Body weight (or more precisely, body fatness) is regulated by nonconscious parts of the brain. As a person loses weight, this regulatory system turns on, activating a "starvation response" that increases hunger, cravings, and attention to food, and reduces fullness at meals. The more weight a person loses, the stronger this response becomes, and it eventually reduces metabolic rate somewhat as well. This causes people to feel like they're fighting their own impulses in order to lose weight and maintain the loss (because they are).

I'm not aware of evidence that this response is affected by time per se. It definitely depends on the amount of weight a person has lost, and weight decreases over time in the early stages of a diet. So it's indirectly connected to time in that way.

The starvation response is strongest when fat loss it at its maximum, and it declines when a person regains fat (https://dx.doi.org/10.1002%2Foby.21653). Because it continues as long as a person remains below their original weight, this can lead to a situation where a person feels like they're still trying hard to diet, even as they are regaining weight. Very frustrating!

One thing that may be directly determined by time is how long we're willing to fight the starvation response. It's easier to fight hunger for one month than it is to fight hunger for six months. The starvation response wears most of us down eventually.

That said, I don't want to make the situation seem hopeless. There are ways to mitigate the starvation response, which I've touched on in other responses and I cover in more detail in my book.

I would go back on Joe Rogan's podcast by myself if he invited me. The debate was too contentious and unstructured for me to get my own ideas across effectively.

Leptin is a hormone produced by fat tissue that the brain uses to gauge the size of body fat stores. It plays an important role in the response to weight loss.

Here's how it works. When a person loses weight, body fat mass declines, and this causes leptin levels to decline. This activates nonconscious parts of the brain tasked with regulating body fatness. The brain then initiates a "starvation response" that includes increased hunger, increased attention toward calorie-dense foods, increased cravings, reduced satiety (fullness), and reduced metabolic rate (https://dx.doi.org/10.1172%2FJCI35055, https://doi.org/10.3945/ajcn.111.012385, https://doi.org/10.1210/jcem.87.5.8628). Your brain instinctively pushes you to eat more food. This is one of the main reasons why weight loss is hard, and often temporary.

That said, as an individual trying to lose weight, knowing your leptin level isn't very useful. It doesn't give you actionable information.

One thing I haven't told you yet is that in addition to responding to changes in body fat mass, leptin also responds to short-term changes in energy balance (energy in vs. out). In other words, if you overeat for a day or two, your leptin levels will increase even though your body fat mass hasn't changed much. Similarly, if you undereat for a day or two, the opposite will happen.

Another way to change energy balance is to exercise. I'm not an expert on the impact of exercise on leptin levels, but my read of the literature is that it takes very strenuous or prolonged exercise, especially over a number of days, to lower leptin levels to the same degree as undereating. For example, running an ultramarathon or going through military combat training reduces circulating leptin quite a bit (https://doi.org/10.1016/S0026-0495(97)90200-6, https://doi.org/10.1097/00005768-200210000-00010). Moderate exercise of the type that most people engage in doesn't seem to have much impact on leptin levels, independently of its impact on body fatness (https://doi.org/10.1152/jappl.1997.83.1.5).

There are many, but here are a few big ones that spring to mind.

1. Obesity, and difficulty losing weight, are caused by a "slow metabolism". That may be true in some cases, but usually the thing sustaining higher weight and preventing weight loss is a high calorie intake. On average, people with obesity actually have a higher metabolic rate than lean people, because they have more tissue to sustain (https://doi.org/10.1093/ajcn/35.3.566, https://doi.org/10.1038/ijo.2012.172). One origin of this myth is that people often underestimate their own calorie intake.

2. Weight loss can only occur by cutting carbohydrate intake and reducing insulin levels, while calorie intake is irrelevant. This whopper is refuted by a large amount of scientific evidence. Cutting either fat or carbohydrate out of the diet causes body fat loss, regardless of insulin levels, and the effect is mostly explained by calorie intake (https://dx.doi.org/10.1016%2Fj.cmet.2015.07.021, https://doi.org/10.1056/NEJM197110072851504). It is true that cutting carbohydrate intake tends to cause weight loss, but that appears to happen because it causes people to eat fewer calories.

3. All we have to do to lose a large amount of weight is eat one less slice of bread per day. The claim is that since a slice of bread contains about 80 calories and a pound of body fat contains about 3,500 calories, reducing food intake by just one slice of bread per day will cause a pound of body fat loss every 44 days, eight pounds of fat loss per year, and 80 pounds of fat loss over a decade. But what if you only had 30 pounds of fat to lose; would you wither away into a skeleton? Just from eating one less slice of bread per day? No, because that's not how weight loss works. As we lose weight, our bodies become smaller and our energy needs decrease, and the gap between the number of calories we're eating and the number we're burning gradually narrows, slowing weight loss until it gradually halts (https://doi.org/10.1016/S0140-6736(11)60812-X). Eating one less slice of bread per day would cause a person to stabilize about eight pounds lighter, very slowly over about three years. Not bad, but also not as effective as claimed.

There are several reasons, but I'll focus on two.

1. Our brains are hard-wired to drive us toward unhealthy foods. We're hard-wired to like sugar, starch, fat, protein, salt, and glutamate (umami). Those substances are detected in our digestive tract as we eat them, and they activate a neural pathway that causes dopamine release in our brains, stamping in a motivational response. Up to a point, the more of these substances are in a food (especially when combined), the more dopamine our brain releases.

This causes our brains to learn to be highly motivated by the sight, smell, and taste of certain foods, as well as the location where we habitually eat them and other cues they're associated with (a combination of classical and instrumental conditioning). So when we smell that pizza coming out of the oven, we begin to crave, and we may even begin to feel hungry when we don't need to eat. This "learning" happens on a nonconscious level.

This doesn't happen as much with plain lentils or raw carrots because they contain less of the things that spike dopamine in the brain. When we examine the foods that people report craving the most and experiencing addiction-like eating behaviors with, they're almost always processed, concentrated combinations of carbohydrate, fat, and/or salt (https://doi.org/10.1371/journal.pone.0117959).

This system worked well for our distant ancestors because it motivated them to seek the foods they needed to survive and reproduce. But in the modern world, those strong motivations are in excess of what we need to meet our calorie needs, especially in the context of expertly designed foods that spike our dopamine levels higher than the simple foods of our distant ancestors.

2. Our brains are wired to resist weight loss attempts. We have a brain system, located primarily in a nonconscious brain region called the hypothalamus, that regulates body fatness (https://dx.doi.org/10.1210%2Fjc.2011-2525). Generally, if a person isn't trying to change her weight, she will be at her brain's preferred weight, which I'll call her "setpoint". The setpoint is determined by both genes and environment.

When she tries to lose weight, the level of a hormone called leptin will decline, and her brain will sense this and activate a sort of starvation response. It will increase hunger and cravings, decrease fullness at a meal, and decrease metabolic rate. This will continue until the fat is regained and she attains her setpoint.

So part of your brain literally thinks you're starving when you lose weight and tries to sabotage your weight loss attempts, and this is a key reason why weight loss is hard and most people regain some or all of the weight they lose. It's much harder to stay lean if you previously had obesity than if you started out lean, because in the former scenario you're below your setpoint.

That said, there are ways to appease the brain systems that put up this obstacle, at least to some degree. Eating simple unrefined food that's less seductive, lower in calorie density, higher in protein, and exercising regularly are a few things that can help.

An important insight into eating behavior and body weight is that our behavior and physiology are guided to a large extent by non-conscious brain functions. Most people tend to intuitively believe that their decisions are fully conscious and willful, but where exactly does hunger come from? Where do cravings come from? We don't decide to experience those; they well up from non-conscious parts of our brains and then we experience their pull on our behavior. Our eating behavior is guided by habits, hunger, fullness, cravings, and other forces that are mostly beyond our direct conscious control (not to say that we have no conscious control at all).

If we aren't trying to achieve a specific dietary goal, like weight loss, most peoples' eating behavior is quite intuitive. We eat the foods we're accustomed to and like the taste of, when we're hungry or in the usual context/time, and stop when we feel full. For the most part, we let the non-conscious brain drive the bus. That's the default way in which humans interact with food, and that's probably how most people have eaten throughout most of human history.

Yet when we want to change something, we often approach eating very differently. We take a top-down approach and try to impose change on ourselves, running roughshod over the signals from our non-conscious brain. Perhaps we stop eating when we're still hungry, or ignore cravings. This takes a lot of effort and feels unnatural. Calorie counting for portion control is an example of this. Some people can do it successfully, but they tend to be unusually motivated and persistent.

The strategy I prefer is to work with the non-conscious brain and try to recruit its help. This is a bottom-up approach. For example, rather than eating less than you would prefer, can you design a diet whose properties naturally lead to a lower calorie intake? Can you design a food environment that stops your brain from triggering cravings as often, rather than trying to fight cravings after they arise? At its best, this allows you to eat and live in a more comfortable, intuitive way, yet still make progress toward your goal.

These strategies are achievable and effective (although to be clear, there is currently no "silver bullet" for obesity). For example, Kevin Hall's group recently showed that simply putting people on a diet of minimally processed foods causes them to eat fewer calories and lose body fat, even though they weren't asked to eat less (https://doi.org/10.1016/j.cmet.2019.05.008). In the same study, people who were put on a diet of highly processed foods experienced the opposite effect. Similarly, Christopher Gardner's group showed that asking people with excess weight to eat whole-food-based low-carb or low-fat diets led them to spontaneously eat fewer calories and lose weight over a one-year period (https://doi.org/10.1001/jama.2018.0245). Rather than forcing themselves to eat less, these people spontaneously and intuitively ate less because of how their new diet interacted with their non-conscious brains. These are just a few examples of this type of strategy-- there are many others.

Simply stated, I recommend three main things as a first pass:

1. Focus the diet on minimally processed foods that are simple and have a lower calorie density. This includes foods like fresh fruit, fresh meats and seafood, beans, potatoes and sweet potatoes, oatmeal, and vegetables. If you want to eat a potato, just eat a potato rather than burying it in calorie-dense flavorings. This principle limits most foods that are made from flour, added fats, and added sugars.

2. Eat more protein.

3. Build a supportive food environment. Make it easy to engage in behaviors that support your goal, and hard to engage in behaviors that don't. That means putting all foods away so they aren't visible between meals at home or at work, and it means banishing from your home the problem foods that tempt you even when you can't see them.

If that isn't enough, you can try building on those strategies by adding other elements such as carbohydrate or fat restriction, calorie counting, or intermittent fasting. Some people may also benefit from keeping a food diary to raise awareness of their eating behavior, managing stress eating, improving sleep, and getting regular physical activity.

Overconsumption of calories is typically the reason weight loss attempts fail. That said, I don't think it's productive to blame oneself, or others, for that. It's just how our brains are wired.

There are a few things that I think commonly contribute to the overconsumption of calories during weight loss attempts.

1. An unsupportive food environment. If you're surrounded by foods and food cues that trigger your eating drive, it will be hard to keep your diet in line with your goals over the long term. Sights and smells that remind your brain of food will trigger your cravings and hunger, and you'll be left struggling against them. Most people give in eventually. Again, this isn't a moral failure, it's just how our brains are wired, so it's best to plan around it.

2. Relying too heavily on willpower to control calorie intake. Eating fewer calories via portion control does cause weight loss, but it can be hard to sustain. You're essential pitting two brain functions against one another: in one corner, there are the brain systems that generate hunger and fullness; in the other corner, there are the brain systems that generate willpower. This internal conflict is hard to sustain indefinitely, and the hunger usually wins in the end.

3. Eating foods that favor overconsumption without realizing it. The research community has largely figured out the properties of food that make them more or less filling per calorie (https://www.ncbi.nlm.nih.gov/pubmed/7498104). Foods that are high in calorie density (calories per weight or volume), delicious, low in protein, and low in fiber tend to cause us to unknowingly eat more. Neither low-fat nor low-carb foods are necessarily filling per calorie, although some of them are. Bread for example seems light and fluffy, but as soon as you chew it, the air goes away and it doesn't occupy much stomach volume. Same for crackers, popcorn, and rice cakes. On the low-carb side, we have bacon, cheese, and especially added fats like oils and butter. It's better to focus on fresh foods that have plenty of water in them, like fresh meats, potatoes, oatmeal, beans, and fresh fruit.

Yes. There are many variants of it. The most-studied type of intermittent fasting involves restriction of food intake one to three days per week. In most studies, subjects are allowed to eat some food on restricted days, often about a quarter of their usual intake. In these studies, intermittent fasting is often compared against an old-fashioned low-calorie diet in people who carry excess weight.

These studies have been collected into meta-analyses, or "studies of studies", to give an overall view of the scientific literature on the topic. The results show that this type of intermittent fasting causes about nine pounds of weight loss over 3 to 12 months, roughly the same as a regular low-calorie diet (https://doi.org/10.11124/JBISRIR-2016-003248, https://dx.doi.org/10.1186%2Fs12967-018-1748-4).

In one study, participants did not eat any food on fast days, which occurred three days per week. After eight weeks, participants who did intermittent fasting had lost about the same amount of weight (16 to 18 lbs) and body fat as subjects who ate a regular low-calorie diet (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5042570/).

Another thing that is sometimes called intermittent fasting is time-restricted feeding (TRF). In TRF, you eat as much as you want each day, but you do it over a shorter period of time than usual. For example, you might restrict your "eating window" to eight hours rather than the usual 11 to 14. Most studies of TRF have been done on people fasting for the month of Ramadan, but some have been more traditional weight loss studies. A meta-analysis suggests that TRF causes modest weight loss, although there haven't been very many high-quality studies yet (https://doi.org/10.1007/s11154-019-09524-w).

As you can see, the evidence suggests that intermittent fasting is a valid tool for weight/fat loss, but it isn't generally superior to an old-fashioned low-calorie diet. In weight loss, the important thing is to create a sustainable calorie deficit, and that can be done in many ways, some of which don't involve paying attention to calorie intake per se.

I'll start by explaining what a "setpoint" is and how it works. This is the level of body fat that our brains/bodies regulate around. Some people naturally settle at a low body fat level, and others naturally settle at a high body fat level. For people who have a high setpoint and therefore naturally carry a lot of body fat, it can be hard to lose fat because the brain resists weight loss attempts. It "wants" you to stay at your setpoint, and it can be very persuasive. This process is regulated by the brain-- mostly a non-conscious part of the brain called the hypothalamus that plays an important role in eating drive. Genes are part of what determines whether someone has a high or low setpoint.

Although genetics influences our susceptibility to obesity, the setpoint isn't an immutable feature we're born with. As we age, most people in affluent countries gain weight and our setpoints rise. Lab animals fed rich, processed diets also see their setpoints rise over time. Similarly, there is strong evidence from animal studies, and suggestive evidence from human studies, that the setpoint can move in the downward direction. Diets that are less processed, less seductive, and higher in protein tend to lower the setpoint. Regular physical activity may also help, to a lesser extent. Bariatric surgery appears to have a powerful setpoint-lowering effect, and some weight loss drugs may as well.

The catch is that if you want to maintain a lower setpoint through diet and exercise, you have to maintain the diet and exercise regimen. If you revert to your old habits, your setpoint and weight will also revert. So it's not akin to wiping the slate clean and then going on with your life. It's a permanent commitment. Current evidence is limited but it suggests that simply maintaining a lower weight for a long period of time is not sufficient to reset the setpoint (https://doi.org/10.1093/ajcn/88.4.906).

I'm not an expert in the eating behavior of children, but I'll share a few general thoughts.

First, I think you're right to focus on the eating behavior of children. Children are still learning their food preferences and eating habits, while adults are less flexible. Childhood is the period during which we learn to like the foods that are particular to our culture (https://doi.org/10.1146/annurev.nutr.19.1.41). To a large extent, as adults we've already "learned" what we are going to learn about food and eating, and it's harder to change. For example, someone who didn't grow up eating strong French cheeses may never learn to like them. Someone who grew up snacking all the time may have a harder time limiting their food intake to meal time.

Most children naturally dislike vegetables and prefer rich, refined, processed foods over foods that are less refined and processed. This is simply how the human brain is wired. Processed foods rich in fat, sugar, starch, and/or salt cause the brain to release a lot of dopamine and trigger a natural motivational response, while less processed foods have lower concentrations of dopamine-stimulating nutrients and many also have aversive bitter flavors. Processed foods also tend to be easier to chew.

That's the bad news. The good news is that we can exploit the brain's wiring to get kids to eat their vegetables. One strategy is to add lots of fat and salt to them. This increases the dopamine response to the dish as a whole, and eventually the motivational response rubs off onto the flavors and textures of the vegetable itself (this is called Pavlovian conditioning). Once they like the flavor of the vegetable itself, you can cut back on the fat and salt. It also helps to offer the food often but not force the child to eat it.

One thing I see a lot that makes me cringe is "kid food". This is simply a euphemism for junk food. I get it the point of it-- it's easier to feed a child foods they'll wolf down without complaint. But let's not fool ourselves about what it is or what it does to a child's health, weight, and food preferences. "Kid food" is how you train a child to have a lifelong preference for unhealthy foods (https://doi.org/10.1146/annurev.nutr.19.1.41).

It's hard to prove, but I think culture plays a role in healthy eating. I try to cultivate what I call "eating hygiene" in my family, including my currently 18-month-old son. I use my French heritage as a guide. Every time we eat, we sit down at the table, even for snacks. No TV, no phones, no toys, no distractions. We rarely give our son food between scheduled snacks and meals. For the most part, our son eats the same thing we eat, and he rarely gets "kid food". The only beverages he gets are water and whole milk. We don't force him to eat anything, but we don't necessarily offer an alternative to foods he doesn't like.

Every family is different, but as a generalization the US doesn't have very good eating hygiene. We eat a lot between meals, we often don't eat together, we often eat distracted by TV and other things, and we tend to eat unhealthy foods (https://dx.doi.org/10.3945%2Fajcn.2009.28962). Again, I can't prove it, but I suspect this contributes to the challenges we have with eating and weight in this country, and to some extent in all countries that have modernized.