It is well documented that inclusion of n-3 PUFAs in high fat diets leads to reduced development of diet-induced obesity in rodents 
. Unfortunately, not all studies where the anti-obesogenic effects of fish oils are studied provide a detailed description of the macronutrient composition. However, in standard commercial available high fat- and very high fat diets, starch is the most abounded carbohydrate source and the amount of sucrose is low or absent. Here we show that a high amount of sucrose in the diet counteracts the obesity-reducing effect of fish oil as well as the well described anti-inflammatory effect in adipose tissue 
. Irrespective of the fatty acid source, mice fed high protein diets remained lean whereas mice fed diets enriched in sucrose became obese and had higher expressions of inflammatory markers in adipose tissue. Collectively, our results demonstrate that a high intake of sucrose abrogates the protective effects of fish oil in development of obesity.
As dietary sucrose, but not protein or fat, stimulates secretion of insulin from pancreatic β-cells, an increased dietary sucrose
protein ratio will translate into an increased insulin
glucagon ratio in the fed state. In this respect the observed higher insulin
glucagon ratio in mice fed the sucrose-based diets than in mice fed the protein-based diets was expected. Increased levels of insulin in fed mice were observed irrespectively of the type of fat in the diet. Insulin is a powerful anabolic hormone that stimulates adipocyte differentiation and adipose tissue expansion 
. Activation of insulin signaling is crucial for the development of obesity 
and insulin receptor substrate-1 (IRS-1) transgenic mice are obese 
. Increased insulin signaling and glucose uptake in adipose tissue in the fed state in sucrose fed mice may thus override the protective effect of fish oil when it comes to protection against obesity-development. It should also be mentioned that although several studies have demonstrated a protective effect of fish oil in obesity-development, it has been reported that inclusion of fish oil increased the amount of adipose tissue mass in hyperinsulinemic ob/ob
Differences in the insulin
glucagon ratio and hence differences in cAMP-dependent signaling may at least in part orchestrate the observed differences in energy homeostasis between the sucrose- and protein-based diets regardless of whether these diet are supplemented with corn oil or fish oil. In the liver, Ppargc1a
is induced in response to elevated levels of cAMP and plays a central role in the control of hepatic gluconeogenesis 
. High circulating levels of insulin combined with a low level of glucagon translate into reduced cAMP signalling in the liver. Thus, the observed increased gluconeogenesis in the fed state in protein fed mice may result from cAMP-mediated stimulation of Ppargc1a
expression. Increased gluconeogenesis in the fed state may contribute to the observed lower energy efficiency in protein fed mice, as 6 ATP molecules are consumed per molecule of glucose synthesized from pyruvate, rendering gluconeogenesis an energy-consuming process. Moreover, concomitant increased expressions of Gpt
suggest that energy consuming processes such as amino acid degradation and ureagenesis are higher in protein than sucrose fed mice. As mammals have no direct storage capacity for protein it needs to be metabolically processed immediately. The high cost of urea production and gluconeogenesis is actually often cited reasons for the higher thermic effect of protein than other macronutrients 
and this may partly explain why diets higher in protein exert a larger effect on energy expenditure than diets lower in protein 
A second mechanism by which a low sucrose
protein ratio in the diet leads to reduced energy efficiency may be related to the observed expression of Ucp1
in iWAT. Increased cAMP-signaling is known to induce adaptive thermogenesis by induction of Ppargc1a
expression and it is well known that the UCP1 protein allows dissipation of energy in the form of heat 
. Of note, acute or chronic upregulation of fatty acid oxidation alone, that is increased fatty acid oxidation without a concomitant uncoupling of mitochondria, has no net effect on whole-body energy expenditure or adiposity 
. Although Ucp1
expression was unchanged in iBAT, whole body energy homeostasis may be influenced by increased expression in iWAT. In fact, increased occurrence of brown-like adipocytes within WAT depots is a feature of mouse strains resistant to dietary obesity, such as the A/J strain 
and reduced adiposity associated with aP2-transgenic expression of Ucp1
is linked to increased energy dissipation in white, but not interscapular brown, adipose tissue 
. Conversely, inhibition of diet-induced expression of Ucp1
in iWAT in Sv129 mice by administration of a general cyclooxygenase inhibitor accentuates obesity-development 
Our finding that inclusion of sucrose abolishes the anti-obesity effect of fish oil seems to contradict a recent study from Sato et al. 
, as these authors demonstrated that inclusion of 5% the n-3 PUFA EPA (eicosapentaenoic acid) into a high fat-high sucrose diet reduced body weight gain in mice. The reason for this discrepancy is not clear, but different dietary compositions as well as doses and type of n-3 PUFAs may account for the different results obtained. The amount of n-3 PUFAs used in this study is slightly higher (6% n-3 fatty acids) than the 5% EPA used by Sato et al.. However, whereas Sato et al. used EPA, the n-3 PUFAs used in our study comprise a mixture (thereof 32±3 g/kg and 18±3 g/kg EPA and DHA (docosahexaenoic acid), respectively). Moreover, in fish oil as used in our study, the n-3 PUFAs are present in the form of triacylglycerols, whereas Sato et al. used purified EPA ethyl ester. It should also be mentioned that the main fat source in the diets used in our study is corn-oil rich in n-6 fatty acids, whereas Sato et al. used anhydrate milk fat containing more than 60% saturated fat. Last, the amount of sucrose used in our study is higher than the dose used by Sato et al. It is worth noting, however, that both the study by Sato et al. and our study demonstrated that sucrose did not reduce the ability of fish oil and/or EPA to prevent diet induced accumulation of fat in the liver.
A strong association between obesity and adipose tissue inflammation exists and obesity is characterized by chronic low-grade inflammation in adipose tissues 
. In light of this it may not be surprising that expression of macrophage and inflammatory marker genes was elevated in obese mice compared to lean mice. Still, as the anti-inflammatory effect of fish oil in adipose tissue is well described 
, it was unexpected that the expression of inflammatory markers was similar in adipose tissue from obese corn oil and the fish oil fed groups. In our study the state of obesity rather than the n-3
n-6 PUFA ratio in both feed and adipose tissues appeared to determine the expression levels of inflammatory markers in adipose tissue.
Chronic low grade inflammation plays an important role in development of insulin resistance 
. Pioneering work by Storlien et al. 
, later confirmed by several others 
has demonstrated that n-3 PUFAs can prevent development of diet-induced insulin resistance in rodents. The insulin sensitizing effect of n-3 PUFAs is generally accepted to be related to the anti-inflammatory effect, recently demonstrated to be mediated by the GPR120 receptor 
. Calculation of HOMA-IR indicated that mice fed high levels of proteins were more insulin sensitive than mice fed sucrose, but no significant differences was observed in an ITT. Similar to expression levels of inflammatory markers in adipose tissue, this was irrespective of whether the diets were supplemented with corn oil or fish oil. It was therefore unexpected that the GTT demonstrated that mice fed corn oil or fish oil in combination with sucrose or protein exhibited impaired glucose tolerance irrespective of whether or not the mice remained lean. It is possible that different mechanisms underlay the impaired glucose tolerance observed in the sucrose and the protein fed mice. It is likely that impaired glucose tolerance in sucrose fed mice is related to the obese state. Of note, in fish oil and protein fed mice, glucose tolerance was impaired even if weight gain and inflammation were maintained at low levels. Further studies are required to elucidate the mechanisms underlying the impaired glucose tolerance in these mice, but the possibility that adaption to a low carbohydrate intake with concomitant high hepatic gluconeogenesis and glucose output should be considered.
Seen as a whole, our study indicate that the sucrose
protein ratio, rather than the n-6
n-3 PUFA ratio in the diet determines development of obesity, adipose tissue inflammation and glucose intolerance. Activation of the NF-κB system appears to represent a link between obesity, inflammation of adipose tissue and insulin resistance 
. Insulin is able to activate NF-κB by phosporylation of IκBα in different cell systems 
, thus, high levels of circulating insulin may activate the NF-κB system also in adipose tissue. Whether increased insulin levels in sucrose fed mice translated into activation of the NF-κB system in adipose tissue in these mice will require further investigation.
Together our results demonstrate that the background diet exerts a crucial influence on 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 promote obesity development in mice. As the intake of sucrose in Western societies is high and increasing dietary intake of n-3 PUFAs is recommended by several health authorities it would be of importance to investigate whether the background diet influences the effect of fish oil also in humans.