Why we get fat – finally understanding the process

If you have ever heard someone saying fat loss or fat gain is ‘simple thermodynamics – calories in, calories out’, or if you have ever heard or read the only way to lose fat is to enter a diet that has a calorie deficit (less calories in than going ‘out’) then this is critical information you cannot afford to miss.

Everyone should be taught this information in school. This couldn’t have come soon enough. It’s the missing piece that I had always been looking for, but never found easily. Completing a recent cross-country drive gave me a chance to listen, uninterrupted, to an excellent audio book on the topic, and I have found it revolutionary.

Its easy to be overwhelmed and confused with all the stories in the media about weight management, obesity, fat loss, people gaining weight, etc. I have read, for years now, so many articles that all hinted at something being a greater cause than all the little minor details that would be written about, and yet no one actually just put the finger on why exactly it was that I or you might have some excess fat on us, while other people don’t.

It wasn’t until this book that I finally ‘got’ it. The sense of relief was immense, and beyond that, the absolute need to tell as many people as I could about it.

So, I will try to keep this as straightforward as possible, because this is important.

At it’s most basic, we gain fat due to carbohydrates opening the pathway to food energy being stored in fat cells.

To go into any more detail would be to add extra information, rather than summarize, however, so the above statement is the basic formula of how the human body gains fat.

Of course, I expect you would like more detail, and I am only too happy to share it with you.

  • Effectively, carbohydrates in food are digested and become glucose (sugar) in the bloodstream. How quickly they are digested, and how much there is of them is what affects how much glucose there is in the bloodstream after eating. This is where the concept of Glycemic Index, and Glycemic Load come from.
  • The more carbohydrates, the higher the blood glucose. The faster they are digested, the faster blood glucose rises.

So this is where insulin comes in. One of insulin’s primary roles in the body is to act as the ‘hall monitor’ in the bloodstream, to ensure that glucose concentrations aren’t getting too high as this can be dangerous.

  • You may have heard of the term ‘insulin spike’ or ‘insulin response’ when people are talking about food – in particular carbohydrates. This is because neither protein nor fat evoke a response from insulin. The importance of that will become obvious soon.
  • Insulin is what tells cells in the liver, muscles and fat to take in glucose from the blood, and store it.

This is important because once the muscles and liver have as much as they can take, if there’s more ‘excess’ glucose in the bloodstream, it all needs to be stored in fat. This is after some is taken in by the fat while the muscles and liver and taking some in – and this depends on a lot of factors, including genetics.

So, when you or I eat a meal with carbohydrates, our insulin comes to our aide and makes sure our blood sugar level doesn’t deviate too far off course. It does this by sending glucose to fat cells, muscle cells and the liver. After some time, blood glucose lowers, and fat cells (and potentially muscle cells) are accessed for their reserves of energy, until eating happens again and the cycle restarts from the beginning.

If we eat many meals with lots of carbohydrates, then our insulin doesn’t get a rest, and we eventually become pre-diabetic, as the cells in the muscles, fat and liver become ‘deaf’ to the insulin, and the body produces more and more to get the same effect.

  • If we eat a meal with protein and fat, insulin remains almost completely dormant. It doesn’t need to control blood sugar, because the protein and fat don’t have the same effect on glucose levels in the blood.

And this is the really important part: Meals with fibre are digested slower, meaning blood sugar doesn’t spike and then crash down, but it steadily remains around where it should be. This means insulin’s job is much easier, and less needs to be produced. This is why slow carb works for so many people. And also why low carb diets are also so successful.

Now that we understand insulin’s job, we need to look at energy in the body.

  • If you eat enough, then you body is supplied with all the energy it needs. If you eat too little, it needs to take energy from fat stores. This is why so many diets work, right?

Well, almost.

  • If you eat less than your requirement, then the body does need to find the energy. However, it can look to the muscles for glucose, and to the liver too, before it goes to the fat. And if you eat another carbohydrate-heavy meal a few hours later, the body is yet again working to control blood glucose levels, though the effect is less if you have eaten less than you might otherwise have.

But there’s still an effect.

So here’s the most important part of this article, taken directly from Wikipedia:

  • Insulin stops the use of fat as an energy source.

Let’s consider that again.

Insulin stops the use of fat as an energy source.

Said differently, in another part of the Wikipedia entry:

  • Decreased lipolysis – forces reduction in conversion of fat cell lipid stores into blood fatty acids.

Therefore, whenever insulin is present, fat cannot be used as energy. Glucose must be used.

Why is this the case?

If we take a look at the earlier example of there being excess glucose in the blood, insulin needs to ensure that it is used by the muscles, and liver, as much as possible, as well as by the fat cells, to adjust the level of blood glucose. Of course it will inhibit any other energy source, when there’s already more than enough glucose to go around, and it might need to store a good portion of it in the fat cells.

So insulin blocks fat entering the bloodstream from the fat cells, because it would put us in danger of having a blood sugar level higher than the safe range.

If you think about this then, reducing portions of commonly-eaten foods, like breads, pastas, cereals and potatoes only reduces the number of carbohydrates, but doesn’t mean insulin will not be alerted to the presence of glucose in the blood. Therefore, it is after digestion, and after the glucose has been used in muscles, the liver and fat, that fat is an acceptable energy source. If the next meal comes in soon after, any fat-burning time is kept to a minimum. Overnight, fat becomes the primary fuel source, as food energy is used up in the body. It’s convenient that way. But in the morning, glucose becomes the primary source, if cereal and milk is eaten, or if toast and jelly, or honey is eaten. Effectively, insulin is woken up, and begins its job for the day.

Controlling how insulin is working is less or more important, depending on if you’re the type of person who stores fat easily, or burns it in your muscles. This choice the body makes about where to direct energy is complex, and is affected by many, many different factors. To suggest that all people process their food the same way is silly, but to suggest that every body stores or burns energy from the bloodstream in the same way is crazy. Everyone is different.

  • Therefore, everyone has a unique circumstance, in which fat loss, weight maintenance or fat gain can be ‘tuned’ according to carbohydrate intake (grams/day).

So, all those calorie controlled diets out there miss an important fact – its the fact that they (accidentally) control carbohydrates that they have such success. Most people who eat 60% carbohydrates will cut out a certain amount of carbohydrates when they control their calorie intake – it just makes sense. 60% of 1500 calories is much less than 60% of 2200 calories. In fact, this example shows that on the calorie controlled 1500 calorie diet, the dieter would be consuming 225 grams of carbohydrates per day, compared with the unrestricted eating which yields 330 grams of carbohydrates per day. That’s a substantial 105 grams per day difference, even if the dieter was unaware of their carbohydrate, protein and fat intake, and was focused only on calories.

That means the body has a much better chance of having times throughout the day to burn fat for energy.

But ultimately, as I’ve learned, its not about calorie control, but carb control and timing. Carbohydrates before activity will be more likely to feed into the muscles, however carbohydrates before sitting watching tv will be more likely to feed into the fat cells, as the muscles will have a full store of glucose (as glycogen) already, and the excess glucose from the bloodstream needs a place to go. With insulin active to keep glucose levels controlled, any fat burning is blocked.

Slow carb offers a simple approach, but incorporating carbohydrate sources that have a lot of fibre, and including protein and vegetables with those carbohydrates, making for a very slow digestion, and a slow entry into the blood. This means many people might be burning fat for energy in between meals, some people only overnight. Importantly, however, it’s much less likely that any excess will need to be stored – because slow carb is about eating enough to feel satisfied. The body is smart, when given the chance, and knows how much it needs. Many people find they eat less than they expect – this is because the body can count fat stores as another ‘food source’, and might pull in 100-300 calories of fat per day, resulting in less appetite.

So now you know.

This isn’t some fad diet advice – this is biochemistry, and this is body-science. Just about every diet system or weight loss pill out there is based on the idea of getting your body to burn more energy in muscles, and store less in fat cells, or for you to burn more energy in your muscles, and therefore direct more energy that way. Many supplements work by mobilizing fatty acids in the fat cells, or inhibiting the fat cells’ ability to store more fatty acids, whilst others work to try to divert more energy to the muscle cells. Some talk about metabolism – many of these increase your burning of energy by raising your temperature and/or heart rate.

Slow carb gives you a chance to give your insulin and blood sugar levels a rest. It also does so in a way that doesn’t leave you starving and dreaming about food all day long, like calorie controlled diets do.

If you’d like to read more about why we get fat – and what to do about it, please click here for Gary Taubes’ book on Amazon.

You might be interested in reading these too:

  1. Baby Boomers’ easy Weight Management with slow carb If you find yourself turning to the TV to learn the latest diet tricks, or picking up magazines in the drugstore to find out what the latest 'wonder' supplement is for staying thin,...
  2. Slow Carb Fat Loss – Secret Weapon Number 2 This secret weapon isn't exactly a food, but it's definitely not a supplement. It's dosage is more like something you'd see on a medicine bottle, but it's totally natural. Recently it's been hyped...
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22 Responses to Why we get fat – finally understanding the process

  • Justin says:

    Gary also has 2 article on the NYT that are a good precursor to his books

    One thing to note is that these are hypothesis (albeit good ones with a lot of anecdotal data) on why causes fat gain and loss. There is also discussion on how 2 other major hormones, leptin and cortisol, could be cause while insulin could only be the effect, I am working on a post right now to compare and contrast these 3 theories.

    That being said reducing your carbs and timing them appropriately will work for almost everyone to lose fat.

    For those of use trying to gain mass, insulin is very anabolic so it is good to understand that side of it as well, I am still looking for an article to explain the best way to use insulin to our advantage for those of use that are lean.

    • Luke says:

      Hey Justin,
      Thanks for your comment – you’re right that currently, there’s no hard science that 100% provides these theories. That said however, there was no scientific evidence that went into the commonly accepted food pyramid and low fat way of eating that North America adopted in the late 70′s, and it’s negative impact has been felt around the world in the last 3 decades. I think, although the Taubes, Atkins, et al school of thought doesn’t have absolute proof, it has enough evidence, and research to back up it’s theories for 99% of the population. I liken it to the fact that we inherently rely on anaesthetics, and know they work, though we still don’t scientifically understand them completely.
      I really look forward to reading your post on the other hormones. There’s a lot of work going into studies around the world now, and it’s endlessly interesting to follow them up and learn what they have to report.

      Insulin is anabolic, in terms of muscle growth. I can recommend reading Mark McManus’ article on this very fact – http://www.musclehack.com/best-bodybuilding-diet-plan-revealed/ – where he describes what is effectively a CKD (cyclical ketogenic diet) with a 36 hours carb-up, involving some math (as compared to a ‘cheat day’). His work and planning has gone into quite a lot of depth, and is worth reading.


      • Mark McManus says:

        @Luke. Thanks for telling people about my site, bud. Great article, too. You really covered all points nice and succinctly. This would be a good first read for newcomers to the subject of carbs and fat storage. :)

        • Luke says:

          Hey Mark, thanks for checking out the article! I’ve really learned a lot from your site and appreciate all your hard work.
          Cheers! Luke

  • Jason says:

    This is one of the reasons I want to tell people to stop worrying about fat free and start worrying about grain free.

    I really need to read this book (although maybe now I don’t have to)! :)


    • Luke says:

      Hey Jason, you’re right that fat free is not worth worrying about. But more than that – fat free is actually making populations fat, and riddled with heart disease and obesity! The low fat idea was based on non-existant results, but beliefs held by certain US influential people, who convinced the government that it was the right way to go to lower heart disease and cholesterol. Unfortunately, basing recommendations on hunches and ideas, not facts, has resulted in problems across the world in the last 3 decades.
      It’s definitely worth reading – very detailed and really pulls back the mystery-curtain of what’s going on in your blood stream, fat cells and related cells, etc.


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  • Erik Smith says:


    Why we get fat and what to do about it, by Gary Taubes, is the greatest read of my life. I used to think that , four hour body diet was my muse.

    I curious to know why you didn’t discuss, Taubes chapter on exercise, carbohydrate addiction, or restriction discussions of the slow carbohydrates and finally the most critical, how the tissues respond to insulin for each person and over time in the same individual?

    I’m my experience while I have lost weight and inches over the past year on the slow carb regimen, it is the very effect of insulin and genetics that have eluded my ultimate goal. it is of my opinion that people who are clinically overweight such as myself ( 6ft 2 in, 212 lbs, BMI of 27%) or obese, are either pre-diabetic, diabetic or within a metabolic syndrome and the fuel partitioning gauge needle is pointing to the right or “F” for fat storage. Thus the insulin response to the cells and it’s sensitivity as well as resistance, is the reason why many of us overweight, obese and some others cannot move beyond a plateau.

    It is of my opinion that we must break the insulin response and it’s cellular up regulation of insulin for those who are in a metabolic syndrome. You must reverse the process.
    How to do this is the answer.

    I know you have thoughts on this as I have seen you struggle with different facets of the 4 hr body plan with regards to weight gain, muscle building, paleo and primal diets and how to achieve your personal goals.

    • Luke says:

      Hi Erik,

      Thanks for your comment.
      Simply, I didn’t want to summarize the entire book, because I think it’s important for people to read it and understand the details for themselves.

      You’re right that for some people, especially those overweight or obese people, the ‘fuel partitioning needle’ points solidly towards Fat Storage, and not Muscle Energy. For many people this creates a vicious cycle of feeling tired, but not being able to energize themselves, regardless of the amount of food consumed. In this scenario, caffeine becomes a very close friend, however this doesn’t work forever, and any sugar/artificial sweetener coming in with the caffeine is destructive too.

      If this is the case, you’re right that reversing the insulin stranglehold on the body is the key answer. The ‘how’ of this may take the shape of slow carb, however for many people in an extreme situation, more appropriate would be the Atkins method of almost eliminating carbohydrates, and allowing the body to switch over to running on fat as its primary fuel. Contrary to media/popular belief this is very healthy, and will reduce cholesterol, and reduce blood pressure, as well as other indicators for heart disease risk.

      Personally, I find, having experimented with carb thresholds to find my personal balance, that my partitioning gauge doesn’t default to muscle energy, in fact it’s easy for me to store fat. Therefore, I am very careful to control insulin in my body. Slow carb is my method of doing this, along with cinnamon, lemon juice, and some healthy fats. In terms of muscle gains, this can make things a little more difficult. I have used alpha lipoic acid, which mimics insulin on the muscle cells, and experimented with carbs in various forms. Most of my muscle gains do come with some fat gains too, so it’s a balancing act. But this is different for different people of course. The irony is that most people who meet me take one look and assume I have a typical fast-processing, muscle energy metabolism (I was just measured @ 9.3% bodyfat with a BodyMetrix device). So I think it is possible to tune your approach to your body and get the results you desire, though it depends on how long the road has been to ‘here’. I am still focused on gaining muscle, without gaining a lot of fat along with it. I am experimenting currently with intermittent fasting as a way to achieve this, though it’s early days to report on the muscle gain aspects of this (there’s definitely success for fat loss using various intermittent fasting results).

      Al the best, and thanks for writing,

  • paris says:


    My partner and I have been on the slow carb diet for a week now. I lost 6.5 lbs by my first cheat day and he lost 5 lbs. Now even though he has never been officially tested, we have an idea that he may be allergic to gluten and wheat. As on our cheat day he indulged in some garlic bread where the rest of the day was a burger in a wrap(also gluten), baked potato with sausage and cheese, and some ice cream, I had nearly the same stuff except I ate a whole pizza and didn’t have a baked potato. I gained back 2.5 lbs but no body fat increase, however, he proceeded to gain over 7lbs of weight in less than 24 hours. Now after one week of doing the diet I managed to lose 5.7 lbs and 2.3% bodyfat, He has only lost a total of 2lbs after 8 days of the diet which means he has lost 4 lbs since his cheat day. he is very disheartened now and is giving up the diet even though i know he will never lose the weight unless he stops eating gluten. a lot of his family struggle with their weight as well and i think that it is all connected to the gluten as they always complain about aching joints and pain like arthritis. this is something a lot of people never even think about before they start trying to lose weight. I was hoping you could do an article about gluten and how this can make people fat and could be the one thing they don’t ever consider to be the reason the diet doesn’t work for them if they are gaining loads of weight on a cheat day.


    • Luke says:

      Hi Paris,
      Thanks for your comment. I appreciate where you’re coming from – it sounds like a difficult situation.

      My suggestion is that your husband follow the slow carb diet for 4 weeks, minimum, before he judges whether or not it will work. Everyone’s bodies are different.
      I also suggest that if you think gluten could be a problem, that it gets followed up by a medical professional who can test him – there’s no use battling it without knowing, but it could also clear it from the table. You don’t know until you’re tested.

      With regards to cheat days – some weight regain is usual, though normally it would be gone by Wednesday or Thursday the following week. I’m not sure how you are measure bodyfat – if it’s with bodyfat scales then I would suggest ignoring the results as they’re highly inaccurate. The best measurement of success is a BodPod or DEXA scan, and failing that, measuring around the body is best. Use mid-upper arm, mid-upper thigh, belly button, hips, shoulders to get a good idea of how things are changing.

      The last note I’ll make is that guys tend to gain some muscle on the diet, and this can make weight loss results look less impressive.

      I’ll do some more research on gluten, as a celiac myself I have a lot of experience with these topics, however I don’t think there’s any conclusive evidence that suggests eating gluten as a celiac leads to weight gain.

      All the best,

  • Claudia says:

    Hey Luke!
    Wondering what your thoughts on how much calorie counting we actually have to do, despite Tim saying we don’t have to count calories at all using this diet.
    I’m just thinking of days when I’m eating a lot of volume of beans and also high volumes of fat through meat (for instance yesterday I had a BBQ and enjoyed lots of meats and veggies but I know that a burger patty itself has 300+ calories).
    Will the cheat day still help us?

    • Luke says:

      Hey Claudia thanks for your comment!
      I still recommend not calorie counting in general. A fatty meat is a slight deviation off the slow carb plan – it suggests low fat meat for a couple of reasons. 1/ most meat is grain fed, resulting in more fat through the muscle tissue and may also carry obesogens (from chemicals) in the fat. 2/ it’s better to get healthy fats from other sources like olive oil, macadamia oil, pure butter, ghee, coconut oil, etc. That said, some animal fat during the week won’t put thing off too much if it’s occasional. But I wouldn’t expect you’d be eating ‘high volumes of fat’ regularly.
      It still doesn’t matter so much that the burger patty has 300 calories, because with beans and leafy vegetables, it all digests slowly and if a few meals like this are eaten, you’re going to feel satisfied but most like still have bodyfat being used overnight (especially) for energy requirements, but avoid storing fat the next day. This is one reason why slow carb works so well.
      Stick with the cheat day routine – it’s actually more about loading up on carbohydrates, like breads, pasta, etc.. but really is a chance to relax and just enjoy any foods you have missed during the week.
      Over time, I think you’ll start to find what works for you and what doesn’t. For example, I experimented with low fat burger patties (frozen) as my staple for 2 or 3 meals per day for a couple of weeks, but noticed that my results weren’t as good. Switching back to chicken, salmon and tuna helped a lot so I don’t eat that much ground beef these days.

      All the best,

  • Claudia says:

    Thanks for the insight Luke! Ok so even if I was filling up on a lot of good fats and therefore Intaking some serious calories so I was no longer at a deficit, the cheat day would still be effective as with the whole slow carb diet ya?


    • Luke says:

      Hi Claudia – not exactly.
      You need to consider the slow carb principles and not follow your own, modified version and expect the same results. Slow carb doesn’t really involve filling up on anything, but following the set meals, 4 times per day if possible, with each one including low fat protein, beans/legumes, and green leafy vegetables. Some good fats may by used incidentally – ie 2 tbsp olive oil as a dressing, maybe some macadamia oil in cooking, however slow carb isn’t about finding foods that fit a certain nutrition profile (like Atkins is), but more following a healthy whole food and meal-based lifestyle.
      You really do need to drop ideas about calories. Most likely you’re not taking into account other methods of burning calories, as no one formula truly calculates this accurately day to day, so it’s far superior in understanding to follow your hunger and energy levels (as long as you’re getting adequate sleep), moderate caffeine intake (it may spike hunger) and include some short high intensity exercise sessions during the week (to a maximum of 3 or 4).
      I’m not sure I’m understanding your linking cheat day’s effectiveness with what you’re doing on the other days, sorry about that. Cheat day is a day to spike calories, to spike carbohydrates and to replenish your body with both so that leptin, a hormone involved in fat loss regulation, doesn’t drop very slow and cause fat loss to slow down.

      All the best,

  • Claudia says:

    Thanks! You make some awesome points.
    Well I suppose here’s the link in my head: if throughout the week I haven’t gone into a decent caloric deficit (via food intake, exercise, absorbtion, heat etc.) then leptin levels wouldn’t have dropped significantly.
    The scenario is what if someone’s “Eating until full” happens to put them at a surplus of calories (even after exercise, heat, absorption etc.)?

    Thanks again!

    • Luke says:

      Hey Claudia. In theory, its very unlikely that eating this food will lead to calorie surplus, if you’re eating enough to cover energy needs. Not so much eating until full though – eating until satisfied. There’s a difference.
      Again, slow carb is definitely not about concentrating on calories and/or any calorie deficit. Try to go a little less ‘Type A’ on slow carb – it needs less maintenance and attention than other diet methods ;)

      On the other hand, if you’re looking for a diet/plan that’s healthy, but needs careful attention, check out Paleo.

      All the best,

  • Claudia says:

    Makes total sense! I guess when you’re eating to satiety the rest takes care of itself :)

    Thanks for all your helpful insight!!

    • Luke says:

      Hey Claudia,

      That’s exactly it! Listen to your body and over time, it will start giving you true feedback that’s accurate.

      All the best,

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