Selenio

Selenium is an essential mineral and micronutrient. It is fundamental to human health and found in many foods. It is found in meat, grain cereals, egg yolk, milk, brazil nuts, mushrooms, garlic and seafood (hence, selenium levels are high in populations with high intake of seafood, like the Inuit population).¹²

Selenoproteins are important constituents of a number of enzymes with a range of functions including antioxidant function, thyroid hormone metabolism, male fertility and immune mechanisms. A decline in blood selenium levels in the UK and other European countries has raised concern about possible public health implications, particularly in relation to cancer and cardiovascular disease. Whilst there is some understanding of the role of these proteins in health and disease (and some interesting theories and research), there are many unanswered questions and much debate about supplementation. More research is repeatedly called for.² La carenza è collegata a una vasta gamma di problemi medici, ma un'assunzione eccessiva può avere effetti tossici e può persino essere cancerogena.³

Quanto selenio è raccomandato?

Le raccomandazioni dietetiche per il selenio variano tra i paesi. Questo perché c'è incertezza su cosa costituisca lo 'stato ottimale di selenio'.⁴ The average daily intake of selenium in in the UK has been falling over a 30-year period. In the UK, the recommended daily allowance (RDA) is 75 micrograms/day for men and 60 micrograms/day for women.⁵In the USA, the RDA is 55 micrograms/day for adults. The World Health Organization (WHO) RDA for selenium is 70 micrograms/day to 350 micrograms/day. Some are recommending supranutritional supplements of 200 micrograms/day to 800 micrograms/day. Toxic effects probably occur at around 40x the RDA.

I livelli di eritrociti, siero e tessuti della glutatione perossidasi (GP) possono essere misurati per valutare lo stato del selenio.⁶Il selenio nei capelli e nelle unghie può essere misurato. Discuti con il tuo laboratorio prima di decidere quale metodo utilizzare.

Dietary recommendations were made over 20 years ago on the basis of blood levels of GP. This intracellular enzyme converts hydroxyl radicals or hydrogen peroxide into water and this requires selenium as a co-factor. Levels of this enzyme have thus been taken to correlate with selenium levels. Since then, over 30 new selenoproteins have been identified and about half of these have an identified biological function.⁷⁸These selenoproteins include:

• Quattro enzimi GP, una classe principale di selenoproteine funzionalmente importanti e le prime ad essere caratterizzate:
  

  • GP classico x 1.

  • GP Gastrointestinale x 2.

  • Plasma GP x 3.

  • Fosfolipide idroperossido GP x 4.

• Thioredoxin reductase (TR), a selenocysteine-containing enzyme important in regulating metabolic activity.

• Selenoprotein P (60% of plasma selenium is held in this form) may have a role in the transport of selenium but also may have other functions.

• Several iodothyronine deiodinase enzymes. These are responsible for conversion of the prohormone thyroxine to the active thyroid hormones (triiodothyronine, or T3).

• Sperm capsule selenoprotein which may be responsible for maintaining integrity of the sperm flagella (and hence motility of sperm).

• Selenoproteina W. Questa è coinvolta nel metabolismo muscolare.

Deficiency of selenium may cause a variety of problems but the health implications of a decline in selenium levels in the UK over 25 years have not been systematically investigated. In general, selenium is thought to be important in a number of varied aspects of health: for a healthy immune system, for a protective effect against some forms of cancer, to maintain and enhance male fertility, for a reduction in cardiovascular mortality and to regulate inflammatory markers in asthma. Research is in progress and there is a lot of interest in the results of this research. Although there are still many unanswered questions, so far selenium has been linked with:

• Contrastare lo sviluppo della virulenza e inibire la progressione dell'HIV verso l'AIDS.⁹22Le sperimentazioni sono limitate e i risultati sono misti, ma è necessaria ulteriore ricerca.¹⁰

• Enhancing sperm motility.¹¹

• Un possibile ruolo nella prevenzione della pre-eclampsia.¹²

• Una possibile relazione tra disturbi dell'umore e carenza di selenio.¹³

• A possible association between elevated selenium intake and reduced cancer risk. The association between selenium status and cancer risk remains enigmatic and epidemiological studies have failed to consistently link low selenium levels with increased cancer risk in men and women.¹⁴ It is possible that there may be gender differences in susceptibility to cancer risk with men being more susceptible to the risk of low selenium. In the Nutritional Prevention of Cancer Trial, daily supplementation of selenium significantly reduced prostate cancer in men.¹⁵ It is thought that the greatest benefit from selenium supplementation will be in low selenium status populations, although the biggest primary and secondary prevention trials - SELECT (= Selenium and Vitamin E Cancer Prevention Iprove) - sono stati condotti negli USA dove lo stato del selenio è buono.¹⁶ Alcuni prevedevano che i risultati SELECT nel 2013 avrebbero mostrato benefici.¹⁷ Tuttavia, la sperimentazione è stata interrotta perché non c'erano prove di un beneficio né dalla vitamina E né dal selenio nella prevenzione del cancro alla prostata.¹⁸ È stata sollevata anche una preoccupazione riguardo a un legame tra l'integrazione di selenio e un aumento del rischio di sviluppare il diabete di tipo 2.¹⁹

• Nell'asma, alcuni studi epidemiologici suggeriscono che carenze di selenio, zinco e altri nutrienti (vitamine A, C, D ed E) possano essere associate allo sviluppo di disturbi allergici e asma.²⁰ Sebbene diversi studi suggeriscano che l'integrazione con selenio possa aiutare i sintomi dell'asma cronica, solo un piccolo studio, secondo la revisione Cochrane, ha mostrato prove di ciò.²¹

• Cardiovascular disease.²²The hypotheses that low selenium concentrations are associated with increased cardiovascular disease and that selenium supplements prevent coronary heart disease are unproven. Selenium supplementation cannot currently be recommended for cardiovascular disease prevention.²³ Deficiency is associated with cardiomyopathy in general, but particularly in Keshan disease, a congestive cardiomyopathy associated with Coxsackievirus and selenium deficiency.²⁴²⁵

• Carenza nei pazienti con traumi e ustioni. Uno studio riporta un miglioramento del recupero con l'integrazione di selenio nei pazienti con traumi e ustioni.²⁶

• La ghiandola tiroidea contiene più selenio di qualsiasi altro tessuto. La carenza di selenio può contribuire a precipitare l'ipotiroidismo e le malattie autoimmuni della tiroide e l'integrazione può contribuire alla prevenzione di queste malattie.²⁷

I fattori di rischio includono:

• Poor dietary intake. Because selenium is found in meat and seafood, vegetarians in areas with low environmental selenium may become deficient.

• Fumo.²⁸

• Stato socio-economico. Livelli più bassi sono stati riscontrati in persone che ricevono benefici statali o con un livello di istruzione inferiore.

• Gli anziani che vivono in case di riposo o case di cura avevano livelli più bassi di selenio rispetto agli anziani che vivono in modo indipendente.

• Gli anziani, in particolare nelle aree dove l'assunzione è bassa.

• Where you live:
  

  • Areas with a low selenium soil environment are associated with deficiency. The elderly in such areas are particularly at risk and one study from New Zealand demonstrated suboptimal levels of selenium in a significant proportion of 103 elderly New Zealand women.²⁹

  • I livelli di selenio sono più alti nel sud dell'Inghilterra che nel nord.²⁸ The selenium status may vary within countries according to factors such as the selenium in the local soil but is also affected by the levels of selenium in imported crops.

• Pazienti in nutrizione parenterale totale (NPT).³⁰³¹

• Pazienti con traumi e ustioni.²⁶

Those at high risk of deficiency should have their selenium status measured. If deficient in selenium, such patients should certainly be given supplements. As with many nutritional components, distinction must be drawn between the basic nutritional requirement to prevent deficiency and supplementation to prevent, for example, chronic disease and cancer.³²³³³⁴ According to a report from the USA, 99% of over 17,000 subjects were 'selenium replete' according to plasma selenium levels and not in need of routine supplementation.³⁵ C'è un diffuso interrogativo sull'uso delle RDA e alcuni dubbi sui metodi di valutazione della carenza in questo come in altri ambiti della nutrizione.³⁶³⁷ Evidence of benefit at supranutritional levels of dietary intake is needed to support what has been called the 'nutraceutical' approach to supplementation. Further studies are needed and are being undertaken to support such beneficial effects.¹⁹³⁴ If such evidence is forthcoming then health professionals will need to use dietary and clinical assessment methods to ensure that those at risk of cancer and inflammatory or infectious disease be appropriately advised about selenium intake.³⁴