Nutrition, evolution and thyroid hormone levels – a link to iodine deficiency disorders?

doi:10.1016/j.mehy.2004.02.033

Medical Hypotheses

Volume 62, Issue 6, June 2004, Pages 871-875

https://doi.org/10.1016/j.mehy.2004.02.033 Get rights and content

Abstract

An increased iodine requirement as a result of significant changes in human nutrition rather than a decreased environmental iodine supply is suggested to represent the main cause of the iodine deficiency disorders (IDD). The pathomechanism proposed is based on the fact that serum concentrations of thyroid hormones, especially of trijodothyronine (T3), are dependent on the amount of dietary carbohydrate. High-carbohydrate diets are associated with significantly higher serum T3 concentrations, compared with very low-carbohydrate diets. While our Paleolithic ancestors subsisted on a very low carbohydrate/high protein diet, the agricultural revolution about 10,000 years ago brought about a significant increase in dietary carbohydrate. These nutritional changes have increased T3 levels significantly. Higher T3 levels are associated with an enhanced T3 production and an increased iodine requirement. The higher iodine requirement exceeds the availability of iodine from environmental sources in many regions of the world, resulting in the development of IDD.

Introduction

Iodine deficiency disorders (IDD) is a term used to denote all the effects of iodine deficiency on growth and development. It is widely held that a deficient dietary intake of iodine substrate represents the main cause of IDD: an inadequate iodine supply precludes the synthesis of sufficient amounts of thyroid hormone [1]. Other factors that may play a role in the development of IDD are goitrogens, such as certain vegetable foodstuffs, trace elements, bacterial and chemical water pollutants and genetic factors [2].

A deficiency of the soil is thought to cause the deficient iodine supply. It has been suggested that a depletion of the soil has arisen from the distant past through glaciation, compounded by the leaching effect of snow, water, and heavy rainfall, which removes iodine from the soil. This deficiency of the soil is thought to lead to iodine deficiency in all forms of plant life and cereal grown in the soil [1].

But apart from a low iodine intake, there may be another, more important factor responsible for the problem: an increased iodine requirement due to significant changes in human nutrition during the last 10,000 years.

There is some evidence in favour of this assumption:

•
Serum levels of thyroid hormones, especially of trijodothyronine (T3), are dietary dependent, or more precisely, they are dependent on the amount of dietary carbohydrate.
•
A high-carbohydrate nutrition is associated with significantly higher levels of T3, compared with very low-carbohydrate diets.
•
Our Palaeolithic ancestors subsisted on a very low-carbohydrate/high-protein diet during a long period of human evolution, a diet that is associated with significantly lower T3 levels.
•
Beginning with the agricultural revolution about 10,000 years ago, a dramatic increase in dietary carbohydrate has occurred.
•
The addition of considerable amounts of carbohydrate to a low-carbohydrate diet is associated with a significant increase in T3 concentrations.
•
Increased T3 concentrations are associated with a higher iodine requirement, that – in many regions of the world – exceeds the availability of sufficient amounts of iodine from environmental sources.

Section snippets

Thyroid hormone levels and nutrition

It is well established that serum levels of thyroid hormones are dietary dependent. Several studies have shown that under isocaloric conditions, variations of diet composition can drastically alter peripheral thyroid hormone metabolism [3], [4], [5], [6], [7].

• This has been demonstrated first in studies on starvation: under total fasting conditions, serum T3 starts to decline within 24 h to reach a plateau approximately 50% below control levels within about 4–6 days, while reverse T3 (rT3)

Palaeolithic nutrition and thyroid hormone levels

Significant changes in human nutrition have occurred during human evolution. While in our primate ancestors, carbohydrates in the form of fruits and berries were an important part of their diet, the diet of bipedal hominids became increasingly carnivorous. Early hominids like the Australopithecines, who lived between 3 and 4 million years ago as well as Homo habilis, who appeared about 2.5 million years ago, were scavengers. They subsisted on a mixed diet, consisting of vegetable materials as

Agriculture – significant changes in human nutrition and in thyroid hormone levels

Beginning about 10,000 years ago agriculture began to develop. The agricultural revolution brought a sharp increase in the consumption of plant food, particularly of cereals which were high in starch. The industrial revolution, and especially the development of high-speed steel roller mills in the 19th century offered the possibility to grind cereals very finely, thereby removing almost all of the fibrous indigestible material, and increasing the yield and palatability. At the same time,

A link to IDD?

While other factors may play an additional role, iodine deficiency undoubtedly is the main cause of IDD. In many regions of the world, the environmental iodine supply is not sufficient to meet the iodine requirement in humans.

What is the reason for this imbalance between iodine requirement and iodine supply? Is it really a depletion of the soil as a result of glaciation, as has been proposed? Granted that a depletion of the soil has occurred during the ice ages, this must have been a very slow

Conclusions

•
Substantial evidence is presented that significant changes in human nutrition during the agricultural and industrial revolution must have altered thyroid hormone levels, resulting in an increased iodine requirement.
•
The mechanism described is well suited to explain the development of IDD as a result of these alterations.
•
Other potential consequences of these alterations in thyroid hormone levels should be subject of future investigations.

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Nutrition, evolution and thyroid hormone levels – a link to iodine deficiency disorders?

Abstract

Introduction

Section snippets

Thyroid hormone levels and nutrition

Palaeolithic nutrition and thyroid hormone levels

Agriculture – significant changes in human nutrition and in thyroid hormone levels

A link to IDD?

Conclusions

Metabolism

Am. J. Clin. Nutr.

Metabolism

Am. J. Clin. Nutr.

Metabolism

Lancet

The thyroid and its diseases

Iodine deficiency, other trace elements and goitrogenic factors in the etiopathogeny of iodine deficiency disorders (IDD)

Biol. Trace Elem. Res.

Dietary-induced alterations in thyroid hormone metabolism during overnutrition

J. Clin. Invest.

The role of dietary fat in peripheral thyroid hormone metabolism

Metabolism