There are, I heard on NPR, about 9 promising Alzheimer's prevention therapies in trial . It's likely that, sometime in the next decade, one of them will be a mega-blockbuster -- the biggest change in medical care since the discovery of insulin.
On the other hand, I doubt we'll see a great obesity prevention therapy, despite the billions pharmas are investing into obesity research.
Why is one relatively easy, and the other so hard? Why is it so hard to come up with a safe and effective male contraceptive, but so easy to render women infertile? The Pill (oral contraceptive pill, aka OCP), after all, made its debut over 40 years ago.
The trick is whether one is fighting nature, or not. Women in pre-industrial societies are naturally infertile for most their reproductive years. They're either pregnant, lactating, or cyclically infertile. These are natural states, OCPs need only send "pregnant-lactating" signals and the body does the rest. On the other hand there is no natural state of male infertility. When you try to render a male infertile, you are fighting hundreds of millions of years of evolution.
Obesity is what happens when a biological system engineered to manage scarcity runs into an environment of abundance. The system crashes. Humans did not evolve to have access to so many calories all the time, like all animals we're engineered to use energy as efficiently as possible. Changing that means defeating evolution.
Cancer appears to be a process that natural selection has carefully tuned to balance biological repair mechanisms. It's darned hard to fight cancer.
Alzheimer's though, is a universal lifelong proces that disables the aged post-fertile human. It does not appear to have any significant evolutionary advantage, it is probably the end-result of a garbage collection process that works well within its operational "design" parameters. To delay the Alzheimer's process doesn't require defeating natural selection, so it's a much easier task that defeating obesity.
So I'm betting on an Alzheimer's preventive therapy long before a really good obesity preventive therapy.
 Note prevention and cure are different. There's not much prospect yet for curing Alzheimer's, though one might hope a good preventive therapy might allow natural reparative functions to operate better.