Cubiertas planas

In western diets 20-25 fold more ω6-PUFA are typically consumed than ω3-PUFA which can result in a greater liberation of pro-inflammatory arachidonic acid metabolites (Simopoulos 2002a). It has been previously hypothesized that the low prevalence of lung disease among Eskimos is a result of their diet high in ω3-PUFA (Horrobin and Manku 1987). This was attributed to the anti-inflammatory role of ω3-PUFA in reducing leukotriene generation. Observational research highlights a relationship between fish intake and decreased risk of asthma (Wong 2005). Despite this, the results from intervention studies have yielded conflicting data.

Significant improvements in daytime wheezing, reductions in concentration of exhaled hydrogen peroxide (marker of airway inflammation), and increase in peak expiratory flow were observed in mild adult asthmatics following 2 months of 200mg·d^-1 of an EPA/DHA mix when compared to a placebo (150 mg·d^-1 of olive oil) (Emelyanov, et al. 2002). However no significant changes in FEV₁ were noted which may be attributed to mild level of asthma exhibited in the participants, and the relatively low daily dose of EPA/DHA. Furthermore the improvements that did occur could not be solely attributed to EPA and DHA as the supplements also contained carotenoids, which can act as antioxidants. As asthma is regarded as a long-term irreversible condition a longitudinal ω3-PUFA supplementation study was conducted over a 12 month period by Dry and Vincent (1991). Twelve asthmatics were randomized to

103 receive 1000 mg·d^-1 of an EPA/DHA supplement, or placebo under double-blind conditions. In the ω3-PUFA group FEV₁ reportedly increased by 23%, significantly greater than that seen in the placebo group (p < 0.005) (Dry and Vincent 1991). This improvement did not however translate to a reduction in medication usage and in addition the authors failed to report the constituents of the placebo. Both of these studies also failed to tightly control for the individuals long-term dietary intake which will likely impact on the habitual ω3:ω6 ratio.

The effects of consuming two different ω3:ω6 ratios (0.1:1 and 0.5:1) each for four weeks in an asthmatic population has been investigated by Broughton et al., (1997). Pulmonary function and urinary 5-series leukotriene excretion were used to assess treatment efficacy following a standard methacholine challenge and baseline saline challenge. The ω3:ω6 ratio of 0.1:1 resulted in a greater bronchoconstrictive response to the methacholine challenges compared to baseline, with a reduction in the provocative dose to induce a fall of ≥ 20% in FEV₁ of 89%. Increasing the ω3 PUFA intake to a ratio of 0.5:1, resulted in a reduction in drop in FEV1 in response to the methacholine challenge compared to baseline and 9 of the 19 participants actually improved. Those showing no reduction in respiratory measures to the methacholine challenge after the period of increased ω3-PUFA intake where referred to as ω3-PUFA responders (Broughton, et al. 1997).

The lower ratio of ω3:ω6 caused an increase in pro-inflammatory 4-series leukotrienes of 13.3 ± 4.5 ng, (p<0.05) in all participants following the methacholine challenge. In contrast, following the higher ω3:ω6 ratio there was no increase in the 4-series leukotrienes. Comparing high and low ω6:ω3 ratios, overall 4-4-series leukotriene excretion were significantly lower (p<0.05) with the high ω3-PUFA intake compared to

104 the low. Leukotriene E5 was markedly increased following the high ω3-PUFA diet, compared with baseline in both responders and non-responders although it was 230%

greater in responders (p<0.05). Higher levels of LTE5 are associated with reduced levels of airway inflammation. With a urinary ratio of LTE₄ to LTE₅ of <1 following the higher ω3 intake, it may help to elucidate the positive mechanisms for alleviating respiratory symptoms (Broughton, et al. 1997). The research highlights the pro-inflammatory nature of skewed ω3:ω6 PUFA in favour of ω6 PUFA can have a significant impact upon inflammation associated with asthma. As such detailed intervention studies that can increase ω3-PUFA intake are warranted in inflammatory conditions such as asthma. Chapter 5 highlights the use of ω3-PUFA as a treatment intervention in asthmatics with EIB.

2.6.2. Prebiotics

Prior to the 1990’s there had been a widespread belief that while food intake may regulate certain metabolic activities associated with micro-organisms, changing the diet had little effect on the overall composition and structure of the microbial communities in the human gut (Macfarlane and Macfarlane 2003, Macfarlane, Steed and Macfarlane 2008). However, it is now recognised that the species composition of the microbiota, as well as many physiological consequences can be modified by small changes in food consumption that includes the introduction of prebiotics into the diet (Macfarlane, Steed and Macfarlane 2008).

The original definition of a prebiotic was proposed by Gibson and Roberfroid (1995) as ‘a non-digestible food ingredient that beneficially affects the host by

105 selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon and thus improves health’. More recently, this has been amended to ‘a prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microbiota that confers benefits upon host well-being and health’ (Gibson, et al. 2004).

Prebiotic oligosaccharides can be classified according to their chemical constituents and degree of polymerisation. In addition to be classified as a dietary prebiotic the ingredient must:

 Resist gastric acidity, hydrolysis by mammalian enzymes and

gastrointestinal absorption;

 Be fermented by the intestinal microflora;

 Stimulate selectively the growth and/or activity of the intestinal bacteria associated with health and wellbeing (Gibson, et al. 2004).

Any food ingredient that escapes digestion in the upper gastrointestinal tract has the potential to be prebiotic, but only certain carbohydrates have shown convincing evidence of prebiotic action. A review of the literature (Macfarlane, Macfarlane and Cummings 2006) highlights that the majority of prebiotic research has focused on inulin, fructooligosaccharides (FOS), and galactooligosaccharides (GOS). FOS and GOS are not sensitive to gastric acid, and do not serve as substrates for hydrolytic enzymes in the upper digestive tract; as such they are capable of stimulating the growth and activity of intestinal bacteria within the lower gut. FOS and GOS have been

106 assessed and achieve all of the above classification criteria (Gibson, et al. 2004, Bouhnik, et al. 2004). Bouhnik et al (2004) compared the prebiotic characteristics of a number of candidate prebiotics in vivo. Following 7 days of consumption, GOS showed the greatest prebiotic effect with a stronger bifidogenic effect than short chain FOS (Bouhnik, et al. 2004). Although this is a rapidly growing research area, the evidence for prebiotic status of other non-digestible carbohydrates (glucooligosaccharides, isomaltooligosaccharides, lactosucrose, polydextrose, soybean oligosaccharides, and xylooligosaccharides) offer preliminary promising data but currently are not sufficient to classify them as prebiotics according to the stated criteria (Bandyopadhyay and Mandal 2014) .

Unlike probiotics where the microbes are introduced to the gut, and actually have to compete against the established colonic communities, an advantage of prebiotics is that they target probiotic bacteria that are already commensal to the large intestine. As such, prebiotics are arguably a more practical and efficient way to manipulate the gut microflora than probiotics. Conversely, if the gut microflora are not present due to disease, ageing, antibiotics, or drug therapy the prebiotic is unlikely to be of benefit and a synbiotic (combined prebiotic and probiotic) approach may be more beneficial (Macfarlane, Steed and Macfarlane 2008).