Why do most people have a problem eating carbs or when do carbs make us fat.

Lots of people talk about how bad eating carbs are and how it makes you fat but not may actually explain how the body handles carbohydrate.

I won’t even come close to explaining it completely and in every situation but I hope to bring you some insight into how it works. This explanation is not meant to be an exact calculation of how much fat someone accumulates per day. It is for example purposes only. I will also not be taking into account any of the inflammation or other potential negatives of eating a high carb diet. This is just to look at how carbs can cause weight gain in context of a high carb diet with moderate fat.

To understand how carbs are dealt with we first need to understand glycogen. Glycogen is the storage form in the body for carbs. We have two major depots for it. Liver and muscle. Glycogen is just very long chains of glucose combined.

Liver is used to provide a steady supply of glucose to the body for things that use alot of it and typically can’t use fat for fuel. Like the brain and red blood cells. There is about 100g of glycogen stored in the liver or about 400 calories. The body can access this glycogen pretty easily and will do so to keep blood glucose levels stable. The glucose derived from glycogen does not typically have much of an affect on blood glucose levels or insulin. It is just metered out as it is needed and will not register on a constant glucose monitoring device other than just very minute up and downs if you zoom in close enough.

Muscles are the other depot. Much larger than the liver stores, the muscles can hold about 400g or 1600 calories worth of glycogen. This glycogen is different. It cannot be readily accessed by the rest of the body. It is reserved for muscles and will only be used there if the level of exercise the body is doing requires glucose. So like running from a bear. In order for it to be returned to the blood it has to undergo a process. One that might be familiar to some. Gluconeogenesis. Most people think this is just where the body creates glucose from protein but it is actually working in other ways and fairly often. So if we have glycogen stored in the muscle and there is an emergency, like low blood glucose, gluconeogenesis could be used to get at the glycogen stored in the muscle. It is a big process though so this is unlikely to happen all that often. In order to get out it has to be converted to various different molecules and then to an amino acid, Alanine, which can then be sent to the liver where it can be converted through multiple reactions back to glucose. It is a process. Just suffice to say this is pretty much off limits to the body except for exercise. If you want to learn how this works here is a very indepth video.

Now that we know the places in which carbs are stored, now we can look at why it is a problem for most people to eat them at recommended levels.

Lets use an example of someone eating a standard diet of 2000 calories per day. We will use the standard recommendations:

  • 15% protein = 300 calories/75g
  • 35% fat = 700 calories/78g
  • 65% carbs = 1300 calories/325g

Let us pretend that this is a brand new human that just appeared on earth and has never eaten anything before. All glycogen is empty. Lets also pretend this new human doesn’t do any exercise. They work from home and don’t workout. They barely move. For simplicity.

Knowing what we know about glycogen we can see that 100g of those carbs will go to the liver that will be used to maintain blood glucose levels. 225g will go to muscles which will be reserved for intense exercise. Like running from a bear. The protein will mostly go to maintaining and repairing lean tissue and making enzymes. A small amount might go to glucose but that will be negligible. For simplicity we will forget about that. The fat will go to adipose tissue for storage and when insulin drops, after about 2 hours in a proper functioning human, the fat will be used for fuel. Some of the byproducts of fat breakdown (glycerol) will contribute to daily glucose needs but like the protein we will forget about that for simplicity.

Lets look at our totals after new human day 1:

  • liver 100g of glycogen
  • Muscles 225g of glycogen

Since we know the liver will be used to maintain blood glucose that will be fine and will empty out over the course of the day. Cool.

The muscle glycogen doesn’t get used because our new human is sedentary.

Day 2:

Same diet. We will only look at carbs from here on out for simplicity.

325g of carbs consumed. So we know the liver will take the 100g easy. That leaves 225g. Well the muscles still have 225g from yesterday and as we mentioned previously, the capacity is about 400g. So that means only 175g of that glucose from the carbs is going to go to muscles.

Lets look at our situation:

  • liver 100g of glycogen
  • Muscles 400g of glycogen (MAXED)
  • Excess of 50g of glucose

What happens to that glucose? Well it undergoes a process called de novo lipogenesis (creation of new fat). That fat will get taken to fat cells for storage and added to the pool along with the 78g from the daily food intake.

We are probably still in good shape. The 78g of dietary fat plus the 50g made from glucose is still probably not going to contribute to any significant weight gain. The body should be able to use that when not eating.

Day 3:

Same diet.

325g of carbs consumed. So we know the liver will take the 100g easy. That leaves 225g. Well the muscles are maxed out at 400g so that 225g is now excess floating in the blood.

Lets look at our situation:

  • liver 100g of glycogen
  • Muscles 400g of glycogen (MAXED)
  • Excess of 225g of glucose

Like the previous day we know that excess is going to be converted to fat via de novo lipogenesis. Now we have 225g plus 78g of fat. 303g (lets call it 300g for simplictiy) is likely enough to add to net weight gain in a sedentary person. How do I know this?

Lets look at some numbers about fat usage:

Levels of fat Oxidation at rest/walking/running

At rest a person uses somewhere between 0.06 and 0.10 g of fat per minute for fuel. This is only going to happen when insulin is at baseline. So when you eat you will not be using fat for fuel. Let’s look at how this might play out:

  • 3 meals a day
  • 2 hours for insulin to return to baseline after each meal
  • 8 hours of elevated insulin
  • 16 hours of baseline insulin

16 hours of baseline insulin means that there will be 960 minutes of fat burning time. Lets give the upper limit of 0.10g/min of fat oxidation.

That gives is 96g of fat used per day. Lets call it an even 100g.

So if we have 300g of extra fat per day coming from the diet and we only use 100g, that means 200g of fat extra will remain in fat cells per day. Since there is 454g in one pound you can see how fast this might add up. That is 1 pound of fat every 2.25 days.

Now keep in mind that nobody is completely sedentary and not every gram of excess glucose gets converted directly to fat but you can see what happens here in the simplest of terms. There is also the fact that the body will attempt to deal with excess energy by increasing metabolic rate just the same as it adapts to lower calories when you cut calories.

If a person is doing this on a consistent basis they might gain 1 pound every 2.25 days for the first week so you might gain 3lbs. The body will start upregulating metabolism in order to deal with the excess energy. After a few weeks you will likely be gaining marginal amounts of weight. A faster metabolism in the context of a high carb diet usually means more hunger and that person might be likely to start eating more. The higher intake will start the process again.

Now if this brand new human were to exercise this would be a different story. Looking at the chart above you can see that running has a potential oxidation rate of 2g of glucose per minute. Since this hypothetical diet has 225g of glucose going to muscles a day, if the person where to simply run for 112.5 minutes a day they would be in balance. This is where the calories in vs calories in kind of works. Again there will be issues with this. The longer hypothetical human does this running, the more adapted the body will get to this and will be used to running this much and eating this much. If hypothetical human stops running they will gain weight pretty quickly.

Back to the non exercising hypothetical human and their hypothetical diet.

If you don’t use that carbohydrate that is stored as glycogen in the muscles the daily carbohydrate will mostly get converted to fat. This starts quickly building up and filling up fat cells. Once fat cells are full this is when things start going haywire.

First blood triglyceride levels go up as glucose is converted to fat (triglyceride is just fat in the blood). Once those levels are too high that process will be inhibited and the glucose will start to build up in the blood. This increases output of insulin from the pancreas and this reduces the amount of fat you will use per day and increases the need for dietary glucose to supply energy.

This is metabolic syndrome. This is a talk for another day.

Keto ON,

Coach Jack