Mercury, genes and the link with Alzheimer’s disease

cropped-IMG_7188-1.jpgMethyl mercury, a pollutant produced by various industrial activities, is a potent neurotoxin that has now caused serious contamination issues within our oceans. As a fat soluble molecule, methyl mercury enters the food chain and accumulates in the flesh of the fish that then may end up in our supermarkets. Consuming larger, longer living fish on a regular basis is now known to pose a serious health hazard, especially for children and pregnant women who are consequently advised to limit (or even avoid) the intake of some species such as fresh tuna or marlin.

The accumulation of mercury within the body can have profound long-term effects on the nervous system, and has been linked to a variety of conditions including Alzheimer’s disease where it is believed to play a part in nerve cell death. Lipoproteins, such as high density lipoprotein (HDL) and low density lipoprotein (LDL), are combinations of lipids (fat) and proteins that function to transport fat around via the blood, a function that is generally associated with cholesterol, and therefore cardiovascular health. However, approximately 1 in 7 people carry a gene that causes their body to produce a particular lipoprotein called apoE4, known to play a significant role in the development of Alzheimer’s disease. Those who inherit the apoE4 gene from one parent are three times more likely than average to develop Alzheimer’s disease, with those who inherit the gene from both parents having a tenfold risk of developing the disease (Donix et al, 2010). There are multiple hypotheses as to why those carrying the apoE4 gene are more likely to develop Alzheimer’s than those who carry the apoE3 or apoE2 genes; one such hypothesis regards the role that these lipoproteins play in mercury transport within the body, as mercury accumulation in the brain hasbeen linked to the progression of Alzheimer’s. Like all proteins, apolipoprotein is made of chains of amino acids. Cysteine is of particular relevance, as this amino acid contains sulphur, a member of a class of substances called ‘mercaptans,’ the Latin name for “mercury capture.” Because apoE2, the protective form of apoE, contains two cysteine amino acids, it is particularly efficient at removing mercury from the system. In contrast, apoE3 has only one cysteine, and apoE4 none, making it the most ineffective at removing excess mercury from the body.

Given that fish oils are thought to offer protection against neuronal death and therefore the onset of dementia, it seems that ingesting them in high doses may negate any beneficial therapeutic effects unless they are highly purified to ensure all heavy metals are removed. The growing omega-3 market means there are more products of differing qualities and strengths, and the processes used to isolate and purify oils can also differ quite significantly. It would certainly be advisable to choose fish oil supplements that have been purified under pharmaceutical grade conditions to ensure the product not only offers the best possible health benefits, but can also guarantee to be contaminant free.

Refrences

Dórea JG. Environmental contaminants as biomarkers of fish intake: a case for hair mercury concentrations. Eur J Clin Nutr. 2010 Sep 1. [Epub ahead of print]

Albert I, Villeret G, Paris A, Verger P. Integrating variability in half-lives and dietary intakes to predict mercury concentration in hair. Regul Toxicol Pharmacol. 2010 Aug 27. [Epub ahead of print]

Why breast really is best

Breastfeeding is both natural and safe, giving your baby the best possible start in life. When born, infants have no gut faecal flora (healthy gut bacteria), microbes that play an important role in immune defence. The initial milk produced when breastfeeding is known as colostrum, and is high in a specific antibody (IgA) that plays a vital role in mucosal immunity. By coating the lining of the gastrointestinal tract, IgA helps to protect the newborn until its own gut flora is established, and its own immune system is functioning optimally. Breastfeeding also allows mother and baby to form a unique bond, both physically and emotionally. Ideally, women should breastfeed in order to allow their child to obtain the benefits that colostrum offer, one that is not provided by formula milk. In fact, the Department of Health actually recommends exclusive breastfeeding for the first 6 months of life, and this can continue to benefit the baby along with solid foods for many months after.
However, choosing to bottle feed rather than breast feed is not always simply for reasons of convenience. Indeed, not all mothers are able to breast feed. Such examples are: babies born prematurely, or women who are HIV positive or who are undergoing chemotherapy. Furthermore, women taking specific medications, either prescription or over-the-counter, are often advised against breastfeeding. It is important therefore that formula milk offers the best possible benefits for the newborn. The preferred food for premature infants is fortified human milk, either from the infant’s own mother or from milk ‘banks’. However, the number of surviving children born prematurely has increased substantially over the last two decades and this increase in survival rates is, in part due, to the progess that has developed in enteral nutrition and the development of formulas specifically designed for premature infants. The goal in supplying to these infants is to achieve growth that is similar to foetal growth but that is coupled with satisfactory functional development. Currently the maximum concentration of AA in preterm infant formulas is 0.6% of total fatty acids, DHA is 0.35% of total fatty acids, and the maximum concentration of EPA is no more than 30% of the concentration of DHA.

For many years, baby formula manufacturers have been fortifying and reformulating their products in an effort to produce variations that are ‘equal’ to that of natural breast milk. Formula milk now comes in 3 forms: ready-made, concentrate and powder. Each has its advantages and disadvantages and these generally condense down to convenience and cost. Baby formulas are derived from either cow’s milk or soybean, and are fortified with iron and Vitamin D (which can be lacking in breast milk) and contain a form of sugar called lactose (although lactose free varieties are available). Different brands will generally contain different levels of sugar and protein and some will be fortified with docosahexaenoic acid (DHA) and arachidonic acid (AA), long chain omega fatty acids that are vital for normal growth and development.

The ongoing debate associated with the benefits of breastfeeding over that of bottle feeding centres, in part, on observational findings that link breast milk to higher scores on cognitive tests. These scores are related to head size; it has therefore been hypothesised that breast milk mediates cognitive effects by promoting brain development, particularly that of white matter growth. The two most abundant long-chain polyunsaturated fatty acids in the brain are DHA and AA, where they have a functional and structural role in infant development. DHA is concentrated in the prefrontal cortex of the brain, an area important for association and short-term memory, and in some retinal cells. Concentrations of these fatty acids in human breast milk are relatively consistent during the first year of life, and studies have shown that breast-fed infants have a greater mean weight percentage of DHA and a greater proportion of DHA in their red blood cells and brain cortex than formula-fed infants. Furthermore, cortex DHA in breast-fed infants increases with age, probably due to the length of feeding.

However, human milk is also rich in EPA, another long chain omega-3 that, whilst present in white and gray matter at relatively low levels when compared with DHA, actually plays an important and significant role in myelination and brain function. EPA also plays an important immunomodulatory role by expressing potent virucidal activity, thereby reducing the risk of mother-to-child transmission of infections.

Given the importance that EPA plays in the structure and stabilisation of nerves and brain function as well as its predominant role in the immune system, it is surprising to me that formula milk development has not progressed to include this fatty acid. The rational for this is that EPA competes with AA and that this may cause displacement of AA within membranes. However the research behind individual physiological roles of each fatty acid has developed immensely in the last few years alone and given the presence of EPA within breast milk would suggest to me that there is a role in the development and welfare of the infant. The importance of DHA and AA for brain development is certainly acknowledged by milk formulation companies who have begun in the last few years to add these important fats to their milk products. The Expert Panel recommended that the maximum concentration of AA be 0.6% of total fatty acids, that the maximum concentration of DHA in preterm infant formulas be 0.35% of total fatty acids, and that the maximum concentration of EPA be 30% of the concentration of DHA.

However, commonly available formulas such as Cow & Gate and SMA only include AA and DHA and therefore the only source of EPA for the newborn baby is via its mother’s breast milk, so fully supporting the concept that breast really is best.

Le Huërou-Luron I, Blat S, Boudry G. (2010) Breast- v. formula-feeding: impacts on the digestive tract and immediate and long-term health effects. Nutr Res Rev. 23:23-36.

Puri BK, Stannard JP. (2006). The essentiality of eicosapentaenoic acid in breast milk during human lactation: implications to formula milk manufacturers. AGRO FOOD IND HI TEC. 17:7-8.