Location: Immunity and Disease Prevention Research2018 Annual Report
Objective 1: Determine the differential metabolism and uptake of n-3 fatty acids from dietary phospholipid and triglyceride forms in different tissues. -Sub-objective 1A: Determine the differential uptake and accretion of PL and TG forms of DHA in different mouse tissues. - Sub-objective 1B: Compare the metabolism of PL and TG forms of DHA in different mouse tissues. - Sub-objective 1C: Determine whether the differential accretion of DHA in brain from the PL and TG forms will result in differences in mouse behavior. Objective 2: Compare the anti-inflammatory and immune-modulating ability of n-3 fatty acids from dietary phospholipid and triglyceride forms in different tissues. -Sub-objective 2A: Compare the anti-inflammatory and immune-modulating ability of PL and TG forms of DHA by using experimental allergic encephalopathy (EAE)as a mouse model of brain inflammation. - Sub-objective 2B: Compare the anti-inflammatory and immune-modulating ability of PL and TG forms of DHA in the prevention of diet-induced NAFLD in mice.
Proposed experiments will compare accretion, metabolism and health effects n-3 fatty acids from the phospholipid (PL) and the triglyceride (TG) forms in different mouse tissues. For objective 1, we will determine the dose dependent accretion and metabolism of the PL-DHA and TG-DHA in the mouse tissues and their effects on the behavioral responses. We hypothesize that tissue accretion of DHA will increase with its increased intake, and at a given dose it will be greater in the PL group than in the TG group for tissues which express mfsd2a transporter (brain and liver) and will be similar in tissues (heart, muscle, and adipose tissue) that do not express this transporter. We also hypothesize that concentration of DHA in different lipid classes and PL subclasses, and that of DHA metabolites will be a function of tissue DHA concentration, and that DHA will improve learning and memory, and decrease fear and anxiety induced stress; effects of PL-DHA will be greater than that of TG-DHA on the behavioral responses tested. For objective 2, we will compare anti-inflammatory and immune-modulating effects of single equivalent dose (based on the results of objective 1) of the PL- and TG-DHA on brain and liver. For studies with brain we will use the mouse model of experimental allergic encephalomyelitis (EAE), and for liver we will use the high fat and high sucrose diets fed mouse model of nonalcoholic fatty liver disease (NAFLD). We hypothesize that both forms of DHA will delay the onset and severity of EAE and NAFLD when compared with the control groups, and PL-DHA will be more effective than TG-DHA. We anticipate our results will support the proposed hypotheses, however, if DHA does not have any effect on the response variables tested, we may repeat some experiments by increasing the duration of feeding. Whether or not the two forms of DHA differ in their effects on the responses tested will be equally useful information. A variety of behavioral, biochemical, molecular, immunohistochemistry, flowcytometery, and other analytical methods will be used. The mouse models and the methods proposed here have been previously used in our or our collaborator’s laboratories.
ARS scientists conducted a study in which female mice were fed one of seven experimental diets for five weeks to compare the metabolic effects of the triglyceride (TG) and phospholipids (PL) forms of dietary docosahexaenoic acid (DHA) (control diet, 1%, 2%, 4% TG-DHA and 1%, 2%, 4% PL-DHA). The fatty acid composition of the adipose tissue, heart and eye were analyzed from these mice. Compared with the control group, the DHA amount was significantly increased in all six DHA groups in liver, adipose tissue, heart, and eye but not in brain lipids. The amount of DHA in all these tissues did not differ between the corresponding concentrations of the PL-DHA and TG-DHA; it also did not differ by the dose of dietary DHA. Both forms of dietary DHA decreased the sum of n6 polyunsaturated fatty acids (PUFA) in all tissues examined, and also the sums of monounsaturated and saturated fatty acids in liver and adipose tissue lipids. Results demonstrate that the effects of dietary DHA on fatty acid composition varied in different tissues and that the PL form did not result in greater tissue accretion of DHA than the TG form, as claimed by some other investigators. The fatty acid data from all these tissues were analyzed and a manuscript has been submitted for publication. Experimental autoimmune encephalomyelitis (EAE) is the most commonly used mouse model of human multiple sclerosis (MS). This model was used to compare the effects of the two forms of DHA on EAE on disease progression in the EAE model. In this study, female mice at four weeks of age were started on a control diet low in n-3 PUFA for two weeks and then divided into five groups and fed the experimental diets (control, 0.3 and 1.0% each of the PL- and TG-DHA) for four weeks. EAE was induced when the mice were 10 weeks old by injecting an antigen to induce an inflammatory response to the brain protein myelin, to mimic the development of human MS. Disease onset typically occurs nine-12 days later and is followed by variable outcomes. The disease can be monophasic or chronic. Therefore, researchers monitored a clinical score of disease progression every day between days nine and 28 post-induction, and analyzed the clinical data for days 9-16, 17-21, and 22-28 separately. Key tissues (including brain and spinal cord) were collected for examination by a pathologist. The fatty acid composition of the brain tissues is currently being measured. For phase one (days 9-16), the clinical score in the 0.3% and 1.0% TG-DHA, and 1.0% PL-DHA groups were significantly lower (indicating less severe disease) than in the control and 0.3% PL-DHA groups. For phase two (days 17-21), the clinical score in 0.3% TG-DHA was significantly lower than in the remaining four groups. For the last phase (days 22-28), the clinical score was lower in both the PL-DHA groups than in the other three groups, although the difference was not statistically significant. These findings suggest that TG-DHA may be more effective than PL-DHA in slowing disease progression in the early phases of EAE, but in the final outcome, PL-DHA may be more effective than TG-DHA. Further studies with greater number of animals and longer duration are needed to discriminate between the protective effects of TG- and PL-DHA against EAE.
1. Absorption of dietary docosahexaenoic acid (DHA) and accumulation in key tissues. Dietary n-3 polyunsaturated fatty acids (PUFA) decrease the incidence of chronic inflammatory diseases. Recent studies comparing krill and fish oils suggest that krill oil may be more efficient than fish oil in the tissue accretion of long chain n-3 PUFA, because a major portion of the n-3 PUFA in krill oil are in the phospholipid (PL) form, while fish oil is mostly in the triglyceride (TG) form. To determine if consumption of the two different forms, PL-DHA and TG-DHA, actually resulted in different tissue levels of DHA, ARS scientists in Davis, California, conducted a dietary study to determine tissue accretion of the PL- and TG-DHA in mice. Compared with the control group, DHA concentration significantly increased with both forms of dietary DHA in the liver, adipose tissue, heart, and eye but not in brain; however, tissue DHA accretion did not differ between the PL- and TG-DHA forms. These results do not support the claim by other researchers that the PL- form of n-3 PUFA present in krill oil results in greater tissue accretion of n-3 PUFA than the TG form found in fish oil. Thus, consumption of krill oil supplements by consumers does not appear to offer an advantage over consumption of fish or fish oil rich in n-3 PUFA.