Culture and lifestyle strongly affect behavior for the outstandingly obvious reason that they are behavior. One's lifestyle is a collection of behaviors and habits that one perpetuates. One's culture is the setting in which one enacts those behaviors and habits which make up one's lifestyle and, due to the nature of setting, culture will, of necessity, effect one's lifestyle and, by connection, behavior. I'm not sure why someone would want to divide lifestyle and behavior, they seem quite the same to me. If one robs banks, one is a bank robber. Likely, the said bank robber lives a life of crime or, as we might say, a life exemplified by criminal acts, which immerses one in a criminal culture, and produces a criminal lifestyle. Behavior and lifestyle are much the same. If one commits crimes, one is exhibiting criminal behavior, and living a criminal lifestyle.
The question of how much culture affects behavior/lifestyle is a much better question. What effect does culture have on behavior? I will use culture loosely (as is the tendency nowadays) and say it includes such things as upbringing and beliefs. This question is relatively simple too. If you are raised to believe watermelons are poisonous, you probably won't eat watermelons. The amalgamation of such culturally influenced behaviors is your lifestyle.
But, where I deviate from much modern thought, is on two points. Firstly, cultural conditioning isn't everything. There is, within all human beings, a natural law, an instinctual knowledge of some primal level of good and bad. It may not be very refined, but it is present. Even cannibals know cannibalism is certainly quite a diabolical, spiritual, and serious thing. The proverbial exception to the rule is sex. Sexual norms in various cultures seem much more "flexible", and much easier to reconcile to human natural law, than other elements of natural law (such as theft, murder, or other similar things).
The other point I deviate from general thinking on cultural conditioning is the compulsory factor. Nobody forced me to become the person I am today, I chose. Cultural conditioning does not force someone to become something. Someone raised in a violently abusive and divorced home (such as my father) does not have to continue the cycle. They choose to continue the cycle. Yes, it may seem the natural choice due to conditioning and negative influences. Yes, it is a difficult choice to make the right(or, at least, what I believe the right choice is) choice. But it can be done. Compulsion is almost nonexistent in the world, exception being physically forcing someone to do something by overpowering their strength. Cultural conditioning and cultural norms do not force us to become who we are. We choose to become who we are, under the influence of cultural norms.