...no to the willpower part, or choice, though this might quickly become tangled in the semantics of those words.
Part of what I believe (and assert) is that understanding, belief, intention—all the self-conscious cognition and knowledge we have around e.g. fat and sugar, has on the whole moderate (or no) influence on our actual behavior,
because it is opposed by and undermined by the other part of our brain, the part that evolved in conditions within which sugar and far were extremely scarce.
The dilemma the developed world has today is that cheap calories are cheap and we live in a condition of abundance, with bodies (brains) evolved for a condition of scarcity.
It's not that any one person might not make a reasoned decision, or successfully inculcate habits that remove them from situations where their own instinctual selves short circuit those decisions...
...it's that on the whole, most people are not very successful at this. Modulo the conditions of upbringing and culture etc... which are relatively modest influences...
It's hyperbolic to say we have no defense, I concede... it might be better to say, it takes some happy mixture of conditions including habit-formation and cultural context, to aid the cognitive self in overcoming the animal self.
The main point though was, we have recapitulated a comparable dilemma with respect to methodically crafting social media (etc etc) to engage the animal self. For fun and profit. And the hidden and deferred costs of this are legion.