High fat and high triglycerides visibly thicken the blood.
Ask any phlebotomist who draws blood all day long, and they will tell you who is dehydrated and who has really high triglycerides.
I've been told that some triglycerides are so high the fat rises to the tops of the tubes!

Some young adults on Accutane can test out at over 1000!

I know there are some links on protein/blood viscosity, but I think they demonstrate less of an effect than fat content.

Amino acids do not clump together like the MGUS peptides. The more clumping and longer the chains, the more viscous the serum becomes.

It is really important to drink enough non-diuretic fluids therefore, both for the kidneys and for keeping the blood fluid.