Apart from Skittles really interesting post, there's a lot of emphasis on how individuals are snacking more and eating less...but hardly anyone has mentioned that the actual contents of your food has not changed for the better.
Time and motion studies show that although children may have played out more, there's not as much difference between the average activity level of an office worker in the 1970's and now. There were more manual workers but most weren't.
Smoking rates have gone down as well, which is great for health, but they do put an (artificial) ceiling on metabolism and nations that smoke a lot are often quite slim!
What I meant by food content is that the food you are eating now is simply not as nutritious as it was. There's lots of evidence that the way meat is reared, especially cheap meats (which were expensive then) makes them flabby and fatty- like chicken, so basically your chicken you eat with potatoes is far more fat than a 1970's chicken.
You can easily see this with carrots- 1970 carrots were small and hard and not that sweet- the new ones are more carby and larger and sweeter. Nicer for our tastebuds, but if everything in your diet is sweeter and fattier- you will get fat! The food industry have been allowed to create entire ranges of what are called 'hyperpalatable' foods- high in fat, sugar and processed meat. Look at bacon- it now has added water, often a sugar solution and salt and the meat itself is fattier and less lean. Processed foods full of glucose syrup are an even more obvious example.
This is not individual people's fault. Even if they ate a similar portion size (and they have crept up) and even if they didn't snack and ate a nice bit of chicken wrapped in bacon with carrots on the side- those foods do not have the same nutrients as they would have had in the past, and that is to all our detriments.
We have been done over by the food industry which has been allowed to reign unchecked. Eating lean meat, unprocessed non-chemically covered vegetables and decent bread without huge amounts of processing and sugar, which was common in the 70's is now a luxury that only wealthy organic lovers can afford. That is outrageous. In my husband's country, which wasn't part of the EU for a long time and so didn't adhere to these standards- the food is measurably better quality, the meat doesn't run with white solution when you try to cook it and the vegetables are far more plentiful (he actually laughed on seeing three small bits of purple sprouting broccoli for £3 in a shop once)- even the poorest people eat a lot of vegetables or grow their own because they have to, and there's no choice, but that has turned out not to be a bad thing. In Nordic countries they have banned transfats- we still get to eat them up here.
The international gov'ts of the world, and in the UK are complicit in allowing the market to make everyone very fat indeed, but no-one seems to want to reign them in. Instead they'd much prefer it if you just all blamed each other for a lack of willpower and inability not to snack, whilst the food industry laughs at us chomping on their hyperpalatable cheap rubbish.