Bottom-line: French scientists conclude omega-6 fats are “remarkable booster of adipogenesis”. Prog Lipid Res. 2006 May;45(3):203-36

Summary: In this comprehensive 33-page review, a team of French scientists reviewed globe-spanning research—leaving no mechanism or relevant study unexplored--from animal and human studies, to food consumption trends over the last 40 years (including increased omega-6 fats in baby formula) and obesity trends.  Their fascinating review concludes that disproportionate of omega-6 fat in the diet, in conjunction with sedentary lifestyles will inevitably lead to fat cell formation and to an increased in the prevalence in obesity and metabolic syndrome.  And they cite plenty of studies and plausible mechanisms, including:

• American Kids are Fatter than French Kids—Omega-6 Factor.  School-age American children have twice the prevalence of obesity compared to European kids. Both groups of kids eat about the same amount of calories from protein, carbohydrates and fat.  The distinction however is that American kids eat twice the proportion of omega-6 fat. 

• Omega-6  Make Rats Fat.  Rats fed a high omega-6 fat diet, gain weight rapidly and weigh more than rats fed a diet with identical calories, but less omega-6 fat.  For example, in one study researchers fedratsone of three high fat diets: soybean oil (omega-6 fat), fish oil, and high saturated fat (palm oil).  The soy oil diet rats had the highest amount of body fat of the group, even when these omega-6 fat rats were exercised. 

Omega-6 Fat Increases Insulin Resistance and Prostacyclin. Insulin resistance makes it harder for the body to lose weight and can lead to diabetes.  Omega-6 fats make the omega-6 eicosanoid, prostacyclin, which triggers the cells to make fat.  Notably, inflammation perpetuates this cycle.  

Comment: This compelling review offers a significant plausibility that high dietary omega-6 fats increase the risk for obesity. But as of yet, there are not any random-controlled-double-blind human intervention studies, to show cause and effect. Note, while you have to pay to get the full text, it is well worth the money. The link to the abstract is below.

Link to Abstract:
Ailhaud G, Massiera F, Weill P, Legrand P, Alessandri JM, Guesnet P.
Temporal changes in dietary fats: role of n-6 polyunsaturated fatty acids in
excessive adipose tissue development and relationship to obesity. Prog Lipid Res. 2006 May;45(3):203-36.