Sixteen studies (n=91,426) were included: 2 RCTs (n=79), 1 controlled before-and-after study (n=20), 4 non-controlled before-and-after studies (n=69), 2 prospective cohort studies (n=72,750), 1 retrospective study (n=2,152), 2 case-control studies (n=874) and 4 cross-sectional studies (n=15,482).
Prevention of ARMD (6 observational studies).
One prospective cohort study (n=72,489) reported no significant association between the intake of individual omega-3 fatty acids, white-meat fish or dark-meat fish and ARMD. It found that eating tinned tuna (RR 0.61, 95% CI: 0.45, 0.83) or all fish (RR 0.65, 95% CI: 0.46, 0.91) and eating any type of fish more than four times a week (RR 0.65, 95% CI: 0.46, 0.91) compared with eating fish three or fewer times per week were associated with a significantly reduced risk of ARMD using multivariate analysis. One cross-sectional study (n=3,654) reported eating fish one to three times per month was associated with a significantly reduced risk of late ARMD compared with eating fish less than once a month, in an adjusted analysis. The study found no significant association between fish intake and a reduced risk of early ARMD.
Two case-control studies (n=21 and n=853, respectively), 1 retrospective study (n=2,152) and 1 cross-sectional study (n=7,883) reported no significant association between fish intake and early or late ARMD.
Slowing of progression of ARMD (1 RCT).
One RCT (n=35) reported a significantly shorter recovery time in patients who received photodynamic therapy plus antioxidant/polyunsaturated fatty acid compared with photodynamic therapy alone at 20 days, but not at 40 or 60 days.
Progression to advanced form of ARMD (1 prospective study).
One prospective study (n=261) reported no significant association between fish intake and advanced ARMD.
Progression of retinitis pigmentosa (1 RCT, 1 comparative before- and-after study and 2 non-comparative before-and-after studies).
One RCT (n=44) reported a non-statistically significantly lower loss of cone function and rod electroretinography (ERG) loss in patients allocated to docosahexaenoic acid (DHA) compared with placebo, but no significant difference between treatments in visual fields, acuity and dark adaptation. Fundal photography showed that progression was significantly less in patients allocated to DHA compared with placebo.
One controlled before-and-after study (n=20 enrolled, n=16 analysed) reported significant improvements in visual acuity in the 16 patients with follow-up data allocated to either lutein or lutein plus omega-3 fatty acids plus vitamin B plus enzymes.
One report of 2 small uncontrolled before-and-after studies (n=6 in total) reported no significant change in ERG results.
Progression of proliferative retinopathy and clinically significant macular oedema in patients with diabetic retinopathy.
One non-comparative before-and-after study (n=48) reported improvements in the number of functioning capillaries, a decrease in presumed vitreous permeability to fluorescein dye, and a reduction in the visual acuity factor in the photostress test. However, the results data were sparse and difficult to interpret.
Prevention and slowing of the rate of progression of age-related cataracts (2 cross-sectional studies). One cross-sectional study (n=2,900) reported that omega-3 plus omega-6 consumption (using broccoli/spinach consumption as a distant proxy measure) was associated with cortical but not nuclear cataract. One cross-sectional study (n=1,045) reported no significant association between omega-3 consumption, obtained from food or oils from fish or seafood, and nuclear cataract.
One non-comparative before-and-after study (n=15) reported that 9 of the 14 patients with pre-existing cataract showed improved vision, measured using Landolt's rings, associated with DHA.
Adverse effects.
One RCT (n=44) reported no withdrawals in patients due to adverse effects. Six patients allocated to placebo and 4 patients allocated to an omega-3 fatty acid-containing intervention reported short-term minor adverse effects.