Yes, that is true... you will likely have less of a background in some things than other applicants. Big deal!
I was a German major in undergrad, and a Health Promotion major in grad school, and I find that I just bring a different set of skills to the table than some other med students. (Also, if you are at a good school where the students are supportive of one another, you can usually find a colleague who will help you out with their extensive undergrad knowledge, if necessary...)
The main thing is, be prepared for the MCAT no matter what your major. You can then use your extensive psych/anthro experience to wow 'em at the interviews.
After that, as Doc Admin points out, you will get more than enough bio/biochem/etc. to keep you busy. Oath. Best of luck to you!