To me it seems critical that we get a realistic and accurate sense of the overall problem, since much of the difficulty is confusion regarding how to think of it in the face of seemingly conflicting information. I am going to offer my understanding of the scientific consensus as a framework.
For all the confusion we often have over various aspects of nutrition and fitness, I think there are a handful of principles that are particularly reliable based on the evidence we have so far.
The reason so many people around the world have been getting so much fatter so quickly is that they are taking in more energy than their bodies need in order to be nourished and satisfied. And they do this for reasons that they are mostly unaware of having to do with reward, preference, palatability, expectancy, reliance on cues, and satiety. Let's call this the mindless surplus intake theory of obesity.
As intuitive as it might seem at first, the mindless surplus intake theory of obesity doesn't have to be true. There could be other reasons for our growing fatness. Our bodies or environment might have changed to make many of us store fat more efficiently from the same amount of energy intake (the broken metabolism theory of obesity). A number of authors in recent years have promoted variations of the broken metabolism theory. The broken metabolism theory sounds scientific and some variations of it have been promoted with long lists of research citations or even with the limited blessing of some researchers, especially when it is combined with reasonable actionable advice as well. In general though it seems to me that the "broken metabolism" theory itself has been pretty thoroughly falsified as a primary driver of obesity. And that fact matters in some important ways.
Our bodies or environment might also have changed to make many of us more impulsive about eating and we have failed to compensate by exercising our willpower adequately (the gluttony theory of obesity). Similarly, our bodies or environment might have changed to cause us to be less motivated to move and we have failed to compensate by exercising our willpower adequately (the laziness theory of obesity). The gluttony and laziness theories are compelling to us individually because we have a strong intuitive sense of the importance of personal responsibility, and self-control is indeed a powerful factor in success across many endeavors. However the laziness and gluttony theories exaggerate the role of deliberate self-control in the myriad decisions we make every day about eating and activity. People successful at controlling their weight use their self-control not to make every decision deliberately but to create better habits for themselves and shape their environment to help them. It is not a failure of willpower that drives obesity. That fact also matters in some very important ways.
Our bodies or environment might also have changed to cause us to need less food while still eating enough to create a surplus (the inactivity theory of obesity). Activity levels contribute significantly to both health and obesity, and especially to the maintenance of healthy weight, but we know the driver of obesity is mostly intake and that just moving more without also cutting back energy intake doesn't reverse weight gain in the obese in general. This is almost certainly a major factor in both health and obesity, but it is clear from the existing evidence that it is not the primary driver of obesity. The trend in fatness corresponds far more closely to changes in intake than changes in activity. Also the experimental evidence shows activity being far more useful for maintaining weight loss than for simply "burning off calories" in most people, largely because we often tend to eat more to compensate for exercising.
In addition, more than one of these might be a factor. But from my reading of the evidence patterns, the primary driver of obesity is now clear. We have been taking in increasingly more energy than we need, mostly because we have been eating more than we would need to fuel our activities and nourish our minds and bodies. And we have been doing this for reasons that do not involve the specific discretionary macronutrients we eat most of (e.g. carbs vs. fats), do not involve us simply failing at deliberate self-control, and do not involve us having inadequate knowledge of which diets are best for weight loss.
What are these mysterious reasons for creeping intake if it is not eating too much fat or too much sugar or too much starch in particular as many authors have claimed?
- Our intake is regulated primarily by mechanisms of reward, preference, palatability, expectancy, reliance on cues, and satiety. We learn what to eat and how much based largely on stimulus cues, what we expect from food, how palatable foods are to us, and how they make us feel.
- Our reward and satiety mechanisms are optimized for regulating our weight under natural stimulus conditions by relying primarily on volume and weight of food and secondarily on energy content. We mostly tend to eat about the same volume of equally palatable and rewarding foods every day.
- Our metabolism is optimized for efficiency of energy storage rather than for maintaining a stable weight.
- We are bad at estimating how much energy we need in order to be nourished and feel satisfied.
- We are bad at estimating how much energy we are taking in in order to compensate for well-engineered distortions of reward and palatability.
- We have come to rely increasingly on cues in our environment to determine how much energy we need and how much we are taking in
- We have come to increasingly exploit our reliance on cues in our environment in order to market food and health and fitness products, and this distorts the cues we rely upon so heavily
- We have come to increasingly rely on strategies which overemphasize small or irrelevant metabolic effects, rely on outdated theories, rely on willpower, ignore the long term, and in general are unsustainable and make us feel like failures when we can't sustain them.
If my understanding of the scientific consensus is correct, the mindless surplus intake theory of obesity reflects it well and can be understood in more detail in terms of the satisfaction theory of intake regulation. This says that we eat mostly what satisfies us because of the way the reward and satiety mechanisms work in our nervous system rather than because of metabolic or nutritional effects. What gets "broken" in obesity in general is that we stop being satisfied with a nourishing amount of food and we keep eating even though we are taking in more than we need. The reasons for this have little directly to do with willpower or carbs or fats and everything to do with the mechanisms of reward, preference, palatability, and satiety.