Spring 2002 Issue — Selenium Yeast: the Micro Nutrient with Macro Impact

Andrew Kendrick

Overseal Foods Ltd

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Introduction

Selenium is fast becoming one of the most reported nutraceutical ingredients both in the scientific press and also in the mainstream media. More specifically high selenium yeast has established itself as a nutraceutical which combines recognised health benefits and a highly effective delivery system.

Selenium's link to health is two fold, firstly to alleviate dietary related deficiencies and secondly to produce a positive heath effect following selenium supplementation to levels in excess of the Reference Nutrient Intake (RNI).

This article will briefly discuss the health benefits of selenium and also discuss the bioavailability advantages that exist in using high selenium yeast as a source.

Selenium in the diet

Selenium is present in tiny quantities within the human body, generally less than 1mg in total. Living tissue can not manufacture selenium and therefore requires that it be obtained from either the diet or our surroundings in the case of lower life forms. Absorption of selenium by plant root is the point where selenium enters the food chain for animal and human consumption. As such, physio-chemical factors that influence this process could be ultimately deemed to impact on the selenium status of specific population groups. Selenium intake levels in the UK have declined considerably over the last 25 years (see Fig 1) but are generally in line with that of other European countries.

Fig 1. Selenium intake in the UK 1974-1997

The classical dietary source of selenium is from grain derived foodstuffs such as oatmeal, cereals and bread. However the effectiveness of this mechanism for selenium delivery is dependent on the selenium content of the soil where the ingredient grains are cultivated. For example, American wheat based products are a better source of selenium than their North European equivalents because of the higher selenium content in the soil of the main area of cultivation - the Upper Great Plains.

In the UK especially, soil acidity and selenium complexion with iron or aluminium conspire to decrease the amount of selenium available to plants for uptake.

Food sources rich in selenium also include oily fish, liver and oatmeal. Socio-economic changes impacting on current dietary patterns are seen as a potential cause of the current selenium deficiency in the UK population. A number of country/authoritative body derived figures exist for daily selenium intake that is recommended for the maintenance of health. These values have been derived from the expert study of clinical trials or biochemical analysis of the plasma selenium level required for selenoenzyme activity. These include Reference Nutrient Intake (RNI) values of 75µg for men and 60µg for women in the UK. These values establish a level of nutrient intake that will prevent deficiency with the majority of the population. It is interesting to note that from the results of the MAFF Food Surveillance Information, current intake within the UK is only approximately 50% of the established RNI.

Selenium and health

Selenium has been recognised as an anti-oxidant for many years and has been assigned a putative role in mammalian metabolism along these lines. More recently its role as a constituent of the class of proteins known as the "selenoproteins" has expanded our understanding of its role in metabolism. A greater understanding of selenium biochemistry has lead to selenium being viewed as a nutrient with a role to play both in the maintenance of good health and also in the prevention of a range of conditions endemic in modern society.

Selenium deficiencies were first identified in livestock in the 1950s and manifested as reproductive impairment, growth depression, white muscle disease and dysfunction of the heart and skeletal muscle.⁽¹⁾

Within humans chronic selenium deficiency has been reported as Keshan disease (heart disease) and Kashin-Beck disease (deforming arthritis) but these vary depending on the geographical area and additional causative co-factors are suspected.⁽¹⁾

Selenium deficiency has also been epidemiologically linked to loss of immune function, susceptibility to viral infections, reproductive problems, mood dysfunction, senility, accelerated cognitive decline, thyroid/hormone metabolism, cardiovascular disease, asthma and cancer mortality.

There have been a number of clinical trials that have looked at the effect of selenium supplementation on human health. The largest and most publicised was the Linxian Study were the diets of 30,000 rural Chinese adults were supplemented with b-carotene, selenium and vitamin E. The study found that there were 13% fewer deaths from cancer and 9% fewer deaths overall in the supplemented population.⁽²⁾ The dramatic impact of this study was due in part to the long-term poor nutritional status of the subject group compared to a traditional Western population.

A number of other clinical trials that used selenium as the functional ingredient are summarised below:

Study/condition	Summary of findings
NPC study	The Nutritional Prevention of Cancer (NPC) study supplemented 1312 individuals (with a history of skin cancer) with 200µg selenium and reported a 50% decrease in cancer mortality rate compared to a control group.(3)
Oxidative (pancreatitis) disease	Sufferers of recurrent pancreatitis were supplemented with 600µg/day of selenium and a decrease in pain and frequency of attacks was reported.(4)
Immune Response	Patients supplemented with 200µg/day sodium selenite were shown to have an enhanced immune response and an 82% increase in natural-killer-cell activity.(5)
Male Fertility	Sub-fertile men were supplemented with 100µg/day selenium for 3 months. 11% of men in the supplemented group subsequently fathered a child compared to none in the non-supplemented group.(6)
Mood	The subject group was split into two which received either a high (225µg/day) or low (33µg/day) selenium supplement. The level of selenium could be correlated to the psychological mood scores at the end of the study with the subjects given higher levels of selenium showing improved, more positive moods.(7)
Arthritis	Arthritis suffers were given 200µg/day for 3 months after which time they reported reduced pain and joint stiffness.(8)

 

Bioavailability of selenium from high selenium yeast

The terms bioavailability has moved from the scientific arena to that of the public domain and consequently as the use of the term has widened the understanding of it has diminished!

In essence bioavailability describes the ability of the digestive system to release a compound from the food matric for the body to absorb in a form suitable for use in metabolism.

A common perception within popular science is that high selenium yeast offers selenium in a more bioavailable form (called selenomethionine) than that of sodium selenite or sodium selenate because it is non-specifically incorporated into protein. On uptake into the body selenomethionine is catabolised into an inorganic pre-cursor before entering the available selenium pool.

The additional problems with selenite and selenate as supplementary sources of selenium is that they can combine with other dietary components such as Vitamin C, zinc and other minerals in such a way that the selenium can become inactive.

High Selenium yeast is prepared by including inorganic selenium within the growth media for uptake and incorporation into cellular protein. After harvesting the yeast is washed to remove residual inorganic selenium and then heat-treated. The heat treatment process is designed to eliminate biological activity of the yeast whilst retaining the nutrition properties of the product.

Due to the complex analytical procedures required to establish the exact form of selenium scope exists for suppliers to offer high selenium yeast as merely a blend of yeast powder and sodium selenite or sodium selenate. However, reputable suppliers will be able to provide the necessary analytical assurances in order that consumers can be protected.

Conclusions

High selenium yeast has gained increasing significance as a nutraceutical ingredient for supplements both alone and also in combination with additional factors such as the increasingly prevalent ACE mixture (Vitamins A, C and E). This is based on the increasing consumer awareness of selenium as a functional ingredient and also the perception of its enhanced bio-availability over inorganic sources. The influence of bio-availability is one which is key to the supplement and functional food industries, as it will ensure that the consumer perceived 'feel good' factor has at least a chance of being realised with a physiological affect, within the body, following uptake of the nutrient in question.

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