The only proven way of currently extending life expectancy in a variety of mammals is undernourishment (not malnourishment) but requires further study to see whether it has undesirable side effects.

Long lived species have less response to famine so, according to Aubrey de Grey, not likely to have a big impact in humans.

Laboratory mice primarily die from cancer, so any treatment that reduces cancer incidence is likely to have an impact on average lifespan even if not affecting the underlying ageing damage.

And when it comes to humans on a standard western diet (not that healthy), research is difficult as reducing calories may be just from normal (too much) to healthy levels. That doesn't necessarily mean reducing calorie intake further will be good for you.