Insulin makes you fat? Not so fast

There is currently the carbohydrate-insulin hypothesis of obesity, which basically states that insulin, due to carb intake, makes you fat.

There are two sides to this.

  • Insulin secretion from carb consumption
  • Total daily insulin levels

Let’s look at the first one.

Carbs make you fat due to insulin

Before we dive in, we have to understand what insulin does.

Insulin has many actions that work on a spectrum. A little bit of insulin will have strong inhibitory or stimulatory effects on certain processes in the body, and negligible effects on others.

As the amount of insulin increases, the more it will inhibit and activate different processes. Insulin doesn’t function like a switch.

One of the supposed “bad” actions of insulin is that it inhibits lipolysis and fatty acid oxidation and promotes the storage of the food you ate.

To clarify, that’s a good thing.

Insulin shuts off lipolysis, because fatty acids inhibit the proper oxidation of glucose. The inability of insulin to shut off lipolysis is found in diabetes.

Insulin doesn’t just store nutrients in the fat, but also in the muscles. Every organ and cell in your body can use glucose as fuel.

Furthermore, insulin speeds up the oxidation of glucose. There is evidence that someone who eats 400-500g of carbs per day burns all 400-500g as fuel. None is being stored. And insulin helps with that.

I did a whole vid on this concept: Earning carbs are for the uninformed!

Let’s say you eat glucose, and you burn 1g of glucose per minute. Now you get a slight bump in insulin and that increases glucose oxidation to 1.5g per minute. Increase insulin even more and you burn 2g per minute.

So that glucose that is being burned for fuel isn’t being stored. If you eat 600g of glucose and 40g of fat per day, you want your body to burn the glucose or where else is it going to get the majority of its fuel?

If you’re adapted to burning glucose, almost none of the glucose will be converted and stored as fat. Most of it will be burned as fuel.

Now we have to look at

GI, GL and satiety index

A central part of the carbohydrate-insulin hypothesis of obesity is the GI index.

The higher the GI, the more it will stimulate insulin.

Another thing to look at is the GL; glycemic load. Since watermelon has a very high GI it’s strange that we don’t get fat eating it right? That’s because according to the GL index, we have to eat a mountain of watermelon for it to make a difference. So since we don’t eat as much, it drops the amount of carbs we eat and also how much insulin is being released.

Then we also get the satiety index. A lot of people hate on potatoes because it has a high GI. “Avoid potatoes, it’s the worse!” is what they usually say. But it turns out the potatoes also have the highest satiety index. Meaning, you can’t eat too much of it without feeling super stuffed, thus limiting total calories in and total weight gain.

But how can we eat a ton of potatoes? Deep fry it or add a lot of butter and salt and voila! It’s freaking delicious and you eat past satiety (plus a boatload of extra calories from the fat). Can you expect to gain weight from french fries or buttered potatoes? Ofc you can! Is it the potatoes fault? Heck no!

With all that said, here is a massive study you can dive into that shows you evidence upon evidence that there is no difference in weight gain or weight loss between groups that eat high vs low GI.

Alright, so GI, GL and insulin index have been discussed, now let’s look at hyperinsulinemia or more broadly, metabolic syndrome.


Hyperinsulinemia is when there is too much insulin in the blood because your pancreas is secreting too much of it, or it’s being cleared from the body too slowly.

This is usually because your tissue is insulin resistant, thus more insulin is required to deliver the message.

Hyperinsulinemia is usually caused by inflammation and stress. Inflammation can be due to a variety of reasons, namely, gut irritation and inflammation (who doesn’t often get bloating, digestive upset and perhaps even diarrhea from junk food), excess iron, xenoestrogens in our environment, nutrient deficiencies (e.g. zinc, selenium, B vitamins, etc.), air pollution, water pollution, toxins in our diet (e.g. omega 6), etc.

As opposed to normal insulin secretion in response the carbs, hyperinsulinemia is much different. Hyperinsulinemia is a problem.

You can check your daily insulin secretion by testing for c-peptide. C-peptide is a byproduct of insulin production. It is produced in equimolar amounts to endogenous insulin but is excreted at a more constant rate over a longer time. The level of C-peptide is a gauge of how much insulin is being produced in the body.

Children with higher BMI Z-Score had a higher fasting C-peptide (age 12) but linear regression showed that children with more steps per day were less likely to have a higher fasting C-peptide irrespective of BMI.” (R)

Hyperinsulinemia can be fixed by eating more (non-processed) protein, whole fruit, low-fat dairy, eggs, shellfish and certain starches (depending on which ones you tolerate), such as potatoes, yams, sweet potatoes, rice, etc.

Different carbs on insulin levels

Too much insulin can cause hypoglycemia. It’s not necessarily the increase in insulin that’s bad, but rather the hypoglycemia that’s caused by it (R). To prevent hypoglycemia, the body released cortisol and glucagon to synthesize more glucose. So when you eat food and see that you have elevated blood sugar. You don’t know if that glucose is coming from food or from newly synthesized glucose.

And we do know that diabetics have elevated cortisol and gluconeogenesis and that’s what’s driving their hyperglycemia.

White bread is very insulinogenic and not very satiating, so that can cause hyperinsulinemia and even hypoglycemia in some people. With what do you feel the best blood sugar control, potatoes or white bread? For me, that’s definitely potatoes, and potatoes actually have a higher GI than white bread.

Sugar and fruit

According to GI charts, pure glucose has the highest GI and fructose the least.

How many times have you heard that high GI foods like sugar, are the most fattening food, since it’s so insulinogenic? So first off, sugar is not nearly as insulinogenic as glucose or white bread.

Fruit, as opposed to sugar, is even less insulinogenic, since it contains minerals (e.g. potassium), fiber and polyphenols that help with the mobilization and utilization of glucose. Fructose actually enhances the uptake of glucose into a cell (insulin-mimetic effect), promotes glycogen storage and also enhances the oxidation of glucose. So you end up with better blood sugar control, ATP production, and glycogen stores.

Fruit is so powerful, that you only have to eat 1 apple about 20 minutes before a meal to HALF the glycemic response from that meal.

But some fruits do have a high GI, but it all depends on how much you eat. And regardless of how insulinogenic it is, you won’t even find one study showing that fruit causes insulin resistance and/or weight gain.


Refined grains are much more insulinogenic compared to unrefined grains. That’s why many studies show that unrefined grains improve insulin sensitivity. That problem came in when people started eating refined and ultrarefined foods.

Before refined foods, almost no one was fat or obese (unless they were royalty and were eating all day long). When fast food came to the market, more and more people became obese. And it’s not necessarily the refined grains that are the problem in of itself, but also the omega 6 fats, xenoestrogens, food preservatives, flavorings, colorants, fortification, etc.

Insulin release

A brief spike in insulin, even if it’s huge, is fine, if it comes back to normal quickly and doesn’t cause hypoglycemia. It’s the prolonged elevation of insulin that becomes an issue. You can easily check your 24-hour insulin release by testing for c-peptide. Some people on keto have a higher c-peptide compared to when adding in carbs such as fruit, honey, etc.

Continuous glucose monitors are great, but you should also take a look at your insulin release (a.k.a. c-peptide).

And just because someone is low GI doesn’t mean you’ll tolerate it well. Oats is low GI, but every time I would have oats for breakfast, I’d be crazy hungry, fatigued and irritable within 30 minutes. If I have milk, fruit or meat instead, I have energy for hours.

Lastly, everyone responds differently to starches.

The response to the standardized meal of bagel and cream cheese consumed by our cohort varied substantially across participants, with glycemic excursions (eg, Figure 2A) (ie, the maximum glycemic elevation from baseline over time after eating a meal33) ranging from 6 to 94 mg/dL (mean, 30.7 mg/dL).” (R)

But not fructose:

However, when comparing the PPGRs (postprandial (post-meal) glycemic responses) of different people to the same meal, we found high interpersonal variability, with the PPGRs of every meal type (except fructose) spanning the entire range of PPGRs measured in our cohort” (R)

Some people can eat white bread and only get a minor increase in blood sugar whereas someone else will get a massive increase. This is much less the case with fructose-containing foods such as fruit, honey, maple syrup, etc (R, R).

You can basically say that it’s a universal rule that starches can mess with a lot of people’s blood sugar, but most/all people will have the same blood sugar control after consuming fructose-rich foods.

Conclusion and summary

It’s not the release of insulin that’s the problem, it’s how long insulin stays elevated. Inflammation and poor food choices eventually lead to hyperinsulinemia.

Certain foods have a high GI, but they also have a low GL and high satiety index. This makes them terrible foods for gaining fat.

But at the end of the day, eating in a surplus will still lead to weight gain, regardless of where your insulin is. And it’s easiest to overeat on fatty starches (think french fries, pies, cookies, burgers, pizza, or any baked goodie really).

In takeaway, personalized nutrition is still key. Someone will have good glycemic control with potatoes and someone else will not. For those that crave starches but get blood sugar issues from them, try consuming a fruit such as an apple before the meal and see if that helps.

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