A good place to start is to read Michael Pollan's Omnivore's Dilemma[1]. I was amazed at how much of our current food originates as corn. Corn is good at fattening cattle in record time, and seems to be having the same effect on humans who eat its derivatives.

[1] https://www.goodreads.com/book/show/3109.The_Omnivore_s_Dile...

Another explanation could be the people in the 70s had a healthier gut microbiome. I don't we think know enough to say for certain but studies have found gut bacteria affects metabolism and can be linked to obesity and diabetes.

Probably to due with less calories spent in manual work. Like walking instead of driving, washing clothes and utensils instead of using electronics, etc.

Calories spent from exertion are generally negligible unless it's a very intense focused kind of exercise done for a long duration. Run a whole mile and all that gets you is one free apple.

100 calories per hour mean an additional 571 calories per day if you spread it across the whole 7 days.

When your body needs 2600 calories per day just to stay at the same weight it is very easy to lose weight by cutting them to 2000 calories. When your body only needs 2000 calories then you need to cut them down to 1400 calories for the same result which may be impossible from a psychological perspective.

One free apple a day over decades makes quite an impact.

You need to eat about 7,700 calories too much to gain a kilogram of weight (https://www.livestrong.com/article/304137-how-many-calories-...). So, eating 25 calories too much each day gains you that kilogram in a year. If you keep doing that between ages 10 and 40, you ‘gain’ 30 kilograms, and chances are you end up obese.

You can counteract that by burning about 25 more calories each day. To do that by walking, you need to walk about half a mile extra each day (https://www.livescience.com/8931-calories-walking-burn-depen...)

The difference in the amount of calories I burn on days when I go to work, and am mostly sedentary, and days when I'm at home puttering around constantly, is massive.

At least as tracked by my Fitbit it can be upwards of 2000 calories difference, and 10-15k steps.

It's unlikely that your TDEE is 4000 calories when you're at home puttering around. These fitness trackers are extremely unreliable at estimating caloric expenditure.

Whether it is accurate in absolute terms is less interesting than what it reports in differing trends. Every indicator that it reports is vastly higher on the days when I am not in the office, chained to my desk. Just being on my feet, walking back and forth all day, up and down and round and round, is a night and day difference.

The composition of a diet makes difference.

May it be that it shifted to increased share of heavily processed food. Also, it can be related to changes in the way we grow, feed or fertilise cattle and plants. This can lead, some doctors say, to leaky gut and inflammation; many of us may suffer from those without being aware of it.

I agree. Feels like multiple environmental factors might be at play.

Lately I’ve been wondering about exposure to PFAs on all sorts of packaging.

I am inclined to agree. At least one study (that I have found) has showed a clear influence of pesticides and pollutants ingested by or exposed to the mother to the child's probability of being obese.

Liu Y, Peterson KE. Maternal Exposure to Synthetic Chemicals and Obesity in the Offspring: Recent Findings. Curr Environ Health Rep. 2015;2(4):339–347. doi:10.1007/s40572-015-0068-6

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5482496/

I thought it was common knowledge by now that 'caloric intake' is mostly useless as a predictor for obesity? Mostly useless, because it says nothing about the amount of energy the body has to expend to metabolize the foods that contain them, and it completely disregards systematic metabolic changes resulting from dietary choices.

Some bodies burn more calories at rest. Some bodies burn more calories when working out. All bodies burn available fat when they take in fewer calories than they burn. It's a physics thing.

A physics thing that is mostly useless if you look at caloric diet success ratio. People feel mere calories restriction as torture. If you want them to keep a diet, you should pay as much attention to what they eat as to how much they eat.

I have an unfounded belief that people do not actually use up most of the calories they eat, they are excreted. Everyone, including both normal-weight ("thin") and obese people eats ingests more calories than is needed by the body given the sedentary lifestyles in modern society, yet some do not gain weight as quickly. Additionally, given how efficient human bodies are, meaning how much work you have to do to burn an apple's worth of calories, it is impossible to suggest that everyone of normal weight burns all excess calories.

I have a well founded belief that obesity cause is sugar and its close relatives. There's no mistery about that. Obesity is almost unknown before sugar is introduced in a country's diet.

Current epidemic is caused because sugar is not only available but practically unavoidable if you consume prepackaged food. It's added to all kinds of food, some of them, like mayo, bread or sausages, totally superfluous. Meanwhile some "experts" keep talking about dietary fat and calories.