Effects of dietary polyunsaturated n-3 fatty acids on dyslipidemia and insulin resistance in rodents and humans. A review

For many years, clinical and animal studies on polyunsaturated n-3 fatty acids (PUFAs), especially those from marine oil, eicosapentaenoic acid (20:5,n-3) and docosahexaenoic acid (22:6,n-3), have reported the impact of their beneficial effects on both health and diseases. Among other things, they regulate lipid levels, cardiovascular and immune functions as well as insulin action. Polyunsaturated fatty acids are vital components of the phospholipids of membrane cells and serve as important mediators of the nuclear events governing the specific gene expression involved in lipid and glucose metabolism and adipogenesis. Besides, dietary n-3 PUFAs seem to play an important protecting role against the adverse symptoms of the Plurimetabolic syndrome. This review highlights some recent advances in the understanding of metabolic and molecular mechanisms concerning the effect of dietary PUFAs (fish oil) and focuses on the prevention and/or improvement of dyslipidemia, insulin resistance, impaired glucose homeostasis, diabetes and obesity in experimental animal models, with some extension to humans.     

The Antiarrhythmic Effect of n-3 Polyunsaturated Fatty Acids: Modulation of Cardiac Ion Channels as a Potential Mechanism

Sudden cardiac death remains one of the most serious medical challenges in Western countries. Increasing evidence in recent years has demonstrated that the n-3 polyunsaturated fatty acids (PUFAs) can prevent fatal ventricular arrhythmias in experimental animals and probably in humans. Dietary supplement of fish oils or intravenous infusion of the n-3 PUFAs prevents ventricular fibrillation caused by ischemia/reperfusion. Similar antiarrhythmic effects of these fatty acids are also observed in cultured mammalian cardiomyocytes. Based on clinical observations and experimental studies in vitro and in vivo, several mechanisms have been postulated for the antiarrhythmic effect of the n-3 PUFAs. The data from our laboratory and others have shown that the n-3 PUFAs are able to affect the activities of cardiac ion channels. The modulation of channel activities, especially voltage-gated Na⁺ and L-type Ca²⁺ channels, by the n-3 fatty acids may explain, at least partially, the antiarrhythmic action. It is not clear, however, whether one or more than one mechanism involves the beneficial effect of the n-3 PUFAs on the heart. This article summarizes our recent studies on the specific effects of the n-3 PUFAs on cardiac ion channels. In addition, the effect of the n-3 PUFAs on the human hyperpolarization-activated cyclic-nucleotide-modulated channel is presented.     

A modified <Emphasis Type="Italic">n</Emphasis>–3 fatty acid desaturase gene from <Emphasis Type="Italic">Caenorhabditis briggsae</Emphasis> produced high proportion of DHA and DPA in transgenic mice

The functions of polyunsaturated fatty acids (PUFAs) have been widely investigated. In mammals, levels of n-3 PUFAs are relatively low compared to those of n-6 PUFAs. Either a lack of n-3 PUFAs or an excess of n-6 PUFAs could potentially cause health problems in humans. Hence, methods to increase the amount of n-3 PUFAs in diet have been intensely sought. In this study, we demonstrated that the n-3 fatty acid desaturase gene (sFat-1) synthesized from revised and optimized codons based on roundworm Caenorhabditis briggsae genomic gene for enhanced expression in mammals was successfully expressed in Chinese hamster ovary (CHO) cells and significantly elevated cellular n-3 PUFA contents. We generated sFat-1 transgenic mice by introducing mammal expression vector DNAs containing the sFat-1 gene into regular mice through the method of microinjection. Fatty acid compositions were then altered and the levels of docosahexaenoic acid (DHA, 22:6n-3) and docosapentaenoic acid (DPA, 22:5n-3) were greatly increased in these transgenic mice. Various types of tissues in the transgenic mice produced many types of n-3 PUFAs, such as alpha-linolenic acid (ALA; 18:3n-3), eicosapentaenoic acid (EPA, 20:5n-3), DPA, and DHA, for example, muscle tissues of the transgenic mice contained 12.2% DHA, 2.0% DPA, and 23.1% total n-3 PUFAs. These research results demonstrated that the synthesized sFat-1 gene with modified and optimized codons from C. briggsae possess functional activity and greater capability of producing n-3 PUFAs, especially DHA and DPA, in transgenic mice.     

Arachidonic and docosahexaenoic acids reduce the growth of A549 human lung-tumor cells increasing lipid peroxidation and PPARs

Polyunsaturated fatty acids (PUFAs) play an important role in both induction and prevention of carcinogenic process. It is well known that several types of neoplastic cells show decreased total PUFA content, contributing to their resistance to chemotherapy and lipid peroxidation. In the light of this, human lung cancer A549 cells, with low PUFA content, were exposed to arachidonic or docosahexaenoic acid to investigate the effect of n-6 and n-3 PUFAs on growth and elucidate underlying mechanisms. The bulk of the results showed that both n-6 PUFAs and n-3 PUFAs decrease human lung-tumor cell growth in a concentration-dependent manner, inducing cell death mainly evident at 100microM concentration. The mechanism underlying the antiproliferative effect of n-6 and n-3 PUFAs appeared to be the same, involving changes in fatty acid composition of biomembranes, production of cytostatic aldehydes derived from lipid peroxidation and able to affect DNA-binding activity of AP-1, and induction of PPAR. From these results it may be hypothesized that n-6 PUFAs, like n-3 PUFAs, are able to inhibit tumor growth.     

Early Life Stress Interacts with the Diet Deficiency of Omega-3 Fatty Acids during the Life Course Increasing the Metabolic Vulnerability in Adult Rats

Early stress can cause metabolic disorders in adulthood. Omega-3 polyunsaturated fatty acids (n-3 PUFAs) deficiency has also been linked to the development of metabolic disorders. The aim of this study was to assess whether an early stressful event such as maternal separation interacts with the nutritional availability of n-3 PUFAs during the life course on metabolic aspects. Litters were randomized into: maternal separated (MS) and non-handled (NH). The MS group was removed from their dam for 3 hours per day and put in an incubator at 32°C on days 1° to 10° postnatal (PND). On PND 35, males were subdivided into diets that were adequate or deficient in n-3 PUFAs, and this intervention was applied during the subsequent 15 weeks. Animal''s body weight and food consumption were measured weekly, and at the end of the treatment tissues were collected. MS was associated with increased food intake (p = 0.047) and weight gain (p = 0.012), but no differences were found in the NPY hypothalamic content between the groups. MS rats had also increased deposition of abdominal fat (p<0.001) and plasma triglycerides (p = 0.018) when compared to the NH group. Interactions between early life stress and n-3 PUFAs deficiency were found in plasma insulin (p = 0.033), HOMA index (p = 0.049), leptin (p = 0.010) and liver PEPCK expression (p = 0.050), in which the metabolic vulnerability in the MS group was aggravated by the n-3 PUFAs deficient diet exposure. This was associated with specific alterations in the peripheral fatty acid profile. Variations in the neonatal environment interact with nutritional aspects during the life course, such as n-3 PUFAs diet content, and persistently alter the metabolic vulnerability in adulthood.     

Co-expression of fat1 and fat2 in transgenic pigs promotes synthesis of polyunsaturated fatty acids

Transgenic Research - Omega-3 polyunsaturated fatty acids (n-3 PUFAs) are essential for the development and health of mammals, such as humans and livestock. n-3 PUFAs must be supplied by diet due...     

Dietary ratio of n-6 to n-3 polyunsaturated fatty acids and periodontal disease in community-based older Japanese: a 3-year follow-up study

The longitudinal relationship between dietary n-6 to n-3 PUFAs ratio and periodontal disease in 235 Japanese subjects for whom data were available for the years 2003-2006 was investigated. PUFAs intake was assessed at baseline with a brief-type self-administered diet history questionnaire. Full-mouth periodontal status, measured as the clinical attachment level (CAL), was recorded at baseline and once a year for 3 years. The number of teeth with a change in the loss of CAL ≥3 mm at any site over a year was calculated as ‘periodontal disease events’. Poisson regression analysis was conducted, with dietary n-6 to n-3 PUFAs ratio as the main predictor, to estimate its influence on periodontal disease events.A high dietary n-6 to n-3 PUFAs ratio was significantly associated with greater number of periodontal disease events. The findings suggest the dietary n-6 to n-3 PUFAs ratio is associated with periodontal disease among older Japanese.     

Differential induction of electrophile-responsive element-regulated genes by n-3 and n-6 polyunsaturated fatty acids

In this study the n-3 polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid and docosahexaenoic acid appear to be effective inducers of electrophile-responsive element (EpRE) regulated genes, whereas the n-6 PUFA arachidonic acid is not. These n-3 PUFAs need to be oxidized to induce EpRE-regulated gene expression, as the antioxidant vitamin E can partially inhibit the PUFA induced dose-dependent effect. Results were obtained using a reporter gene assay, real-time RT-PCR and enzyme activity assays. The induction of EpRE-regulated phase II genes by n-3 PUFAs may be a major pathway by which n-3 PUFAs, in contrast to n-6 PUFAs, are chemopreventive and anticarcinogenic.     

Sucrose counteracts the anti-inflammatory effect of fish oil in adipose tissue and increases obesity development in mice

Background

Polyunsaturated n-3 fatty acids (n-3 PUFAs) are reported to protect against high fat diet-induced obesity and inflammation in adipose tissue. Here we aimed to investigate if the amount of sucrose in the background diet influences the ability of n-3 PUFAs to protect against diet-induced obesity, adipose tissue inflammation and glucose intolerance.

Methodology/Principal Findings

We fed C57BL/6J mice a protein- (casein) or sucrose-based high fat diet supplemented with fish oil or corn oil for 9 weeks. Irrespective of the fatty acid source, mice fed diets rich in sucrose became obese whereas mice fed high protein diets remained lean. Inclusion of sucrose in the diet also counteracted the well-known anti-inflammatory effect of fish oil in adipose tissue, but did not impair the ability of fish oil to prevent accumulation of fat in the liver. Calculation of HOMA-IR indicated that mice fed high levels of proteins remained insulin sensitive, whereas insulin sensitivity was reduced in the obese mice fed sucrose irrespectively of the fat source. We show that a high fat diet decreased glucose tolerance in the mice independently of both obesity and dietary levels of n-3 PUFAs and sucrose. Of note, increasing the protein∶sucrose ratio in high fat diets decreased energy efficiency irrespective of fat source. This was accompanied by increased expression of Ppargc1a (peroxisome proliferator-activated receptor, gamma, coactivator 1 alpha) and increased gluconeogenesis in the fed state.

Conclusions/Significance

The background diet influence the ability of n-3 PUFAs to protect against development of obesity, glucose intolerance and adipose tissue inflammation. High levels of dietary sucrose counteract the anti-inflammatory effect of fish oil in adipose tissue and increases obesity development in mice.     

Obesity polymorphisms identified in genome-wide association studies interact with n-3 polyunsaturated fatty acid intake and modify the genetic association with adiposity phenotypes in Yup’ik people

n-3 Polyunsaturated fatty acids (n-3 PUFAs) have anti-obesity effects that may modulate risk of obesity, in part, through interactions with genetic factors. Genome-wide association studies (GWAS) have identified genetic variants associated with body mass index (BMI); however, the extent to which these variants influence adiposity through interactions with n-3 PUFAs remains unknown. We evaluated 10 highly replicated obesity GWAS single nucleotide polymorphisms (SNPs) for individual and cumulative associations with adiposity phenotypes in a cross-sectional sample of Yup’ik people (n = 1,073) and evaluated whether genetic associations with obesity were modulated by n-3 PUFA intake. A genetic risk score (GRS) was calculated by adding the BMI-increasing alleles across all 10 SNPs. Dietary intake of n-3 PUFAs was estimated using nitrogen stable isotope ratio (δ¹⁵N) of red blood cells, and genotype–phenotype analyses were tested in linear models accounting for familial correlations. GRS was positively associated with BMI (p = 0.012), PBF (p = 0.022), ThC (p = 0.025), and waist circumference (p = 0.038). The variance in adiposity phenotypes explained by the GRS included BMI (0.7 %), PBF (0.3 %), ThC (0.7 %), and WC (0.5 %). GRS interactions with n-3 PUFAs modified the association with adiposity and accounted for more than twice the phenotypic variation (~1–2 %), relative to GRS associations alone. Obesity GWAS SNPs contribute to adiposity in this study population of Yup’ik people and interactions with n-3 PUFA intake potentiated the risk of fat accumulation among individuals with high obesity GRS. These data suggest the anti-obesity effects of n-3 PUFAs among Yup’ik people may, in part, be dependent upon an individual’s genetic predisposition to obesity.

Electronic supplementary material

The online version of this article (doi:10.1007/s12263-013-0340-z) contains supplementary material, which is available to authorized users.     

Effects of polyunsaturated fatty acids on hibernation and torpor: a review and hypothesis

Polyunsaturated fatty acids (PUFAs) can have strong effects on hibernation and daily torpor in mammals. High dietary PUFA contents were found to increase proneness for torpor, decrease body temperatures, prolong torpor bout duration, and attenuate hibernation mass loss. The mechanism by which PUFAs enhance torpor and hibernation is unknown, however. On the basis of a review of the literature, and on reexamining our own data on alpine marmots, we propose that effects on hibernation are not due to PUFAs in general, but to shifts in the ratio of n-6 PUFAs to n-3 PUFAs in membrane phospholipids. Specifically, high ratios of n-6 to n-3 PUFAs increase the activity of the Ca2+-Mg2+ pump in the sarcoplasmic reticulum of the heart (SERCA) and counteract Q10 effects on SERCA activity at low tissue temperatures. Therefore, high n-6 to n-3 PUFA ratios in cardiac myocyte membranes appear to protect the hibernating heart from arrhythmia, which in hypothermic nonhibernators is caused by massive increases in cytosolic Ca2+. The resulting reduced risk of cardiac arrest during hypothermia may explain why increased dietary uptake of n-6 PUFAs, but not of n-3 PUFAs, can strongly enhance the propensity for hibernation, and allows heterotherms to reach lower body temperatures, with associated increased energy savings. Therefore, at least for herbivorous hibernators, such as marmots, linoleic acid (C18:2 n-6)--the dietary source of all n-6 PUFAs--appears to represent a crucial and limited resource in natural environments.     

Dietary ratio of n-6/n-3 PUFAs and docosahexaenoic acid: actions on bone mineral and serum biomarkers in ovariectomized rats

Hypoestrogenic states escalate bone loss in animals and humans. This study evaluated the effects of the amount and ratio of dietary n-6 and n-3 polyunsaturated fatty acids (PUFAs) on bone mineral in 3-month-old sexually mature ovariectomized (OVX) Sprague-Dawley rats. For 12 weeks, the rats were fed either a high-PUFA (HP) or a low-PUFA (LP) diet with a ratio of n-6/n-3 PUFAs of 5:1 (HP5 and LP5) or 10:1 (HP10 and LP10). All diets (modified AIN-93G) provided 110.4 g/kg of fat from safflower oil and/or high-oleate safflower oil blended with n-3 PUFAs (DHASCO oil) as a source of docosahexaenoic acid (DHA). Fatty acid analyses confirmed that the dietary ratio of 5:1 significantly elevated the amount of DHA in the periosteum, marrow and cortical and trabecular bones of the femur. Dual-energy X-ray absorptiometry measurements for femur and tibia bone mineral content (BMC) and bone mineral density showed that the DHA-rich diets (HP5 and LP5) resulted in a significantly lower bone loss among the OVX rats at 12 weeks. Rats fed the LP diets displayed the lowest overall serum concentrations of the bone resorption biomarkers pyridinoline (Pyd) and deoxypyridinoline, whereas the bone formation marker osteocalcin was lowest in the HP groups. Regardless of the dietary PUFA content, DHA in the 5:1 diets (HP5 and LP5) preserved rat femur BMC in the absence of estrogen. This study indicates that the dietary ratio of n-6/n-3 PUFAs (LP5 and HP5) and bone tissue concentration of total long-chain n-3 PUFAs (DHA) minimize femur bone loss as evidenced by a higher BMC in OVX rats. These findings show that dietary DHA lowers the ratio of 18:2n-6 (linoleic acid)/n-3 in bone compartments and that this ratio in tissue correlates with reduced Pyd but higher bone alkaline phosphatase activity and BMC values that favor bone conservation in OVX rats.     

Adaptive thermogenesis by dietary n-3 polyunsaturated fatty acids: Emerging evidence and mechanisms

Brown/beige fat plays a crucial role in maintaining energy homeostasis through non-shivering thermogenesis in response to cold temperature and excess nutrition (adaptive thermogenesis). Although numerous molecular and genetic regulators have been identified, relatively little information is available regarding thermogenic dietary molecules. Recently, a growing body of evidence suggests that high consumption of n-3 polyunsaturated fatty acids (PUFA) or activation of GPR120, a membrane receptor of n-3 PUFA, stimulate adaptive thermogenesis. In this review, we summarize the emerging evidence that n-3 PUFA promote brown/beige fat formation and highlight the potential mechanisms whereby n-3 PUFA require GPR120 as a signaling platform or act independently. Human clinical trials are revisited in the context of energy expenditure. Additionally, we explore some future perspective that n-3 PUFA intake might be a useful strategy to boost or sustain metabolic activities of brown/beige fat at different lifecycle stages of pregnancy and senescence. Given that a high ratio of n-6/n-3 PUFA intake is associated with the development of obesity and type 2 diabetes, understanding the impact of n-6/n-3 ratio on energy expenditure and adaptive thermogenesis will inform the implementation of a novel nutritional strategy for preventing obesity.     

Polyunsaturated fatty acids in male and female reproduction

In Westernized societies, average consumption of n-6 polyunsaturated fatty acids (PUFAs) far exceeds nutritional requirements. The ratio of n-6 to n-3 PUFAs is generally >10:1 whereas on a primitive human diet it was closer to 1:1. Diets fed to intensively farmed livestock have followed a similar trend. Both n-6 and n-3 PUFAs can influence reproductive processes through a variety of mechanisms. They provide the precursors for prostaglandin synthesis and can modulate the expression patterns of many key enzymes involved in both prostaglandin and steroid metabolism. They are essential components of all cell membranes. The proportions of different PUFAs in tissues of the reproductive tract reflect dietary consumption. PUFA supplements (particularly n-3 PUFAs in fish oil) are promoted for general health reasons. Fish oils may also benefit fertility in cattle and reduce the risk of preterm labor in women, but in both cases current evidence to support this is inconclusive. Gamma-linolenic acid containing oils can alter the types of prostaglandins produced by cells in vitro, but published data to support claims relating to effects on reproductive health are lacking. Spermatozoa require a high PUFA content to provide the plasma membrane with the fluidity essential at fertilization. However, this makes spermatozoa particularly vulnerable to attack by reactive oxygen species, and lifestyle factors promoting oxidative stress have clear associations with reduced fertility. Adequately powered trials that control for the ratios of different PUFAs consumed are required to determine the extent to which this aspect of our diets does influence our fertility.