Water....

Chemicals through foods

I think Cancers are also a factor of longevity - unlike what you have heard - human beings never used to live to old age in large numbers. I know you have a grand mother/grand father somewhere you point at to say watu wa zamani walikuwa wanaishi sana.... they are more the exception than the norm.
Secondly, the kind of things people eat these days are not good... my suspicion sana sana is mafuta ya transformer being used to fry chips! plus all manner of things that find their way into food production.

And food additives? Once I learnt that they use bleach in the manufacture of blue band I said never again! And this is what they encourage our kids to eat!

What about child cancers?

@Shak, They also bleach unga/sugar to make it "more white" - Benzoyl peroxide &Calcium peroxide.

@Chemirocha yes Chlorine is used to sanitize food.

My paternal grandparents died at old age

They had 7 children, 4 of whom have since passed at sub 65

2 from cancer

Google is your friend. Bleach is used to give it a pale colour

what about their relatives - your grant parents age mates. Hao tunaona ndio walikuwa maSurviver! People used to die like flies.

QUOTE
Why You Should Avoid Margarine, Shortening and Spreads
There are a myriad of unhealthy components to margarine and other butter imposters, including:
Trans fats: These unnatural fats in margarine, shortenings and spreads are formed during the process of hydrogenation, which turns liquid vegetable oils into a solid fat
Trans fats contribute to heart disease, cancer, bone problems, hormonal imbalance and skin disease; infertility, difficulties in pregnancy and problems with lactation; and low birth weight, growth problems and learning disabilities in children.
A U.S. government panel of scientists determined that man-made trans fats are unsafe at any level. (Small amounts of natural trans fats occur in butter and other animal fats, but these are not harmful.)
Free radicals: Free radicals and other toxic breakdown products are the result of high temperature industrial processing of vegetable oils. They contribute to numerous health problems, including cancer and heart disease.
Synthetic vitamins: Synthetic vitamin A and other vitamins are added to margarine and spreads. These often have an opposite (and detrimental) effect compared to the natural vitamins in butter.
Emulsifiers and preservatives: Numerous additives of questionable safety are added to margarines and spreads. Most vegetable shortening is stabilized with preservatives like BHT.
Hexane and other solvents: Used in the extraction process, these industrial chemicals can have toxic effects.
Bleach: The natural color of partially hydrogenated vegetable oil is grey so manufacturers bleach it to make it white. Yellow coloring is then added to margarine and spreads.
Artificial flavors : These help mask the terrible taste and odor of partially hydrogenated oils, and provide a fake butter taste.
Mono- and di-glycerides: These contain trans fats that manufacturers do not have to list on the label. They are used in high amounts in so-called "low-trans" spreads.
Soy protein isolate: This highly processed powder is added to "low-trans" spreads to give them body. It can contribute to thyroid dysfunction, digestive disorders and many other health problems.
Sterols: Often added to spreads to give them cholesterol-lowering qualities, these estrogen compounds can cause endocrine problems; in animals these sterols contribute to sexual inversion.

My guess would be water. I lost my dear grandpa a while back to stomach cancer too. In my shags area, clean, treated and piped drinking water was not a reality until some time in the mid 1990s. At the same time, I recall coming across posters and calendars with all manner of agrochemicals (kina round-up, etc) ever since my days as a toddler. The primary economic activity has always been coffee and tea farming.

When it rains, these chemicals will obviously find their way into the local streams via surface run-off, the very streams they relied on for drinking water. Noticed the same all around Meru, Embu, Kirinyaga and adjacent areas. Cancers of the GIT are quite common.