Bioaccessibility of selenium, selenomethionine and selenocysteine from foods and influence of heat processing on the same

Highlights

• Bioaccessibility of Se in common food grains & GLV was determined.

• Bioaccessibility of Se in the foods examined ranged from 10% to 31%.

• Concentration of bioaccessible SeMet in these foods ranged from 0.03 to 36.2 ng g⁻¹.

• Heat processing significantly decreased the bioaccessibility of Se, SeMet & SeCys₂.

• This is the first report on the bioaccessibility of Se, SeMet & SeCys₂ from these foods.

Abstract

Selenium (Se) is an essential nutrient with diverse physiological functions. The selenium content of commonly consumed cereals, pulses and green leafy vegetables (GLV) was determined. Bioaccessibility of Se, and its organic forms selenomethionine (SeMet), and selenocysteine (SeCys₂) was also examined, and the effect of heat processing on the same was studied. The bioaccessibility of Se in cereals ranged from 10% to 24%, that of pulses was between 12% and 29%, and of GLV, 10–31%. The concentration of SeMet in the dialysates of the cereals, pulses and GLV ranged from 5.15 to 28.7, 2.7 to 36.2, and 0.03 to 5 ng g⁻¹, respectively. The concentration of SeCys₂ in the dialysates of the foods examined was negligible. Heat processing significantly decreased the bioaccessibility of Se, SeMet and SeCys₂. This is the first report on the bioaccessibility of Se and its major organic forms from commonly consumed staples, and the effect of heat processing on the same.

Introduction

Selenium (Se) is now recognized as an essential micronutrient. Se covalently binds to multiple compounds of biological importance such as selenium salt, selenium derivatives of sulfur amino acids and methylated derivatives of seleno amino acids. (Finley, 2006). Se has gathered interest because of its antioxidant and anticancer properties, and is considered to be of fundamental importance to human health (Thirty, Ruttens, De Temmerman, Jacques, & Pussemier, 2012). It exerts its biological functions through selenoproteins. Around 25 selenoprotein genes in humans are known, which encode selenoproteins having a variety of functions (Castellano et al., 2008, Kryukov et al., 2003). Se is an important component of several major metabolic pathways, including synthesis of thyroid hormone metabolism, antioxidant defense systems and immune functions (Gandhi, Nagaraja, Prabhu, 2013). It is a key trace element required in small amounts in humans for the function of a number of Se dependent enzymes such as glutathione peroxidase (GPX), thioredoxin reductase and iodine deiodinase. GPX acts against lipid peroxidation and against free radicals, thioredoxin reductase is involved in nucleus redox status, while iodine deiodinase is involved in thyroid hormone metabolism (Combs, 2001, Kaur et al., 2014).

In nature, Se exists in two chemical forms – organic and inorganic. Inorganic forms of Se can be found with different minerals as selenite, selenate and selenide as well as in the metallic (SeO) form (Barcelo and Poschenrieder, 2011, Meplan, 2011). Se in foods is an integral part of various organic compounds including amino acids selenomethionine (SeMet) and selenocysteine (SeCys₂). SeCys₂ is mainly available in animal products and SeMet is present in plant foods such as cereals, pulses and green leafy vegetables (GLV) (Whanger, 2002). SeCys₂ obtained directly from the diet or from the degradation of SeMet cannot be utilized, whereas it must be synthesized in the body from the amino acid serine (Sareen & Jack, 2012). SeCys₂ is required for Se dependent enzyme functions.

Plant foods, especially cereal grains, are the major dietary sources of Se in most of the developing/under-developed countries throughout the world, owing to their high intake. The richest animal sources of selenium are organ meat and sea foods (Moreda, Moreda, Romaris, Dominguez, & Rodriguez, 2013), followed by muscle meat, dairy products, and vegetarian sources such as cereals, and fruits and vegetables.

Mustard, mushrooms, Allium spices (onion and garlic), broccoli, and Brazil nuts have the ability to accumulate Se from soil to significantly high levels. The reason for the high content of Se in Brazil nuts is that the protein of these nuts is rich in sulfur amino acids, and SeMet is known to non-specifically replace methionine (Fairweather-Tait, Collins, & Hurst, 2010). Similarly, mushrooms too are rich sources of protein, which could be one of the reasons for their high content of Se (Bhatia et al., 2013). The chemical and physical properties of Se and sulfur are reported to be similar. Therefore, plants tend to synthesize SeMet when Se is available, since they cannot distinguish between Se and sulfur (Lyons, Papazyan, & Surai, 2007). This could be the reason for the high content of Se in sulfur-rich foods such as the Allium and Brassica vegetables.

Bioavailability of Se varies according to the chemical form of the element, being significantly higher for organic forms. The bioavailability of Se is reported to be influenced by certain dietary factors. Vitamins E and A are reported to increase Se bioavailability, while heavy metals and dietary fiber tend to inhibit the same (Dumont et al., 2006a, Fairweather-Tait, 1997). Requirements of Se may increase with a high consumption of saturated fatty acids because of the need of the antioxidant activity of Se (Mahan & Escott, 2000). Dietary fatty acids can influence the susceptibility of cells to oxidative stress, perhaps due to changes in the fatty acid composition of the cellular membranes (Estadella et al., 2013). Since Se is a potent antioxidant, increased consumption of this trace element may be desirable to prevent oxidative stress resulting from a high intake of saturated fatty acids.

In order to estimate the adequacy of Se in the diets, information, not only on the total Se content of the foods, but also on the different forms of Se (organic and inorganic) present in the food would be necessary. An adequate dietary intake of Se would not necessarily mean that the human body could absorb the whole amount ingested (Kapolna, Shah, Joseph, & Fodor, 2007). Since the bioaccessibility of Se depends on the source, dietary advice concerning improvement of Se intake depends on the characterization of this trace element from foods. Hence, the form in which Se is bioaccessible is of fundamental importance (Templeton et al., 2000).

Only a limited number of reports are available on the bioavailability of Se in foods, and these studies are mostly restricted to foods containing high concentration of this trace element. The foods studied include fish and other sea foods (Cabanro, Madrid, & Camara, 2004), selenized yeast (Dumont et al., 2004, Hinojosa et al., 2006), selenized garlic (Dumont et al., 2006b, Moreda et al., 2013), selenized wheat and meat (Govasmark et al., 2010), mushrooms (Chansler et al., 1986, Bhatia et al., 2013), milk (Lihua, Shen, Peter, Joop, & Hendrik, 1996), Brazil nuts (Dumont, De Pauw, Vanhaecke, & Cornelis, 2006), and foods grown in Se-rich soil (Kumar & Krishnaswamy, 1997). Kapolna et al. (2007) have studied the bioaccessibility and bioavailability of Se in raw and cooked Allium vegetables. Although several studies have indicated that Se from wheat is highly bioavailable, and wheat has been recommended as a good source of this element, these studies were carried out in the United States, where the Se content of the soil is high (Thirty et al., 2012).

Selenium content of cereals, pulses and spices commonly consumed in India is reported to vary widely, depending on their variety and locality where they are grown (Kumar and Krishnaswamy, 2003, Sumit et al., 2012). However, there is no data on the bioaccessibility of Se and its predominant organic forms (SeMet and SeCys₂) from food grains and GLV commonly consumed in India. It would also be relevant to see if the bioaccessibility of selenium is also influenced by heat treatment, as practiced in domestic cooking. Such information would be useful to make recommendations for the optimum intake of this trace mineral.

The present investigation deals with the determination of Se content and bioaccessibility of Se, SeMet and SeCys₂ in commonly consumed cereals, pulses and GLV. The influence of heat processing (pressure cooking and microwave cooking) on the bioaccessibility of Se, SeMet and SeCys₂ from these foods has also been studied in this investigation.