You're spot on with respect to diet. I believe Singapore has the highest rate of diabetes in the developed world.
With respect to supposed excellent medical care, some problems in Singapore that I noticed:
- There is little access to preventative medical care. If you do get preventative medical care it tends to be very expensive (scan all the things!). In my home country, I would get a yearly test for glaucoma as my grandmother was affected and it's genetic, but in Singapore this is quite unheard of (I tried once and was subjected to a battery of tests that cost hundreds of dollars). In my home country most people get yearly dental care, but there's no such thing in Singapore.
- People change doctors a lot, which I think is problematic. However, you're kind of forced to change doctors every time you change companies or if your company switches to a different health insurance provider.
- Strong drugs are available over the counter, no questions asked. GPs also happily give out way too many drugs for minor issues.
- People in Singapore can end up losing all their money if they get a serious illness.
- Strong promotion of pseudo-medicine, such as TCM. It is legitimized even by otherwise respect universities, which may have TCM research centers. After a GP visit, it's also common that you get some pseudo-scientific medicine from the secretary handling your payment (e.g. 'don't eat chicken').
I'm not so much arguing against your hypothesis. However, I worry that optimizing for life span might not mean that people are healthier.
In fact, in some countries, death is sometimes chosen over necessarily prolonging life:
- Euthanasia
- Getting very expensive, painful/dulling medical procedures that end up prolonging your life is discouraged by doctors in some countries