Cardiovascular disease and type 2 diabetes are associated with obesity and are also linked to inflammation and oxidative stress 1 . Weight loss is effective in reducing these conditions 2 , however, as only 1 in 5 overweight people successfully maintain weight loss 3 , alternative dietary strategies to improve health without weight loss are desirable.

Most of each day is spent in the postprandial state. Increases in blood glucose and/or triglycerides following a meal stimulate oxidative stress, impair endothelial function, and cause a rise in circulating inflammatory factors 4 . Research suggests that the negative postprandial responses are exaggerated in obesity and diabetes 4 5 6 . The repeated acute stresses induced by food ingestion (particularly high fat and/or high calorie meals) may contribute to acute cardiac events and/or stimulate further development of atherosclerosis 7 8 . Although most clinical evaluation of cardiovascular disease risk is based on fasted blood values, muting of the excursions in atherogenic factors during the postprandial period could have important health consequences 9 .

Previous research has shown that chronic ingestion of specific fats, such as saturated fats, increase cardiovascular disease risk while other fats, including long chained omega 3 fats (n-3FA), reduce risk 10 11 . The differential effect of these fats on chronic disease risk has been hypothesized to be mediated by unique effects on blood lipids, hemostasis, endothelial function, or inflammation 12 . For example, Tholstrup et al 13 observed different postprandial lipemic responses when they evaluated six meals differing in type of fat. In general, the longer and more saturated fats caused delayed and lower increase in plasma fats. One laboratory recently reported that acute monounsaturated fat ingestion caused more impairment of flow mediated vasodilation than a high saturated fat meal 14 and another group showed that inclusion of a fish oil supplement helped to preserve endothelial function following a high fat meal 15 . Another group also reported improved vascular reactivity following a fish oil meal compared to a mixed fat meal, with a potential role for reduced oxidative stress 16 .

Oxidative stress is hypothesized to be a significant mediator of impairment in postprandial endothelial function as well as a stimulator of the inflammatory response following a high fat meal 17 18 . For example, Nappo et al 19 and Carroll et al 20 reported that ingestion of antioxidant vitamins blunted or eliminated the postprandial rise in inflammatory factors in type 2 diabetics. Since specific types of FA can differentially affect oxidative stress due to differences in chemical susceptibility to oxidation, it is of interest to determine whether acute ingestion of fats differing in number of double bonds influences the postprandial inflammatory response. Bellido et al 21 observed an increase in activation of a redox-sensitive transcriptional factor, NF-kB, in peripheral blood mononuclear cells following a meal high in butter or walnuts but not olive oil. This is intriguing and requires additional study to determine specific fatty acid effects, and information on the effects of meals specifically enriched with n-3FA. The acute effects of n-3FA on oxidative stress and inflammation have not been extensively studied, especially in overweight individuals who may be more susceptible to inflammatory stimuli.

In summary, diets high in n-3FA are typically associated with lower systemic markers of inflammation in many epidemiological studies 11 , but there has been limited research with conflicting results concerning the acute effect of n-3FA ingestion on inflammatory response after a meal. As most of the day is spent in the postprandial state, and obese individuals experience greater oxidative stress and inflammatory responses to high fat meals compared to lean individuals, it is of interest to determine whether meal composition affects the postprandial inflammatory response. The objective of this study was therefore to clarify the role of different sources of fat in a high fat meal on inflammation and oxidative stress in overweight and obese adults.

The key findings of this study were that the type of fat in a high fat meal differentially influenced levels of the redox sensitive transcription factor NF-κB and the soluble adhesion molecule ICAM-1 in overweight and obese individuals. The observation in our study that a meal containing a substantial dose of n-3FA (DHA +EPA) led to higher levels of NF-κB than a meal high in saturated fat is intriguing since generally epidemiological and in vitro studies support an anti-inflammatory effect of these fats 12 . For example, pre-exposure to n-3FA reduced pro-inflammatory responses in vascular endothelial cells 27 . However, other in vitro studies have reported that exposure to EPA increases or prolongs cell-stimulated NF-κB activation 28 29 It is not necessarily surprising that the acute responses to n-3FA are different than to chronic ingestion because repeated supplementation allows for the incorporation of n-3FA into cell membranes. n-3FA partially replace arachidonic acid and result in the production of the less prothrombic and inflammatory n-3FA-derived metabolites than those produced from arachidonic acid 11 30 . Cell exposure after acute ingestion is likely too brief to result in substantial membrane incorporation. Thus, the targets are more likely rapid reactors such as transcriptional factors. NF-κB is a redox sensitive transcription factor that regulates the gene expression of many inflammatory proteins and has recently been shown to contribute to endothelial dysfunction in overweight and obese adults 31 .

Interestingly, while the activation of NF-κB is considered a pro-inflammatory signal, we did not observe a subsequent increase in circulating inflammatory marker concentration following the high n-3FA meal (O3FA). Aljada et al 24 observed an increase in nuclear NF-κB binding in PBMC as well as expression of enzymes that activate this transcriptional factor within an hour of ingestion of a mixed meal. This was followed by a 28% increase in plasma CRP 3 h after the meal but no change in sICAM. The lack of association between upregulation of NF-κB by O3FA and inflammatory proteins in our study may be related to an insufficient duration of assessment or lack of measurement of other important transcriptional factors (e.g. AP1) or inflammatory factors (e.g. IL-6). For example, an increase in ICAM-1 following a butter-rich meal was not detected until 9 h postprandial 21 , and another study showed postprandial effects at the mRNA level 32 . Evidence that other transcriptional factors may be affected by fatty acids was provided by a study demonstrating that the saturated fat-induced IL-6 release from cultured cells was independent of the NF-κB pathway 33 .

Few clinical trials have examined the acute inflammatory and signaling responses to different fatty acids in humans. Similar to our study, Bellido et al 21 reported that a high fat meal containing polyunsaturated fats increased NF-κB. However, unlike our study; they noted the same effect 3 h after ingestion of a high saturated fat meal. It is possible that differences in fat sources played a role, as they used butter as the saturated fat source, while we used palm oil. Butter contains substantial amounts of medium chain fatty acids and half the proportion of palmitate as palm oil. This difference in palmitate content of the fats could help explain differences in results since in vitro studies have shown that lower levels of palmitic acid induce activation but higher levels can result in suppression of NF-κB 33 .

The discrepancy between the view of n-3FA as anti-inflammatory 34 35 and our observed increase in NF-κB following acute n-3FA could be related to an adaptive effect that occurs with repeated exposure to n-3FA. It is possible that acute increases in NF-κB following n-3FA consumption are transient and occur when the individual is relatively naïve to n-3FA (low habitual dietary intake). Acute activation of NF-κB in this situation could serve to upregulate antioxidant status over time 36 and buffer sensitivity to inflammatory stimuli. This is similar to the effects of acute bouts of exercise which are shown by some studies to acutely increase inflammation and oxidative stress, but chronically reduce systemic inflammation 37 . Future research into the mechanisms by which the high n-3FA meal increased NF-κB acutely and whether this response changes over time with repeated ingestion is necessary to better understand the health effects of dietary n-3FA.

Chronic ingestion of n-3FA has been reported to increase indicators of oxidative stress in some studies 38 39 but not others 40 41 . Few studies have evaluated the effects of acute ingestion of n-3FA on oxidative stress but the presence of many double bonds susceptible to oxidation theoretically provides a rationale for increased acute oxidative stress. Hall et al 42 observed a 48% increase in plasma 8-epi 6 h following ingestion of high fat meal containing 5 g of n-3FA. Plasma concentration of this indicator of lipid peroxidation did not significantly increase following our high n-3FA meal but the average level tended to rise at 4 h postprandial. It is possible that our sampling period or our lower dose of n-3FA were insufficient to detect a change in this marker of oxidative stress.

ICAM-1 is constitutively expressed on the surface of endothelial cells and its release into the blood increases in response to inflammatory stress 43 . Our study supports the observed increase in ICAM-1 following acute saturated fat ingestion by lean individuals consuming a meal high in butter 21 and another report of lower postprandial ICAM-1 (AUC) for an olive oil meal compared to a higher saturated fat meal 44 . Chronic consumption may also influence postprandial response to these fats as a high monounsaturated fat diet for 12 weeks prior to an acute monounsaturated fat challenge had a favorable effect on postprandial ICAM-1 levels compared to 12 weeks of a higher saturated fat diet prior to a high saturated fat meal in metabolic syndrome patients 45 . Although the predictive role of ICAM-1 in CVD risk is unclear, it has been associated with future myocardial infarction risk 46 , thus, the mechanism and implications of a postprandial increase in ICAM-1 deserve further study.

In regards to postprandial effects of high fat meals on triglyceride (TG) levels, our findings are contrary to Zampelas et al who showed that a meal high in fish oil reduced postprandial TG 47 . On the other hand, Jackson et al did not show differences in acute postprandial TG levels following an n-3FA rich meal in comparison to other fats 48 . Clearly, there is insufficient evidence in this area; and our results add to the little information available regarding the acute effects of n-3FA on postprandial TG. Interestingly, we also noted lower levels of postprandial TG following SFA compared to MFA. This is in line with Pacheco et al 44 who also noted lower TG over an 8 h period following a high palmitic sunflower oil meal compared to an olive oil meal.

One criticism of our study could be a relatively small number of subjects. However, a number of other studies reporting postprandial inflammatory response differences among different meals used a similar sample size 20 21 24 , and since all subjects served as their own controls, variability between treatments was minimized. It is unlikely that testing more similar subjects would provide a different result for those factors that showed almost parallel responses following meals (e.g. VCAM-1, CRP). We did, however, calculate the number of subjects that would be necessary to achieve statistical difference between meal trials with 80% power for TNF-α and 8 epi. A total of 359 and 74 subjects would have been needed to allow determination that the trials caused differential responses (area under the curve) in TNF-α and 8 epi, respectively. This was not feasible and suggests that any difference in effect of the fats on these variables is very modest, if it exists at all, and that the variability in the measures is substantial. It is recommended that future studies including these measures attempt to reduce variability by choosing subjects with similar baseline inflammatory markers to attempt to address this. Our results may also only be applicable to overweight and obese individuals, although this is quite a large proportion of the population.

Overall, consumption of a high fat meal in our overweight subjects resulted in mixed effects on inflammatory markers, most of which were not robust. While some studies have shown that high fat meals lead to acute increases in some markers of inflammation and oxidative stress 19 49 , others report inconsistent responses 6 50 51 . It is possible that a longer sampling period or measurement of gene expression and other gene products that may be more responsive to postprandial stresses was needed. For example, one lab reported significant increases in mRNA for TNF-α 52 , transcriptional factors (AP-1, Egr-1), and matrix metalloproteinases 53 following acute glucose ingestion and another reported increased IL-6 mRNA following high fat meals 32 . In addition, studies reporting robust inflammatory responses to meals typically used subjects with insulin resistance 54 , diabetes 19 20 , or elevated baseline inflammation 55 . Our subjects may have been too healthy to trigger a major inflammatory state following the meals.

While most of the inflammatory factors measured had modest or no change following the meal, ICAM-1 and NF-κB responded differently by meal type. These results suggest that type of fat in meals can differentially influence postprandial inflammation and endothelial activation. Additional work is needed in this area to determine the long term effects and/or whether adaptation occurs with repeated ingestion of these fats.

BMI: body mass index; n-3FA: long chained omega fats (EPA or DHA); O3FA: high long chained omega fatty acids (EPA + DHA) meal; SFA: high saturated fat meal; MFA: high monounsaturated fat meal; PUFA: polyunsaturated fatty acids; CRP: C-reactive protein; TNF-α: tumor necrosis factor-α; ICAM-1: soluble intercellular adhesion molecule-1; VCAM-1: soluble vascular cell adhesion molecule-1; 8-epi: 8-epi-prostaglandin-F_2α; NF-κB: nuclear factor - κB.

The authors declare that they have no competing interests.

Study conception and design (ATP, JWR, YWL); generation, collection, analysis and interpretation of the data (ATP, JWR, YWL), drafting and revision of manuscript (ATP, JWR), funding (JWR), approval of final manuscript (ATP, JWR, YWL).