The chronic effects of fish oil with exercise on postprandial lipaemia and chylomicron homeostasis in insulin resistant viscerally obese men

The problem of obesity has become high priority globally. Nearly two thirds of deaths and 46% of the worldwide burden of disease are due to non-communicable, obesity-associated diseases [1]. In 2008 more than one third of the world's adults were overweight, which is a doubling of the prevalence since 1980 [2]. The phenotype of visceral obesity is associated with a cluster of related conditions called metabolic syndrome (MetS), which encompasses both lipid and non-lipid related disorders. These disorders include insulin resistance, pro-inflammatory and pro-thrombotic states and the emergence of an atherogenic lipoprotein profile. Amongst the latter, changes to postprandial lipoprotein kinetics and an increased fasting chylomicron remnant concentration are commonly reported and may contribute to increased coronary artery disease risk in viscerally obese subjects [3, 4].

In MetS, higher rates of chylomicron production coupled with reduced clearance by high affinity pathways (i.e. the LDL receptor) can contribute to an accumulation of pro-atherogenic chylomicron remnants in plasma [5‐9]. Moreover, several studies from our laboratory suggest that restoration of chylomicron homeostasis in insulin resistant obese subjects is more difficult to achieve than for hepatically derived lipoproteins. We found that an acute insulin infusion for six hours postprandially substantially lowered the apo B100 area under the curve but not the chylomicron apo B48 response [10]. In another study, chronically enhancing insulin sensitivity with the provision of metformin, or a peroxisome proliferator receptor antagonist (rosiglitazone) had no effect on fasting apo B48, or postprandial hyperchylomicronaemia [11]. Likewise, a 10 kg weight reduction following moderate energy restriction did not improve chylomicron homeostasis in a similar cohort [12]. Similarly, in dyslipidaemic diabetic subjects, others have found that the fasting apo B48 concentration was not significantly lowered by fluvastatin alone, but when fish oil was concurrently administered, the change was significant [13]. Based on these findings, we suggest that interventions designed to regulate lipoprotein kinetics may need to be more aggressive to modulate the chylomicron fraction, compared to interventions that reduce hepatically derived apo B.

There is interest in unsaturated fatty acids (FA), particularly the n-3 FA from fish oil in lipoprotein metabolism since they suppress secretion of apo B100 by the liver and promote plasma clearance via high affinity pathways [14‐16]. Evidence from animal experiments shows that the chylomicron secretion rate may be up-regulated by habituating to a higher fat diet [17], and the type of fat can also influence chylomicron secretion [18] or fasting concentrations [19]. The n-3 FA could result in secretion of fewer chylomicrons because they significantly suppress intestinal TAG production in several experimental models of the intestine [20, 21]. Furthermore receptor-mediated uptake of chylomicron remnants is significantly enhanced if the particles are enriched in n-3 FA [22]. However, how these n-3 related mechanisms translate into changes in human chylomicron homeostasis is yet to be resolved [23].

Human studies by Gill and colleagues [24] also suggest that postprandial lipoprotein metabolism is improved by exercise in otherwise healthy subjects. They reported in middle-aged men that a single bout of treadmill walking substantially enhanced apo B100 metabolism of the VLDL₁ fraction concomitant with a slight reduction in postprandial apo B48 concentration. It is reasonable to suggest that a more sustained exercise regimen will have a greater effect on apo B48 metabolism due to progressive improvements in muscle vascularisation and greater conversion of TAG-rich lipoproteins to the high- uptake remnant form [25]. Improvements in insulin sensitivity associated with exercise may also stimulate high affinity remnant clearance pathways that are sufficient to promote chylomicron remnant clearance. However, the effects of a chronic exercise regimen, specifically on postprandial apo B48 metabolism in insulin resistant subjects are not reported.

Therefore we intended to investigate the combined interventions of fish oil and exercise on chylomicron metabolism in viscerally obese, insulin resistant men using a chronic study design. The protocol aimed to achieve an exercise volume consistent with optimising metabolic effects [26], and participation rates. The second part of our hypothesis is based on evidence that the n-3 FA have potential to modulate lipoprotein kinetics, at the level of clearance and possibly also secretion. Furthermore acute exercise may also alter apo B48 concentrations [24], but the long-term effect of exercise combined with fish oil, on chylomicron metabolism has not been previously explored in these subjects.

Twenty nine subjects were initially recruited. To begin with 16 subjects participated in the fish oil group and after week four, two subjects withdrew for personal reasons. Data for BF and FO are thus reported for 16 subjects and 14 subjects for the FOX intervention. Thirteen subjects completed the entire intervention for the placebo group (BP, P and PX).

Baseline measurements indicate that the subject characteristics were generally similar between the randomised control and intervention groups, although the latter were modestly younger (p = 0.011) (Table 2). The baseline HOMA score suggested insulin resistance, whilst plasma TAG and HDL cholesterol concentrations indicated dyslipidaemia [27].

This paper reports on the effects of fish oil and chronic exercise on chylomicron and lipid metabolism in men with insulin resistance. Subjects taking fish oil showed a reduced postprandial TAG response to the moderate fat-containing meal after four weeks. Following the 12 week walking program the fish oil group exhibited an even greater postprandial TAG reduction, and a lower fasting TAG concentration. The combined interventions did not decrease the total or LDL cholesterol concentrations, however by the end of the treatment both groups had increased HDL-cholesterol concentrations compared to baseline and each individual treatment at week four. Most significantly our findings suggest that the combined effect of the chronic exercise and fish oil was sufficient to alter the basal metabolism of chylomicrons in these viscerally obese subjects.

The reduction of fasting TAG that occurred when the interventions were combined corresponds with suggestions in the literature that both exercise and fish oil could individually reduce VLDL production [15, 35], mainly by suppressing hepatic lipogenesis. The combined interventions also lowered the postprandial TAG response. A reduction in fasting VLDL production could have facilitated the lower postprandial TAG_IAUC response as a result of less competition with the exogenous TAG for lipid clearance mechanisms. The lipaemic reduction is also likely because of increased lipoprotein lipase activity in adipose tissue following fish oil [36] or in the leg muscle after exercise [24]. As a result of chronic training adaption, exercise is also suggested to promote lipid clearance by increasing capillary vascularisation [37, 38], as was expected after 12 weeks of regular walking. Since chylomicrons are the preferred substrate for lipoprotein lipase, it is likely that our combined interventions affected chylomicron delipidation in the early postprandial phase and then the VLDL that predominates later in the postprandial excursion [5]. Hydrolysis from both hepatic and intestinal lipoproteins is likely to be a pivotal mechanism in the postprandial TAG reduction.

The effect of fish oil significantly lowered the TAG_IAUC response even without exercise. In animals and humans, fish oil accelerates catabolism of VLDL to LDL [36] through enhancing blood circulation to capillary lipases, lipase binding or expression [39‐41]. Such catabolic changes correspond to our results of an increased LDL cholesterol concentration initially after four weeks of fish oil. Others reporting similar findings after four and six weeks of fish oil have proposed that this increase might be in the LDL₁ fraction rather the small dense particles [42, 43] which suggests that measuring plasma lipids following fish oil in the longer term (i.e. chronic interventions) may be more physiological useful than shorter studies. Interestingly we also reported increases in glucose concentration and HOMA score with the four weeks of fish oil, findings which are also reported in the literature. Woodman et al. [44] measured interim blood glucose concentrations at three weeks during their six week n-3 FA trial and discuss a transient adverse trend in glycaemic control, which was corrected by the end of the trial. As did Mori and colleagues [45], who suggest that the variability in the effect of fish oil on glycaemic control may arise from varying degrees of insulin sensitivity between subjects and the presence of other disorders including hypertension and obesity. By the time our subjects had undergone the additional exercise at week 16, the fish oil-associated HOMA increase was abolished, consistent with earlier findings [46]. However based on the present results, it would be prudent to suggest that insulin resistant patients monitor their glycaemic control when commencing fish oil supplements, and incorporate regular, moderate intensity exercise into their lifestyle.

Our key finding was that fish oil plus chronic exercise did modulate the basal metabolism of apo B48, a finding not reported elsewhere in the literature. This finding is important since chylomicron homeostasis is more difficult to modulate than that of hepatic lipoproteins [25]. The fasting apo B48 concentration represents the difference between the constitutive secretion rate and the clearance rate of the lipid-poor remnants. Suggestions from cell, animal and human studies is that fish oil and exercise could modulate both the clearance of apo B48 remnants [22, 47‐52], and also play a role in reducing apo B48 secretion [53].

Our results have shown that a combined fish oil and exercise intervention may be effective in reducing fasting apo B48 in otherwise free-living, insulin resistant subjects. The interaction between fish oil and exercise may be hinged on the fact that our subjects were insulin resistant throughout the study. Insulin resistance may have contributed to an over-secretion of intestinal apo B [5] as well as compromised the receptor-mediated uptake of apo B48 in our subjects. Another feature of insulin resistance is the association with inflammatory markers (e.g. C-reactive protein, prostaglandin PGE₂, pro-inflammatory cytokines) [54] and high oxidative stress [55]. Liver cell culture studies suggest that this stress may impact chylomicron remnant uptake [52]. Therefore it is plausible that by improving our subjects' oxidative status through 12 weeks of regular exercise [56, 57], and long-term fish oil [58‐61] we may have somewhat corrected the defective uptake of chylomicron remnants by the liver in separate but related mechanisms. A further suggestion from cell culture studies is that when the oxidative status of the liver was improved, the degree of improvement to remnant uptake was even more enhanced when remnants were enriched with n-3 FA [52]. This suggestion might allude to why in our human study the basal apo B48 concentration was only lower when the fish oil and chronic exercise were added together.

We therefore conclude that an effective therapeutic strategy for the chylomicronaemia in subjects with MetS is the combination of chronic, regular exercise and a relatively low dose of fish oil. We also note that these benefits occurred even without insulin resistance being completely normalised. The mechanisms through which these two interventions work are likely to be different, which may be why they were efficacious only when used in combination. While our study was not intended to test the mechanisms, further investigation is warranted to decipher the reasons that chronic exercise and fish oil reduce basal apo B48 concentration in insulin resistant men.