Does eating extra fat make you fat?


Does eating extra fat via fat bombs and Bulletproof Coffee make you fat? Here’s the short answer. Yes and no.

If you are slender, then eating fat will likely not make you fat. If you are obese or overweight then yes, eating more fat will likely make you fat. Let me explain. The answer, of course, has less to do with calories and more to do with physiology.

FatBombsLet’s back up a bit. Under a ketogenic / Low Carb High Fat diet, people are encouraged to eat the large majority of calories as fat. Generally, they should eat real food, until full. Some people have taken this to mean that they should add extra fat to everything they eat – witness the popularity of ‘Fat Bombs’ – treats or foods with very high fat content or Bulletproof Coffee – coffee with the addition of extra oil (MCT, coconut etc). There has been some people who find this slows down weight loss and others that feel it does not. What’s happening?

Insulin is the major driver of weight gain. When you gain body fat, the body responds by increasing secretion of a hormone called leptin, which tells the body to stop gaining weight. This is a negative feedback loop, designed to prevent us from becoming too fat. This is a survival mechanism because obese animals who cannot move properly will get eaten. So why doesn’t it work for us?

Insulin and leptin essentially are opposites. One tells the body to store body fat and the other tells it to stop. If we continue to eat fructose, causing insulin resistance and persistently high insulin, then we will also persistently stimulate leptin. Like all hormones, a persistently high hormone level leads to down regulation of hormonal receptors and the development of resistance. So persistently high leptin levels eventually lead to leptin resistance, which is exactly what we see in common obesity. So, lean people tend to be leptin sensitive and obese people tend to be leptin resistant.

The physiology of eating fat

FatBombs2 copyLet’s now think about the physiology of eating dietary fat. Remember there are only two fuels for the body – you either burn sugar or you burn fat. When you eat carbohydrates, it goes to the liver, through the portal vein and stimulates insulin, which tells the body to start burning sugar, and store the rest as glycogen or fat.

Dietary fat, on the other hand, does no such thing. It is absorbed in the intestines as chylomicrons, goes through the lymphatic system to the thoracic duct and directly into the systemic blood circulation (not the portal circulation of the liver). From there it goes into the fat cells to be stored. In other words, the fat does not affect the liver, and therefore does not need any help from insulin signaling and goes directly into fat stores.

FatBombs3So, doesn’t that mean that eating fat makes you fat? No, no at all. Let’s take the lean person (leptin sensitive) first. Remember the story of Sam Feltham’s 5000 calories/day experiment? He ate an enormous number of calories per day, and still did not gain weight (53% fat, 10% carb). As you eat lots of fat, it will get stored into fat cells, but insulin does not go up. As fat mass goes up, leptin does as well. Since the lean person is sensitive to leptin, he will stop eating in order to let his body weight go back down. If you force-feed him, as Sam did, the metabolism can ramp up to burn off those extra calories.

What happens if an overweight person overeats fat?

Now, the situation for the obese, leptin resistant person. As you eat lots and lots of fat, insulin does not go up. However, that ‘fat bomb’ does indeed go directly into your fat stores. You respond by increasing leptin levels in your blood. But here’s the difference. Your body doesn’t care. It’s resistant to the effects of leptin. So, your metabolism does not go up. Your appetite does not go down. None of the beneficial weight loss effects of eating that ‘fat bomb’ happens. And yes, you will need to eventually burn off that extra fat you’ve taken in.

The practical implication is this. If you are lean and leptin sensitive, then eating more dietary fat, like cheese, will likely not make you gain weight. However, if you are trying to lose weight, and have some problem with obesity/ insulin/ leptin resistance, then adding extra fat to your meals is NOT a good idea. Once again, you can see that we do not need to go back to that outdated, and useless notion of calories. Obesity is a hormonal, more than a caloric, imbalance.

What can you do instead? Well, eating more carbs is not a good idea. Neither is over eating protein. Nor is eating more fat. So, what is left? That’s what we call fasting.

At this point, you might worry about nutrient deficiency. That is why so many people talk about nutrient density. How can you get the maximum nutrients for the minimum calories? I see this as muddled thinking. Ask yourself this – are you worried about treating obesity or nutrient deficiency? If you choose obesity, then worry about obesity. You don’t need more nutrients, you need less. Less of everything.

If you are instead worried about nutrient deficiency, then treat the nutrient deficiency, but let’s be clear – THIS HAS NOTHING TO DO WITH THE TREATMENT OF OBESITY. If you are worried about, say, Vitamin C because you have scurvy, then by all means, take foods dense with Vitamin C. But it will not make a bit of difference for the treatment of obesity.

The issue of obesity and the issue of nutrient deficiency are completely different. Do not confuse the two. I treat obesity, not beriberi disease. So I worry about reversing hyperinsulinemia/ insulin resistance/ leptin resistance. If you are leptin resistant, then no, adding more fat does not make you lose weight.

Fat bombs, for you, are not a good idea.

Jason Fung


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  1. Jane
    Just wanted to add that the first time I can across the concept of fat bombs was in the context of people who were stalling on lchf doing a FAT FAST (my emphasis). The idea was that on a fat fast you would eat nearly all of your calories from fat but the total calories would be restricted to 500/600 as it would in some intermittent fasting. I try and only eat fat bombs as a meal replacement (no addition) when time is really a factor.
  2. 2 comments removed
  3. Rose
    The information is confusing. As I am obese, and shouldnt be eating a high fat diet?
    But this what I have been doing for 1 week and it works for me.
    The first 3 days, I had zero carbs. I went into Ketosis. (Old school Dr. Atkins) I then adjusted my macros, so that I only have 10 carbs a day, Again Old School Dr. Atkins, meaning 40 years ago) by doing that I stay in ketosis, dark purple. I do 25 percent protein, and 70 percent fat. On 1200-1400 calories. I do intermitment fasting. At 10am I drink a keto coffee, which does not take you out of fasting!!! Then between 7-10, I eat the rest of my micros. I am obese, I am 63 female, 5'10", was 234 last Sunday, today 226. I know this will not continue as fast, but being in ketosis does indeed burn fat stores. My weight goal is 145. Dr. Atkins plan used to be (and it works) no carbs until you are in ketosis, the 10 for a few weeks, then add 5 a week, as long as you stay in ketosis, never over 100.
    The one think that was not taken into account was caloric count, again old school, if you consume more calories than you need, they will turn to fat, consume less than you need, you will lose weight. And PS if you burn up 500 extra calories, no you cannot eat more. LOL. Good luck everyone. PS. I am never hungry.
  4. gina
    Well explained Kathy. That is my understanding too.
    It would be helpful for people if there was an infographic showing energy resources of the body 'in an order'. We have to make our body to go to the "stored fat" to lose weight and that is only possibleby cutting on carbs and daily fat.
  5. Hg
    Is it necessary to count macros? I've read that one should maintain their fat intake for ketosis, but on this site , it states not to if you're 40 plus.
  6. Hg
    Do you count macros?
  7. Peter Biörck Team Diet Doctor
    We recommend avoiding to much macro counting. Most people will lose weight and get a other health benefits by just counting grams of carb per day and keep them low. :)

    More reading can be found here:

  8. Karen
    Now I’m really confused. You say to avoid too much macro counting and only count carbs. However, elsewhere on the site it says do not eat too much protein and the above article says don’t eat too much fat, if you are fat. I only need to lose 20-30 lbs so I’m not obese. I do get concerned with eating too much fat and your menu items are so high in calories that I’m sure I wouldn’t lose weight as they are more calories than I normally eat on any given day.
  9. marilyn polifroni
    Hi My bad cholesterol is a little high. I am not understanding how eating high fat , bacon , etc etc will not make it go higher . I have been doing IF and keto for 3 months... love it !!! But I am concerned about my Cholesterol... there must be something that I am not understanding. Any help would be appreciated . thanks
  10. Antonie
    Thanks Jason Fung.

    What medical tests do you recommend to a person who is in ketosis but can not get out of obesity?


  11. Eve
    What the heck ?! I'm 86kg and 163cm so I'm considered obese and now I have to cut down on carbs and fat !? What's left ? Just protein. But I can't eat too much of that either because it'll kick me out of ketosis. I'm so confused :-(
  12. Evelyn
    So is the LCHF diet not for fat people ?
  13. Sheree
    For anyone who finds this later via Internet search like I did:
    "High-fat diet" means that a high PERCENT of your calories comes from fat. If you're only eating 400 calories, and 360 of them are fat, that's still a high-fat diet.
    Also, you can be in ketosis and have high insulin at the same time, because protein raises your insulin too. Even on a LCHF diet, you have to be aware of the insulin index of your food.
  14. Eileen
    What I take from all of this is, start with a LCHF diet, with emphasis on the low carb. If that isn't enough, you're possibly getting too much protein and/or fat, too; the solution to which is to ... stop eating so much. This article explains why too much fat is counterproductive if your body is already too fat -- the mechanism strikes me as being similar to the carb factor: too much carb, it gets stored as fat. Too much fat, it gets stored as fat. Too much of anything, the solution is simply, knock it off. Is this really all that confusing? We obese people just have to let go of the idea that we can ever be at that place that we can just stuff ourselves silly and still lose weight. Many of us are not only eating too much; we are thinking about, and desiring after, food too much, too. The hardest thing about making any change is ... giving up the part we most want to keep. Food has meant something to many of us (well, certainly to me, anyway) that is not really about nutrition. I have to let go of whatever it is food means to me over and above health and nutrition. It's not easy to make my peace with that -- but until I'm eating healthfully, I can't get there at all. So back to square one: LCHF first, to move dramatically toward health; modify as needed (watch fat & protein consumption; learn to eat to satisfaction rather than "full;" fast intermittently as needed) until I get the weight loss I desire; then begin the slow, sometimes painful, process of figuring out the rightful place of food in my life. It's a long and often difficult road, but I'm glad, at least, to finally be on it.
  15. 1 comment removed
  16. Eileen
    Also -- for what it may be worth: I fast from 7 pm to 11 am every day, and I don't eat between meals (except for a cup of coffee once or twice during the day). Sometimes, I get hungry (at least I imagine I do); I figure this is just the price I have to pay for having spent a lifetime eating too much. I try to keep an even temperament throughout, so my family doesn't have to pay the price while I try to figure this out. If you have struggled with this your whole life, this is genuinely hard, and we don't do each other any favors by trying to cover up that fact. We all struggle with *something,* right? I try to be grateful that this is my burden, and not something else. At least I know what to do to make it better.
  17. Guyomar Pillai
    Reading the comments, I think people are confused. He isn't advocating a low-fat diet. He's saying that it's a misconception that the weight loss is coming from adding extra fat to your diet. It's about keeping carbs very low, protein moderate, and using fat (usually in combination with protein, in meat for example) for satiety. His point is not to add 500-calories keto coffees and shakes, or delicious treats, because it's easy to eat too much of these. I know that I can overdo a keto cheesecake easily (combination of the alternative sweetener + fat) but even if I enjoy a burger, I won't overeat it the way I would a sweet. I think he's pushing back against the misinformation (that I see frequently in keto/low-carb community groups) that adding fat is going to make you lose weight in itself. This isn't true, and for people trying to lose weight with the leptin resistance he explained, don't just add fat because it's tasty or because you want to meet some magical macro. Also, fasting and mere caloric restriction are not the same. With fasting, insulin gets very low, whereas with caloric restriction, it never falls enough because you're taking in some food, even if it's less than usual. Insulin doesn't fall low enough with caloric restriction for you to start using your own fat.
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