biochemistry - How does beta branching stop alpha helices from forming?

I am told that beta branching interferes with alpha helix formation.

Problem is that I don't see how beta branching has anything to do with alpha-helix formation. Beta-branches are on the outside of alpha helices and internal hydrogen bonding holds the helix together.

My book (Lehninger) also doesn't say anything about beta branching interfering with alpha helix formation.