.18– 0.27 mg kg−1 DM can sustain normal growth in cattle, milk yield in cows and wool growth in sheep in the summer in New Zealand.

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Biochemical Signs of IodineDeprivation

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Effects of iodine and selenium deficiencies, on cGPX

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Comparison

T3 > T4T1 > T2 T4 > T3

T2 > T1

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Responses of cashmere goats to oral doses of PTU (a goitrin)

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Clinical Manifestations of ThyroidDysfunction

Goitre Thyroid enlargement (in newborn animal)

Impaired fertility Infertility, sterility, poor conception rate, decline in libido and deteriration in semen quality

Impaired embryo and fetal development

Disorders of the integument

Post-natal mortality and growth retardation

Low milk yield

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Goiter in lamb

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Diagnosis of Iodine Deficiency Disorders

Thyroid morphology, histology and composition (Thyroid weight)

Iodine in the bloodstream (Serum iodine, PBI and T4 in bloodstream)

T4 assays

T3 assays

Iodine in urine and milk: Where thyroid dysfunction is induced by CG, urine iodine will

probably be deceptively high and milk iodine deceptively low.

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T3 & T4 state in the first week after birth

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The best diagnosis method

Simple microplate ELISA assays for TSH are now widely used to predict goitre prevalence in infants and should be adopted for livestock.

Adaptive increases in ORD activity clearly indicate imminent thyroid dysfunction and have potential diagnostic value in livestock.

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Iodine Toxicity

Exposure to excess iodine paradoxically results in hypothyroidism because of feedback inhibition of T3 synthesis.

pig & poultry > sheep > cow > horse

Tolerance

300-400 mg/kg DM 50 mg/kg DM

25-50 mg/kg DM5 mg/kg DM

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