A large study in 2018 from Harvard University combined the results of 26 different studies examining the effects of walnut consumption on blood lipid levels and other risk factors for cardiovascular disease. What they found was that those eating one portion of walnuts a day (28g) had lower total cholesterol, LDL-cholesterol and triglyceride levels than those eating less than this amount. Walnut consumption did not affect body weight or blood pressure.
The results were also not affected by funding sources (walnut company vs no walnut company funding)
Why would this be? Walnuts contain plant sterols which prevent cholesterol being absorbed. The high omega-3 fats increase the clearance of LDL-cholesterol from the blood. They also contain polyphenols and tocopherols, which have antioxidant and anti-inflammatory properties.
So it may be that walnuts are one of the best nuts to include in the diet.