Plasma lipoproteins in hyperlipidemic subjects eating iodine-enriched eggs

Iodine fortification of foods and condiments, other than salt, for preventing iodine deficiency disorders

BACKGROUND

Iodine deficiency disorders (IDD) affect close to 1.9 billion people worldwide, and are a major public health concern in many countries. Among children, iodine deficiency is the main cause of potentially preventable deficits of central nervous system development and impairment of cognitive function, as well as goitre and hypothyroidism in people of all ages. Salt iodisation is the preferred strategy for IDD prevention and control, however, in some instances where salt is not the major condiment, alternate vehicles for iodine fortification have been considered.

OBJECTIVES

To assess the effects of fortifying foods, beverages, condiments, or seasonings other than salt with iodine alone or in conjunction with other micronutrients, on iodine status and health-related outcomes in all populations.

SEARCH METHODS

Studies were identified through systematic searches of the following databases from their start date to January 2018: Cochrane Public Health Group Specialised Register; CENTRAL; MEDLINE; MEDLINE in Process; Embase; Web of Science; CINAHL; POPLINE; AGRICOLA; BIOSIS; Food Science and Technology Abstracts; OpenGrey; Bibliomap and TRoPHI; AGRIS; IBECS; Scielo; Global Index Medicus-AFRO and EMRO; LILACS; PAHO; WHOLIS; WPRO; IMSEAR; IndMED; and Native Health Research Database. We also searched reference list of relevant articles, conference proceedings, and databases of ongoing trials, and contacted experts and relevant organisations to identify any unpublished work. We applied no language or date restrictions.

SELECTION CRITERIA

Studies were eligible if they were randomised or quasi-randomised controlled trials (RCT) with randomisation at either the individual or cluster level (including cross-over trials), non-randomised RCTs, or prospective observational studies with a control group, such as cohort studies, controlled before-and-after studies, and interrupted time series. We included studies that examined the effects of fortification of food, beverage, condiment, or seasoning with iodine alone, or in combination with other micronutrients versus the same unfortified food, or no intervention. We considered the following measures: death (all-cause), goitre, physical development, mental development, cognitive function and motor skill development, cretinism, hypothyroidism, adverse effects (any reported by trialists), urinary iodine concentration, thyroid-stimulating hormone (TSH) concentration, and serum thyroglobulin concentration. We included all populations, including pregnant women, from any country.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed study eligibility, extracted data, and assessed risk of bias of included studies.We used random-effects meta-analyses to combine data and generate an overall estimate of treatment effect, when more than one study examined the same outcome measure. The overall effect estimate was calculated as the mean difference (MD) or standardised mean difference (SMD) between the intervention group and the comparison group for continuous outcomes, and as odds ratio (OR) for dichotomous outcomes. We assessed the level of heterogeneity through the I² statistic. We conducted post-hoc subgroup analyses to explore possible sources of heterogeneity, and sensitivity analyses to check the robustness of the findings from the primary analyses. We assessed the quality of the evidence for each outcome using the GRADE framework.Where it was not possible to pool the results in a meta-analysis, we provided a narrative summary of the outcomes.

MAIN RESULTS

Eleven studies met the criteria, providing 14 comparisons, and capturing data on 4317 participants. Seven studies were RCTs, three were cluster non-RCTs, and one was a randomised cross-over design. Seven studies were carried out among school children (N = 3636), three among women of reproductive age (N = 648), and one among infants (N = 33). The studies used diverse types of food as vehicle for iodine delivery: biscuits, milk, fish sauce, drinking water, yoghourt, fruit beverage, seasoning powder, and infant formula milk. Daily amounts of iodine provided ranged from 35 µg/day to 220 µg/day; trial duration ranged from 11 days to 48 weeks. Five studies examined the effect of iodine fortification alone, two against the same unfortified food, and three against no intervention. Six studies evaluated the effect of cofortification of iodine with other micronutrients versus the same food without iodine but with different levels of other micronutrients. We assessed one study to be at low risk of bias for all bias domains, three at low risk of bias for all domains apart from selective reporting, and seven at an overall rating of high risk of bias.No study assessed the primary outcomes of death, mental development, cognitive function, cretinism, or hypothyroidism, or secondary outcomes of TSH or serum thyroglobulin concentration. Two studies reported the effects on goitre, one on physical development measures, and one on adverse effects. All studies assessed urinary iodine concentration.The effects of iodine fortification compared to control on goitre prevalence (OR 1.60, 95% CI 0.60 to 4.31; 1 non-RCT, 83 participants; very low-quality evidence), and five physical development measures were uncertain (1 non-RCT, 83 participants; very low-quality evidence): weight (MD 0.23 kg, 95% CI -6.30 to 6.77); height (MD -0.66 cm, 95% CI -4.64 to 3.33); weight-for-age (MD 0.05, 95% CI -0.59 to 0.69); height-for-age (MD -0.30, 95% CI -0.75 to 0.15); and weight-for-height (MD -0.21, 95% CI -0.51 to 0.10). One study reported that there were no adverse events observed during the cross-over trial (low-quality evidence).Pooled results from RCTs showed that urinary iodine concentration significantly increased following iodine fortification (SMD 0.59, 95% CI 0.37 to 0.81; 6 RCTs, 2032 participants; moderate-quality evidence). This is equivalent to an increase of 38.32 µg/L (95% CI 24.03 to 52.61 µg/L). This effect was not observed in the meta-analysis of non-RCTs (SMD 0.25, 95% CI -0.16 to 0.66; 3 non-RCTs, 262 participants; very low-quality evidence). Sensitivity analyses did not change the effect observed in the primary analyses.

AUTHORS' CONCLUSIONS

The evidence on the effect of iodine fortification of foods, beverages, condiments, or seasonings other than salt on reducing goitre, improving physical development measures, and any adverse effects is uncertain. However, our findings suggest that the intervention likely increases urinary iodine concentration. Additional, adequately powered, high-quality studies on the effects of iodine fortification of foods on these, and other important outcomes, as well as its efficacy and safety, are required.

The incorporation of iodine in thyroid hormone may stem from its role as a prehistoric signal of ecologic opportunity: An evolutionary perspective and implications for modern diseases

To optimize fitness under conditions of varying Darwinian opportunity, organisms demonstrate tremendous plasticity in their life-history strategies based on their perception of available resources. Higher-energy environments generally promote more aggressive life-history strategies, such as faster growth, larger adult size, greater genetic variation, shorter lifespan, larger brood sizes, and offspring ratio skewed towards the larger-sized gender. While numerous mechanisms regulate life-history plasticity including genetic imprinting, methylation, and growth factors, evidence suggests that thyroid hormone plays a central role. Given the pivotal adaptive role of thyroid hormone, the teleology of its dependence on dietary iodine for production remains unexplained. We hypothesize that iodine may have emerged as a substrate for production of thyroid hormone in prehistoric ecosystems because the former represented a reliable proxy for ecologic potential that enabled the latter to modulate growth, reproduction, metabolic rate, and lifespan. Such a scenario may have existed in early marine ecosystems where ocean-surface vegetation, which concentrates iodine for its antimicrobial and antioxidant properties, formed the basis of the food chain. Teleologic parallels can be drawn to the food-chain accumulation of antimicrobials that also exhibit antioxidant properties and promote adult size, brood size, and offspring quality by modulating central hormonal axes. As each higher species in the food chain tunes its life-history strategy based on iodine intake, the coupling of this functional role of iodine with its value as a resource signal to the next member of the food-chain may promote runaway evolution. Whereas predators in prehistoric ecosystems successfully tuned their life-history strategy using iodine as a major input, the strategy may prove maladaptive in modern humans for whom the pattern of iodine intake is decoupled from resource availability. Iodine acquired through sodium iodide supplementation may independently contribute to some biologic dysfunctions currently attributed to sodium.

Effects of insulin resistance and obesity on lipoproteins and sensitivity to egg feeding

OBJECTIVE

This study was undertaken to determine if insulin resistance without and with obesity influences LDL response to dietary cholesterol and saturated fat.

METHODS AND RESULTS

We fed 0, 2, and 4 egg yolks per day to 197 healthy subjects in a 4-week, double-blind, randomized, crossover design. Subjects were dichotomized on body mass index (<27.5 and > or =27.5 kg/m2) and insulin sensitivity (insulin-sensitivity index > or =4.2x1.0(-4) and <4.2x1.0(-4) min(-1) microU/mL), yielding insulin-sensitive (IS, n=65), insulin-resistant (IR, n=75), and obese insulin-resistant (OIR, n=58) subjects. Mean fasting baseline LDL cholesterol (LDL-C) levels were higher in IR and OIR subjects (3.44+/-0.67 and 3.32+/-0.80 mol/L) than in IS subjects (2.84+/-0.75 mmol/L) (P<0.001). Progressive triglyceride elevations and HDL-C decreases were seen across the 3 groups. Ingesting 4 eggs daily yielded significant LDL-C increases of 7.8+/-13.7% (IS) and 3.3+/-13.2% (IR) (both P<0.05) compared with 2.4+/-12.6% for OIR (NS). HDL-C increases were 8.8+/-10.4%, 5.2+/-10.4%, and 3.6+/-9.4% in IS, IR, and OIR, respectively (all P<0.01).

CONCLUSIONS

Insulin resistance without and with obesity is associated with elevated LDL-C as well as elevated triglyceride and low HDL-C. The elevated LDL-C cannot be explained by dietary sensitivity, because the LDL-C rise with egg feeding is less in IR persons regardless of obesity status, probably attributable to diminished cholesterol absorption. The results suggest that dietary management of insulin resistance and obesity can focus more on restricting calories and less on restricting dietary fat.

Thyroid function and other clinical chemistry parameters in subjects eating iodine-enriched eggs

Iodine-enriched (IE) eggs are produced by chickens fed a diet containing kelp. These eggs, which contain an average of 711 micrograms iodine/egg, have been reported to reduce plasma cholesterol in humans and laboratory animals. A modified form of these eggs is under consideration for marketing in the United States. 104 hyperlipidaemic subjects were placed on a low-fat diet for 12 wk. Between wk 4 and 12, approximately half of the subjects were randomized to a dietary control group (n = 53) or a group who ingested one IE egg/day in addition to this diet (n = 51). Some subjects in both groups continued in the study for an additional 4-8 wk. No significant adverse clinical effects were observed or reported, with the exception of one subject who reported an allergic-like reaction soon after beginning egg ingestion. All clinical chemistry values remained within normal limits, and comparisons between the egg group and controls were not significant. Three subjects (two in the egg group and one in the control group) had elevated thyroid stimulating hormone levels during the experimental period. All thyroid function tests remained within normal limits in the remaining subjects. Thus, ingestion of one IE egg of the type used in our study appears to be relatively safe and devoid of clinically significant, short-term adverse effects in healthy individuals.

Long-term compliance with a lipid-lowering diet

Compliance with a lipid-lowering diet was sustained in only 30% of patients following an initial three-month intervention. Additional strategies are required to support long-term adherence.