Mammals are unable to synthesize short chain polyunsaturated fatty acids ( n-3, and n-6 fatty acids ) and so these are considered essential fatty acids as they need to be consumed in the diet otherwise deficiency diseases appear.
The diet therefore has a combination of these sc-PUFAs present, and historically the ratio of n-6:n-3 PUFAs in the human diet was said to be much lower than in the present USA diet of about 16:1. The effect of those sc-PUFAs in the diet is complex, but generally n-6 are considered pro-inflammatory, and n-3 anti-inflammatory.
A number of studies have looked at the effect of n-3 PUFAs on disease activity in rheumatoid arthritis, and in this recent study of high dose fish oil (FO) vs low dose in new onset rheumatoid arthritis treated with triple therapy, the FO group did better. FO contains a large amount of long chain or marine n-3 PUFAs.
FO was associated with benefits additional to those achieved by
combination 'treat-to-target' DMARDs with similar MTX use. These
included reduced triple DMARD failure and a higher rate of ACR
It remains to be seen whether high dose FO has benefits in people other than those with rheumatoid arthritis, and currently NICE have stopped recommending the consumption of oily fish or FO for cardiovascular disease prevention as there is now insufficient evidence of efficacy.
The adverse effects of a diet high in n-6 PUFAs was recently shown in a re-examination of the Sydney Heart Study. 458 men aged 30-59 years with a recent coronary event were asked to reduce saturated fat intake, and to take n-6 PUFAs instead from safflower oil and safflower oil polyunsaturated margarine. The results were
The intervention group (n=221) had higher rates of death than controls
(n=237) (all cause 17.6% v 11.8%, hazard ratio 1.62 (95% confidence
interval 1.00 to 2.64), P=0.05; cardiovascular disease 17.2% v 11.0%,
1.70 (1.03 to 2.80), P=0.04; coronary heart disease 16.3% v 10.1%, 1.74 (1.04 to 2.92), P=0.04). Inclusion of these recovered data in an updated meta-analysis of linoleic acid intervention trials showed
non-significant trends toward increased risks of death from coronary
heart disease (hazard ratio 1.33 (0.99 to 1.79); P=0.06) and
cardiovascular disease (1.27 (0.98 to 1.65); P=0.07).
and they concluded that
An updated meta-analysis of linoleic acid intervention trials showed
no evidence of cardiovascular benefit.
Although there has been some criticism of this study in that the margarines of that time were likely high in transfats which could have contributed to the excess cardiovascular mortality, the suggestion is that it is much safer to ensure that you keep the ratio of n6:n3 PUFAs as low as feasible.
 Proudman SM, James MJ, Spargo LD, [..], Cleland LG. Fish oil in recent onset rheumatoid arthritis: a randomised, double-blind controlled trial within algorithm-based drug use. Ann. Rheum. Dis. 2013 Sep 30. doi: 10.1136/annrheumdis-2013-204145. PubMed PMID: 24081439.
 Ramsden CE, Zamora D, Leelarthaepin B, [..], Hibbeln JR. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis. BMJ. 2013 Feb 4;346:e8707. PubMed PMID: 23386268.