One has to follow diet similar to what his ancestors had and also their daily routine. Our body releases enzymes, acids etc similar to what our ancestors bodies did, based on family diet. Until 1-2 generations earlier, people used to look at food habits (which also alter our DNA over generations) of both families before marriages were performed. Now, nuclear families, marriages between people of different food habits, living in different climates where different soil produces different food etc have lead to children with birth defects, obesity etc.
Not everyone will suffer, as most human bodies will adapt and adjust to different foods over few days or weeks. But why put that extra pressure on body and make it adapt ? Unless you are in army and posted at cold mountains or deserts, or need to work in alien climate for livelihood. People who live at native places have healthier bodies.
Another major reason is : Our foods have not changed since grandparents era but our habits have changed. We still eat all those fats, sweets, spices and also the quantity but our physical movement is just 10-20% of what grandparents did. Most urban jobs are asking you to sit for 8+ hours (in a car or bike or at an office). For all festivals, people will prepare or buy all types of sweets but just eat and sit.
Also increased alcohol, tobacco usage among youth (treating it as social symbol), more caffeine (both coffee and tea were introduced in past 300 years). We do not have pure form of tea like china or japan has. We mix boiled milk, sugar, cardamom, ginger, lemon and what not..
Live naturally and closer to nature