Dietary influence on the insulin function in the epididymal fat cell of the Wistar rat. III. Effect of the ratio carbohydrate to fat

Newly weaned male Wistar rats were fed semipurified diets containing sunflowerseed oil or palm oil. The fatty acid compositions of the phospholipid and triacylglycerol fractions either from whole epididymal fat pads or from fat cells isolated from the pads appeared to be nearly similar. Feeding the diets for a period of only 3-4 weeks after weaning was found to be sufficient to produce a difference in insulin response and insulin binding in the epididymal fat cells. A reduction in the dietary fat content from 30 to 15% of the energy had only minor effects on fat pad weight and on the fatty acid composition of the phospholipid- and triacylglycerol fractions isolated from the epididymal fat pads. There was no effect either on body weights or on average fat cell diameter but there was an increase in insulin response in the isolated fat cells, however. The difference in insulin response after a diet rich in polyunsaturated fatty acids (sunflowerseed oil) as compared to one rich in saturated fatty acids (palm oil) remained after the reduction of fat content.     

Dietary influence on the insulin function in the epididymal fat cell of the Wistar rat. II. Effect of type of carbohydrate

Newly weaned male specific pathogen-free Wistar rats were fed semipurified diets containing sunflowerseed oil or palm oil and various types of carbohydrates, i.e. sucrose, maize starch or potato starch for at least 3-4 weeks. Fat cells were isolated from the epididymal fat pads to study the sensitivity to insulin. The response to insulin of the cells was always higher for sunflowerseed oil than for palm oil. Diets containing polysaccharides were found to induce a higher response as compared to sucrose-containing diets. The changes in insulin response due to the type of carbohydrate were not accompanied by changes in insulin binding to the cells, or in the fatty acid composition of the membrane phospholipids. The fatty acid compositions of the triacylglycerol fractions from the epididymal fat pads reflected very well those of the dietary fats. In addition, the type of dietary carbohydrate had also some effect on this composition. The fecal fatty acid content was low and very similar after diets containing either sucrose or maize starch and sunflowerseed oil or palm oil, indicating that the difference in insulin function between the two dietary fats cannot be caused by a difference in fat absorption in the digestive tract. These findings, as well as the insulin concentrations in the blood samples after the various diets, suggest that the changes in insulin response due to the type of dietary carbohydrate are caused by postreceptor effects.     

Effects of the type of dietary fatty acid on the insulin receptor function in rat epididymal fat cells

Feeding young rats diets containing sunflowerseed oil (SSO) or palm oil (PO) induced several differences in the properties of the isolated epididymal fat cells: insulin stimulated deoxyglucose uptake 127% over the basal value in cells of the SSO group but only 47% in those of the PO group; the insulin concentration giving half maximal stimulation differing only slightly; insulin binding to the cells was higher in the SSO group; Scatchard analysis revealed that this was due to a significantly higher number of low-affinity binding sites, and the epididymal fat pad showed a concomitant change in the fatty acid pattern of the phospholipids, reflecting to a limited extent the differences in the composition of the diets. Neither the average diameters of the isolated fat cells, nor the serum insulin level at the time of sacrifice of the rats differed for the two types of dietary fat. These results indicate that a diet high in linoleic acid (SSO) induces a better response of fat cells to insulin than a diet high in saturated fatty acids (PO).     

Influence of dietary fat on the lipid composition of perirenal adipose tissue in rats

The influence of dietary fat on the composition of perirenal adipose tissue was studied in Wistar rats fed three experimental semisynthetic, isocaloric diets containing different qualities of fat (olive oil, butter and medium chain triglycerides + corn oil). Under these experimental conditions, the saturation index reflects the percentage of fatty acids supplied by each diet; this index was highest in animals fed the diet containing butter and lowest in the group in which olive oil was the dietary fat source. The amount of linoleic acid (the major component of the diunsaturation index) supplied by the diet is directly paralleled by levels of this fatty acid in perirenal adipose tissue, whereas the monounsaturation index in adipose tissues, considered an indicator of the dietary supply of monounsaturated fatty acids in the rat, failed to show a clearly proportional relationship between intake and perirenal adipose tissue levels.     

Modulation of fluid absorption and the secretory response of rat jejunum to cholera toxin by dietary fat.

To study the effects of dietary fat on jejunal water and ion absorption and on cholera toxin-induced secretion, 3 week old Sprague Dawley rats were fed isocaloric diets. Forty per cent of the total calories were given as fat, as butter (high saturated fat), olive oil (high monounsaturated fat), or corn oil (high polyunsaturated fat), with one group on low fat (10% of calories) standard laboratory diet as controls. During in vivo jejunal perfusion studies we found that (i) a polyunsaturated fat (corn oil) supplemented diet improves jejunal absorption of water and electrolytes and these changes are independent of the observed concentrations of luminal prostaglandins; (ii) high dietary fat appreciably reduced the secretory response to cholera toxin, probably without fundamentally changing the mechanism by which cholera toxin induces secretion. We conclude that dietary fat composition altered the permeability and transport characteristics of the small intestine. This observation might have relevance to some human diarrhoeal disorders.     

The effect of intestinal resection and dietary fat quality on the nutritional utilization of fat

The influence of the quality of dietary fat on digestive and metabolic utilization of fat, serum levels of cholesterol and triglycerides and the relative percentages of serum fatty acids was studied in rats in which 50% of the distal small intestine (DSI) was resected and in sham-operated controls. Nutritive parameters were measured 1 month after surgery. The fat components of the diets were: olive oil, butter and equal parts of medium chain triglycerides (MCT), olive oil and sunflower seed oil. There was no changes in digestive utilization of fat in shamoperated controls with either dietary sources of fat, olive oil or butter. The removal of 50% of the distal small intestine led to a decline in the digestive utilization of fat (ADC) in both test diets. When dietary fat was supplied as equal parts of MCT, olive oil and sunflower seed oil instead of 100% olive oil (diet C), digestive utilization of fat was enhanced in resected rats. When ursodeoxycholic acid (UDCA) was added to diet C, digestive utilization of fat remained high, reaching values as elevated as those in sham-operated controls. At the metabolic level, resection of half the DSI led to a significant decrease in serum triglycerides and cholesterol levels, independently of type of dietary fat tested. The type of diet, as well as intestinal resection, do not led to an essential fatty acid deficiency in serum lipids; it is only a reflect of the supplemented diet fat contents.     

Substituting dietary saturated fat with polyunsaturated fat changes abdominal fat distribution and improves insulin sensitivity

Aims/hypothesis: British dietary recommendations are to decrease total fat intake to less than 30 % of daily energy intake and saturated fat to less than 10 %. In practice, it is difficult for people to make these changes. It may be easier to encourage people to switch from a diet rich in saturated fatty acids to one rich in polyunsaturated fatty acids. Methods: A total of 17 subjects – six people with Type II (non-insulin-dependent) diabetes mellitus, six non-obese and five obese people without diabetes – were randomised to spend two 5-week periods on a diet rich in saturated or in polyunsaturated fatty acids, in a crossover design. At the start of the study and after each dietary period, we assessed abdominal fat distribution using magnetic resonance imaging, insulin sensitivity using hyperinsulinaemic-euglycaemic clamps and fasting lipid parameters. Results: Dietary compliance, assessed by weekly 3-day dietary records and measurement of biochemical markers, was good. Energy and fat intake appeared to be reduced on the diet rich in polyunsaturated fatty acids although body weights did not change. Insulin sensitivity and plasma low density lipoprotein cholesterol concentrations improved with the diet rich in polyunsaturated fatty acids compared with the diet rich in saturated fatty acids. There was also a decrease in abdominal subcutaneous fat area. Conclusion/interpretation: If this result is confirmed in longer-term studies, this dietary manipulation would be more readily achieved by the general population than the current recommendations and could result in considerable improvement in insulin sensitivity, reducing the risk of developing Type II diabetes. [Diabetologia (2002) 45: 369–377] Received: 3 August 2001 and in revised form: 26 November 2001     

Consumption of saturated fat impairs the anti-inflammatory properties of high-density lipoproteins and endothelial function.

OBJECTIVES: The purpose of this study was to investigate the influence of dietary fatty acids on the anti-inflammatory properties of high-density lipoproteins (HDL) and vascular function. BACKGROUND: The effect of dietary fatty acids on atherogenesis remains uncertain. METHODS: Fourteen adults consumed an isocaloric meal containing either a polyunsaturated or a saturated fat on 2 occasions. The effects of post-prandial HDL on endothelial cell expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) were determined. Flow-mediated dilation (FMD) and microvascular reactivity were assessed before and 3 and 6 h after the meal. RESULTS: Plasma triglycerides, insulin, and nonesterified fatty acids rose after the meals. The HDL collected 6 h after the saturated meal were less effective than HDL isolated from fasting plasma in terms of their ability to inhibit expression of ICAM-1 and VCAM-1, whereas HDL collected 6 h after the polyunsaturated meal had an inhibitory activity that was greater than that of HDL collected from fasting plasma (p < 0.004 and p = 0.01 for comparison of effect of meals on ICAM-1 and VCAM-1, respectively). Post-hyperemic microvascular flow significantly increased at 3 h after the polyunsaturated meal by 45 +/- 14% and by 21 +/- 11% after the saturated meal. The FMD decreased 3 h after the saturated meal by 2.2 +/- 0.9% (p< 0.05 compared with baseline) and by 0.9 +/- 1% after the polyunsaturated meal. CONCLUSIONS: Consumption of a saturated fat reduces the anti-inflammatory potential of HDL and impairs arterial endothelial function. In contrast, the anti-inflammatory activity of HDL improves after consumption of polyunsaturated fat. These findings highlight novel mechanisms by which different dietary fatty acids may influence key atherogenic processes.     

An inhibitory effect of dietary polyunsaturated fatty acids on renin secretion in the isolated perfused rat kidney

The influence of dietary modification of polyunsaturated fatty acids (PUFA) on renin secretion, renal vascular tone and prostanoid excretion was studied in isolated perfused rat kidneys under basal conditions and in response to angiotensin II. After a four-week regimen of diets enriched with safflower oil, linseed oil or saturated fat, providing 20% of total energy intake (20 energy %), the animals which were fed linseed oil showed a significant fall in the proportion of arachidonic acid in renal phospholipids and a reduction in urinary prostaglandin excretion. In comparison with the other dietary groups, linseed oil feeding also resulted in a consistently lower renal vascular tone with increasing doses of angiotensin II. Under basal conditions both the PUFA-fed groups had significantly lower renal venous renin secretion rates relative to the saturated fat-fed control group. Infusion of angiotensin II (10 ng/min) suppressed renin secretion and abolished significant differences between the groups. As both the control group and the safflower oil group excreted similar levels of urinary prostaglandins, these results suggest that dietary enrichment with 20 energy % PUFA alters renin secretion by a prostaglandin-independent mechanism and that this may contribute to the lower blood pressures observed in these animals compared with the saturated fat-fed control group.     

Diet and risk of Type II diabetes: the role of types of fat and carbohydrate

Although diet and nutrition are widely believed to play an important part in the development of Type II (non-insulin-dependent) diabetes mellitus, specific dietary factors have not been clearly defined. Much controversy exists about the relations between the amount and types of dietary fat and carbohydrate and the risk of diabetes. In this article, we review in detail the current evidence regarding the associations between different types of fats and carbohydrates and insulin resistance and Type II diabetes. Our findings indicate that a higher intake of polyunsaturated fat and possibly long-chain n-3 fatty acids could be beneficial, whereas a higher intake of saturated fat and trans-fat could adversely affect glucose metabolism and insulin resistance. In dietary practice, exchanging nonhydrogenated polyunsaturated fat for saturated and trans-fatty acids could appreciably reduce risk of Type II diabetes. In addition, a low-glycaemic index diet with a higher amount of fiber and minimally processed whole grain products reduces glycaemic and insulinaemic responses and lowers the risk of Type II diabetes. Dietary recommendations to prevent Type II diabetes should focus more on the quality of fat and carbohydrate in the diet than quantity alone, in addition to balancing total energy intake with expenditure to avoid overweight and obesity. [Diabetologia (2001) 44: 805–817]     

The fatty acid composition of chylomicrons influences the rate of their lipolysis in vivo

BACKGROUND AND AIMS: Previous work in our laboratory has shown that chylomicron triacylglycerol is lipolysed in vitro by lipoprotein lipase more rapidly when the particles are enriched with n-6 polyunsaturated as compared to saturated, monounsaturated or n-3 polyunsaturated fatty acids. It is possible, however, that this does not reflect the situation in vivo, where the active enzyme is bound to the vascular endothelium. The aim of the present study is to investigate the effect of the fatty acid composition of chylomicrons on their lipolysis in the rat in vivo. METHODS AND RESULTS: [3H]Oleate-labelled chylomicrons derived from palm, olive, corn or fish oil (enriched in saturated, monounsaturated, n-6 polyunsaturated and n-3 polyunsaturated fatty acids respectively) containing > 90% of the label in triacylglycerol were injected intravenously into functionally hepatectomised rats and blood samples were taken at time intervals up to 40 min. The radioactivity in serum triacylglycerol decreased significantly more rapidly when corn oil as compared to palm, olive or fish oil chylomicrons were used. Conversely, the radioactivity in serum free fatty acid derived from corn oil chylomicrons showed a faster increase than that derived from the other three types of particles. CONCLUSIONS: These results indicate that chylomicrons enriched with n-6 polyunsaturated fatty acids are converted to chylomicron remnants in vivo more rapidly than those enriched with saturated, monounsaturated or n-3 polyunsaturated fatty acids. This provides a partial explanation for the differential rate of removal from the blood of cholesterol carried in chylomicrons of different fatty acid composition demonstrated in previous work from this laboratory.     

Differential influence of different dietary fatty acids on very low-density lipoprotein secretion when delivered to hepatocytes in chylomicron remnants

The influence of dietary fats carried in chylomicron remnants on the hepatic secretion of very low-density lipoprotein (VLDL) was investigated using chylomicron remnant-like particles (CRLPs) and cultured rat hepatocytes as the experimental model. Chylomicron remnant-like particles containing triacylglycerol (TG) from palm, olive, or corn (enriched in saturated, monounsaturated, or n-6 polyunsaturated fatty acids) oil, respectively, were incubated with cultured hepatocytes for 5 hours. The medium was then removed and replaced with medium without CRLPs; and the secretion of TG, cholesterol, and apolipoprotein B48 during the following 16 hours was determined. Secretion of TG into the d less than 1.050-g/mL fraction containing VLDL was unaffected by olive CRLPs, but was significantly increased in cells exposed to palm or corn CRLPs in comparison with both olive CRLPs and control incubations without CRLPs. Secretion of apolipoprotein B48, however, was not changed by any of the CRLP types. Apolipoprotein B messenger RNA levels were decreased by olive and corn CRLPs, and 3-hydroxy-3-methylglutaryl coenzyme A reductase messenger RNA abundance was increased by palm CRLPs; but expression of other genes involved in the regulation of VLDL secretion was unaffected. These findings demonstrate that CRLPs enriched in saturated fatty acids or n-6 polyunsaturated fatty acids increase the secretion of TG in VLDL, possibly because of the secretion of larger particles, whereas those enriched in monounsaturated fatty acids have no effect. Thus, different dietary fats have differential effects on VLDL secretion directly when delivered to the liver in chylomicron remnants.     

Diabetogenic impact of long-chain omega-3 fatty acids on pancreatic beta-cell function and the regulation of endogenous glucose production

In healthy individuals, peripheral insulin resistance evoked by dietary saturated lipid can be accompanied by increased insulin secretion such that glucose tolerance is maintained. Substitution of long-chain omega-3 fatty acids for a small percentage of dietary saturated fat prevents insulin resistance in response to high-saturated fat feeding. We substituted a small amount (7%) of dietary lipid with long-chain omega-3 fatty acids during 4 wk of high-saturated fat feeding to investigate the relationship between amelioration of insulin resistance and glucose-stimulated insulin secretion (GSIS). We demonstrate that, despite dietary delivery of saturated fat throughout, this manipulation prevents high-saturated fat feeding-induced insulin resistance with respect to peripheral glucose disposal and reverses insulin hypersecretion in response to glucose in vivo. Effects of long-chain omega-3 fatty acid enrichment to lower GSIS were also observed in perifused islets suggesting a direct effect on islet function. However, long-chain omega-3 fatty acid enrichment led to hepatic insulin resistance with respect to suppression of glucose output and impaired glucose tolerance in vivo. Our data demonstrate that the insulin response to glucose is suppressed to a greater extent than whole-body insulin sensitivity is enhanced by enrichment of a high-saturated fat diet with long-chain omega-3 fatty acids. Additionally, reduced GSIS despite glucose intolerance suggests that either long-chain omega-3 fatty acids directly impair the beta-cell response to saturated fat such that insulin secretion cannot be augmented to normalize glucose tolerance or beta-cell compensatory hypersecretion represents a response to insulin resistance at the level of peripheral glucose disposal but not endogenous glucose production.     

Monounsaturated fat decreases hepatic lipid content in non-alcoholic fatty liver disease in rats

AIM： To evaluate the effects of different types of dietary fats on the hepatic lipid content and oxidative stress parameters in rat liver with experimental non-alcoholic fatty liver disease （NAFLD）. METHODS： A total of 32 Sprague-Dawley rats were randomly divided into five groups. The rats in the control group （n = 8） were on chow diet （Group 1）, rats （n = 6） on methionine choline-deficient diet （MCDD） （Group 2）, rats （n = 6） on MCDD enriched with olive oil （Group 3）, rats （n = 6） on MCDD with fish oil （Group 4） and rats （n = 6） on MCDD with butter fat （Group 5）. After 2 mo, blood and liver sections were examined for lipids composition and oxidative stress parameters.
RESULTS： The liver weight/rat weight ratio increased in all treatment groups as compared with the control group. Severe fatty liver was seen in MCDD ＋ fish oil and in MCDD ＋ butter fat groups, but not in MCDD and MCDD ＋ olive oil groups. The increase in hepatic triglycerides （TG） levels was blunted by 30% in MCDD ＋ olive oil group （0.59 ±0.09） compared with MCDD group （0.85 ±0.04, P 〈 0.004）, by 37% compared with MCDD ＋ fish oil group （0.95 ±0.07, P 〈 0.001）, and by 33% compared with MCDD ＋ butter group （0.09 ±0.1, P 〈 0.01）. The increase in serum TG was lowered by 10% in MCDD ＋ olive oil group （0.9 ±0.07） compared with MCDD group （1.05 ±0.06）. Hepatic cholesterol increased by 15-fold in MCDD group [（0.08 ±0.02, this increment was blunted by 21% in MCDD ＋ fish oil group （0.09 ±0.02）]. In comparison with the control group, ratio of long-chain polyunsaturated fatty acids omega-6/omega-3 increased in MCDD ＋ olive oil, MCDD ＋ fish oil and MCDD ＋ butter fat groups by 345-, 30- and 397-fold, respectively. In comparison to MCDD group （1.58 ±0.08）, hepatic MDA contents in MCDD ＋ olive oil （3.3 ±0.6）, MCDD ＋ fish oil （3.0 ±0.4）, and MCDD ＋ butter group （2.9 ±0.36） were increased by 108%, 91% and 87%, respecti     

The impact of dietary fat composition on serum leptin concentrations in healthy nonobese men and women

The recently discovered hormone leptin is primarily secreted by adipose tissue and serves as an internal signal indicating the size of body fat stores. The aim of the present study was to investigate the impact of the dietary fatty acid composition on serum leptin concentrations. Therefore, serum leptin levels were measured by RIA in healthy nonobese men (n = 30) and women (n = 25). First, all participants received a baseline high-fat diet, rich in saturated fat, for 2 wk and were then randomly assigned to one of three high-fat dietary treatments, which contained refined olive oil (rich in monounsaturated fatty acids, n = 19), rapeseed oil [rich in monounsaturated fatty acids and alpha-linolenic acid (18:3n-3), n = 17], or sunflower oil (rich in n-6-polyunsaturated fatty acids, n = 19) as the principal source of fat for 4 wk. On the rapeseed oil diet, serum leptin concentrations increased slightly in men [+0.25 ng/ml, T(9) = -2.778, P = 0.021], but decreased distinctly in women [-4.70 ng/ml, T(6) = 5.083, P = 0.002]. Both the olive oil and the sunflower oil diet did not affect serum leptin concentrations. Thus, it is proposed that serum leptin levels were affected by the high amount of alpha-linolenic acid in rapeseed oil. However, questions remain as to why this diet differently affected serum leptin in men and women.     

Effects of synacthen on lipid metabolism in the perfused epididymal fat pad of the rat

Rats were treated with Synacthen, a synthetic corticotrophin analogue, to induce hypercorticism. The epididymal fat pad was selectively cannulated and perfused. In fasted rats acetone ether powder lipoprotein lipase (LPL) activity rose during treatment to levels found in fed controls. In fed animals no further rise in LPL activity was observed during Synacthen treatment. However, the heparin-elutable LPL activity did not change during this treatment in fasted nor fed animals. Pharmacologic levels of insulin in the perfusion medium caused an increase in heparin-releasable LPL activity as a percentage of total fat pad LPL activity (15% v 48%). Hydrolysis of chylomicrons was higher in fasted three days treated animals then in controls (10 +/- 4% v 2 +/- 2%). In this group a higher uptake of liberated free fatty acids was found (2.6 +/- 1.5% v 1.0 +/- 0.5% in controls). The increase in hydrolysis rate and uptake of fatty acids in the treated fasted animals could not be explained by an increase in releasable LPL activity. Fatty acid release from the fat pad was lower in treated animals than in controls (fasted and fed), basally as well as after adrenalin stimulation. The observation that the epididymal fat pad retains its weight during hypercorticism may therefore be ascribed to an increased influx of fatty acids from increased hydrolysis of TG-rich particles and to an inhibited efflux of fatty acids from the adipocyte. The discrepancy between the LPL activity extractable from an acetone ether powder and the heparin releasable LPL activity suggests impairment of the transport of LPL from the adipocyte to the heparin releasable pool at the endothelium.     

Dietary fat and heart failure: moving from lipotoxicity to lipoprotection

There is growing evidence suggesting that dietary fat intake affects the development and progression of heart failure. Studies in rodents show that in the absence of obesity, replacing refined carbohydrate with fat can attenuate or prevent ventricular expansion and contractile dysfunction in response to hypertension, infarction, or genetic cardiomyopathy. Relatively low intake of n-3 polyunsaturated fatty acids from marine sources alters cardiac membrane phospholipid fatty acid composition, decreases the onset of new heart failure, and slows the progression of established heart failure. This effect is associated with decreased inflammation and improved resistance to mitochondrial permeability transition. High intake of saturated, monounsaturated, or n-6 polyunsaturated fatty acids has also shown beneficial effects in rodent studies. The underlying mechanisms are complex, and a more thorough understanding is needed of the effects on cardiac phospholipids, lipid metabolites, and metabolic flux in the normal and failing heart. In summary, manipulation of dietary fat intake shows promise in the prevention and treatment of heart failure. Clinical studies generally support high intake of n-3 polyunsaturated fatty acids from marine sources to prevent and treat heart failure. Additional clinical and animals studies are needed to determine the optimal diet in terms of saturated, monounsaturated, and n-6 polyunsaturated fatty acids intake for this vulnerable patient population.     

Effect of dietary hydrogenated fish oil on the plasma lipoprotein profile and on the fatty acid composition of different tissues on the rat

Dietary fatty acids are actively incorporated into membrane lipids, and fat intake can modify the composition and the biochemical activity of cellular membranes and the pattern of plasma lipoproteins. Industrial hydrogenation of polyunsaturated oils leads to the formation of isomeric trans fatty acids which are incorporated into cellular membranes when they are present in the diet. The trans fatty acid amount present in hydrogenated oils depends on the degree of hydrogenation, being high for partially hydrogenated oils and low for highly hydrogenated oils. Hydrogenated fish oil is widely used in some countries for the production of margarine and industrial fats. This study compares the fatty acid composition of plasma, erythrocytes, subcutaneous adipose tissue, and hepatic microsomal membranes and the plasma lipoprotein profile after feeding rats with a synthetic diet containing either fish oil, partially hydrogenated fish oil, or highly hydrogenated fish oil. It is observed that the tissue content of monounsaturated fatty acids increases and that the content of polyunsaturated fatty acids decreases after an increase of the degree of hydrogenation of the dietary fat. Tissues from animals fed partially hydrogenated fish oil show significant amounts of trans fatty acids only. The plasma triacylglyceride composition and the lipoprotein profile are also altered by the degree of hydrogenation of the dietary fat. Triacylglycerides decrease after highly hydrogenated fat feeding only. Total cholesterol and low-density lipoprotein cholesterol are significantly increased after partially hydrogenated fat feeding. Although no direct evidence is presented, this effect may be attributable to the high content of trans isomers of this dietary fat which nutritionally may behave as saturated fatty acids.     

Dietary fish oils limit adipose tissue hypertrophy in rats

Total body mass, fat pad mass, and fat cell size were examined after feeding rats diets containing 20% triglycerides from fish oil or lard. Although food consumption, weight gain, and fat balance on the two diets were similar, lard-fed rats had 77% more fat in perirenal fat pads and 51% more fat in epididymal fat pads compared with fish oil-fed rats. There was no difference between the two groups in fat cell number in each region; however, adipocytes were significantly smaller in fish oil-fed rats. Thus dietary fish oil appears to limit triglyceride accumulation in adipose tissue and thereby limit fat cell trophic growth.