So our reserve fat is sitting there all fat, dumb and happy and “Whaaaa, I’m disappearing!” What happens to trigger this? And how does that stored fat get into the digestive system? Asking for a umm “friend”.
In animals, adipose tissue (fatty tissue) is the body’s means of storing metabolic energy over extended periods of time. Adipocytes (fat cells) store fat derived from the diet and from liver metabolism. Under energy stress these cells may degrade their stored fat to supply fatty acids and also glycerol to the circulation. These metabolic activities are regulated by several hormones (e.g., insulin, glucagon and epinephrine). Adipose tissue also secretes the hormone leptin.[7]
It doesn’t. The digestive system isn’t the goal; the blood is.
The digestive system isn’t just the alimentary canal (the mouth, throat, esophagus, stomach, large and small intestines, and anus); it also consists of the liver, pancreas, and gallbladder which produce digestive fluids, enzymes, and hormones which help convert sugars and lipids and regulate uptake. Blood is the medium by which nutrients are distributed through the circulatory system but it doesn’t really regulate nutrient uptake or availability itself.
Metabolic functions can essentially be grouped into three macronutrient categories: carbohydrate, protein, and fat metabolism. Sugars and more complex carbohydrates are decomposed into glucose (‘blood sugar’ when it is in your circulatory system) which is regulated by the liver by either storing glucose as glycogen or releasing it into the bloodstream when blood sugar levels decline. This helps maintain stable blood glucose levels and is the primary source of energy to cells throughout the body. Proteins are mostly broken down into polypeptides or amino acids by proteases (an enzyme) which does release some energy but primarily serves to provide structural elements to build up other proteins that are useful to the body. This also produces ammonia (NH3) which the liver processes into urea to be filtered by the kidneys and excreted through urine.
Fat metabolism is the most completed system because it is both the storage mechanism and a critical supply of cholesterols which, despite all the bad press they get, are essential structural and signaling components of cell membranes in all animals. Fats are broken down by digestive fluids and enzymes into fatty acids (in biochemistry terms a carboxylic acid with an aliphatic chain) and glycerol (a simple tri-hydroxyl compound that forms the background of all dietary fats). The liver plays a crucial role in processing these fats, either by using them for energy or storing them for later use. The liver also synthesizes cholesterol, a vital component of cell membranes, hormones, and bile acids. Despite its bad reputation in popular culture, cholesterol is essential for the body’s functioning. The liver also helps convert excess carbohydrates and proteins into fats through a process called lipogenesis (‘fat-making’.) allowing these nutrients to be stored fats can later be used to provide energy when needed. The breaking down of fat stores is called lipolysis which makes these nutrients available for integration or to produce glucose. The latter generally happens when the body is being starved of carbohydrates (either through using up glycogen stores during endurance activities or because there is no intake of food), and the fats are converted via the processes of beta oxidation and glyconeogenesis which is why intermittent fasting is sometimes recommended for weight loss.
Here is a pretty straightforward explanation that doesn’t have any basic errors:
Stranger
Fat storage isn’t necessarily a bad thing. The ability to store fat is critical to long term survival. Without stored fat, you couldn’t survive a major famine, or an extreme winter food shortage. Your body needs to store fat so you’re prepared for a long-term food storage situation. However, when was the last time you didn’t eat for two weeks, let alone two months? Some people can fast for long periods of time, think a year, just drinking water, because they have sufficient fat stored. As mentioned already, carbohydrates provide you with a steady supply of glucose, so if someone eats carbs all day, their blood sugar remains high and their body has no reason to burn body fat. In fact, your body will use any excess glucose to create more body fat. So how do you switch to burning fat instead of glucose? By consuming less calories than you burn, and specifically less carbs. Less carbs mean less glucose, which means lower blood sugar, and in turn less insulin. Insulin is a hormone that tells your body to utilize glucose for energy instead of fat. Maintaining a negative energy balance, and keeping carbohydrates, and therefore insulin, low, forces your body to burn fat. I don’t know anyone who hasn’t lost weight doing this for a long enough period of time.
Fascinating information here. The Dope always provides!
My wife is a T1D, type 1 diabetic, and when her blood sugars get so low that she starts to lose consciousness, I have to inject glucagon which, to my rudimentary knowledge, signals to her liver to release its stored glucose and release glycogen into her blood stream. Voila! She regains lucidity and then is able to eat some carbs. Fortunately this doesn’t happen very often, but the first few times this happened it was pretty scary to see her fade from that conscious lucidity, with her skin becoming wet with perspiration and her breathing becoming strained.
Those glucagon shots (essentially like an EpiPen but it does not deliver epinephrine (adrenaline)), they’re a life saver for T1Ds.
I thought type-1 diabetes typically resulted in high blood sugars. Do you mean if your wife overshot by taking too much insulin?
Right. Her pancreas doesn’t produce any insulin so her bloodsugar is high without it. You don’t usually feel too bad with regular walking-around-too-high bloodsugar. When it drops low is when you get the symptoms @Bullitt describes. And yes it’s because the careful manual mathing and dosing you do to inject yourself with insulin went askew and in that moment you had too much and it dropped your bloodsugar too low.
A pound of fat has about 3500 calories in it. When there is a caloric deficit triglycerides are removed from fat cells and converted to fatty acids and glycerol. Fatty acids are used by tissue outside the brain, while glycerol can be converted to glucose in the liver. The excess carbon from the breakdown of fatty acids are exhaled as carbon dioxide.
There are alpha and beta receptors on fat cells and if they are activated in certain ways this can enhance lipolysis (the breakdown of bodyfat), which is why many drugs people have used for weight loss end up activating the sympathetic nervous system, as this enhances lipolysis. Various other drugs (HGH, IGF-1, thyroid) can enhance the signaling pathways the alpha and beta receptors use to signal lipolysis.
Supposedly the body can use about 31-35 calories per pound of fat per day (meaning you could burn up all your bodyfat in about 3 months if your caloric deficit was large enough). But I think that’s how much you can pull from a pound of fat before you start breaking down lean body mass alongside fat. If you combined a large caloric deficit combined with pharmaceuticals and exercise, you could probably pull far more than 31-35 calories a day from a pound of bodyfat. Low intensity cardio supposedly is best for mobilizing body fat to use as energy as a % of calories burned.
Of course losing weight is the ‘easy’ part. Keeping it off is what is difficult because about a dozen or more biochemical changes happen that makes it easier for your body to regain the fat after you’ve lost it. We spent the last 4 billion years evolving not to starve to death, and our bodies can’t tell the difference between an intentional diet and a life threatening famine, so as you lose weight your body changes a wide range of hormones and genes to make it extremely easy to regain the weight.
There are nutrients that the body does not store or only stores a small amount. These are called vitamins and failure to ingest them will cause problems. The most critical one is probably vitamin C. Someone not eating for a year, no matter how much fat they have, will suffer from and probably die of scurvy. So that non-eating dieter needs to take some multi-vitamins in addition to the water.
And your body my react differently when using fat for energy when you’re sedentary versus when you’re exercising. If you’re sitting around and your body feels it’s using too much fat for energy, it may signal you to start eating. But if you’re exercising, your body may not send those eating signals though it’s using fat for energy. It’s focused on the activity and not concerned that it’s using the fat stores. For instance, it’s not unusual to do something like go on a long hike and not have an appetite at all during the hike. That’s another way that exercise can help you lose fat. You’re burning through your fat reserves with less hunger feelings than if you were just sitting around restricting calories.
Fair enough, but we were talking about body fat and glucose metabolism. Of course, your body needs vital nutrients to stay healthy, which can be obtained through supplements.
According to studies I’ve read, 70+% of dieters who lose 10 or more pounds will regain them back, typically with an extra pound or two, within 18 months of ending their diet. I think it also has to do with the fact that old habits die hard, and giving up sweets and high-carbohydrate foods long enough to lose weight won’t magically keep it off forever. As soon as a dieter falls back to their previous eating habits, the weight returns with a vengeance.
Transitioning from “dieting mode” to “maintenance mode” for me was much harder than losing forty pounds in the first place. Mainly because I lost weight too fast and didn’t give my body a chance to adjust to it. Fortunately, I worked with a dietician who helped maintain the weight loss long enough for my body’s set point to reset, which was about six weeks. She also taught me some new dietary and lifestyle habits that I will need to maintain for the rest of my life in order to keep the weight off permanently. It’s been off for over two years so far.
I’m convinced that some people are the human equivalent of “Easy keepers”: equines that will become morbidly obese on anything but the least carb-rich fodder.
But there is a whole cascade of biochemical changes that happen when you lose weight.
T3 drops
Leptin drops
Ghrelin increases
PYY goes down
CCK goes down
The reward centers in the brain become more sensitive to reward signals from food
Amylin goes down
GLP-1 goes down
Your fat cells become less efficient at using bodyfat for fuel and more efficient at storing bodyfat
etc etc. I think there’s at least a dozen changes that happen.
The end result is that appetite goes up, metabolism goes down, food becomes more rewarding, its easier for your fat cells to gain fat and harder for them to use fat for energy. Its why the more weight you lose, the harder it becomes to lose more. Your body is fighting back because it thinks its starving. If you watch shows like the biggest loser, people lose a ton of weight at first, but the weight loss gets slower and slower the longer they do it.
We’ve spent 4 billion years evolving to not starve to death (by comparison, the higher reasoning parts of our brains are only 3 million years old. Sexual reproduction is only 1.5 billion years old). Not starving to death is one of the most central aspects of our biology, and as of 2025 medicine doesn’t know how to help people permanently adapt to a lower bodyweight after weight loss in a way that works on a society wide level.
While I agree regaining lost weight is common for most people, it’s not inevitable. If you reset your setpoint, maintain a negative energy balance, stay active and burn 600+ move calories per day, and curtail carbs, it’s possible not to regain back the lost weight. You won’t add any body fat if you can keep insulin as low as possible, for as long as possible. Is that easy to do? Not at all, but i’m proof it’s possible. I eat a plant-based, whole food, low-carb (<75 g per day) diet. I’m 70 years old, 5’5” tall, and I consume 1700 calories per day. I track everything I eat, and I don’t eat anything with added sugar. I restrict my eating to 7 hours a day using TRE. After losing 40 pounds, my weight has remained stable for over two years and counting. I don’t think I am an anomaly, I just do what I need to do to keep my weight where it should be. If I were to revert back to my old eating and lifestyle habits i’m certain I would regain the weight I lost. I’ve seen many people fall into that trap and yo-yo diet for decades.
Not long enough to be persuasive. Come back in five years. Maybe ten.
I will be happy to report back in 10 years if I live that long.
I fully agree its possible to lose weight and keep it off permanently. Its just the exception. Telling people to change their lifestyle to treat obesity works the same way telling people to never have sex outside of marriage works to prevent STDs and out of wedlock births. It works if people do it, but most people can’t do it long term since we evolved to do the opposite.
Anti-smoking efforts have been extremely effective. In the 1960s, over 40% of people smoked. Now its about 11% in the US. Anti-smoking efforts work fairly well.
But telling people to change their lifestyle really doesn’t work on a society wide level to cure obesity. Obesity rates keep going up. They’ve recently started to slightly decline (from 46% to 45.6%) but that’s because GLP-1 medications are resetting people’s biological set point so it is easier to stabilize at a lower bodyfat percentage.
Agreed. I realize I represent a small minority of the population, but if people are willing to educate themselves and have the drive to do it, it’s possible to lose weight and not regain it. I don’t see GLP1 receptor agonists as a real solution since people eventually stop using them, and their appetite, along with weight, returns. There are even ways to override the benefits of weight-loss surgery. I have seen that happen with multiple people.
Do I see a majority of people changing their diet and lifestyle habits to achieve long-term weight loss? Not in a million years. The food industry excels at producing foods that directly contribute to obesity. We are taught to eat a low-fat, high-carb diet by the government, and most doctors never talk to their patients about their weight issues, and frankly, many wouldn’t know what to tell them if they did.
If you research longevity like I did, one of the first things you learn is that you need to achieve and maintain a healthy weight. So many people are addicted to sugar and live on fast food it’s no wonder we’ve gotten to where we are today. We should focus on teaching kids healthy eating and lifestyle habits, rather than feeding them unhealthy treats like ice cream and cookies to keep them happy. Hopefully, the next generation or two will learn from the errors of our ways before it’s too late.