| the thermo diet

Is Fructose Killing You?

By Tyler Woodward

In recent years, mainstream media has demonized fructose on just about any account possible. From causing insulin resistance & non-alcoholic fatty liver disease to being a major driving factor contributing to childhood obesity. But does this hold true?


What Is Fructose

Is Fructose As Bad As They Say?

Fructose Debunked

Starch Vs. Sugar

What About High Fructose Corn Syrup?

What Is Fructose?:

What Is Fructose


Fructose is a monosaccharide or a “simple sugar”, similar in composition to the other simple sugars glucose or maltose. When you combine 1 glucose with 1 fructose you get sucrose, or as you likely know it, table sugar. Fructose is also known as the “fruit sugar” and is found naturally in foods such as fruit, honey, maple syrup, avocados and even in small amounts in vegetables.


Fructose tastes twice as sweet as glucose which is why it’s often used as a sweetener in many foods and beverages. You can imagine comparing the sweetness of a ripe fruit, containing sucrose (both fructose and glucose), to a baked potato which is made up of purely glucose.

When most people demonize fructose they are demonizing the added sugar that is put into a lot of our food today, but is it really as bad as they make it out to be?

Glucose Vs. Fructose:

Glucose vs. Fructose


There are 3 major differences between glucose & fructose:

  1. Chemical Makeup - The predominant difference between glucose and fructose is that glucose contains 6 carbons and makes a 6-carbon ring, while fructose contains 5 carbons and makes a 5-carbon ring. 
  2. Metabolism -  Fructose is only processed by the liver, while glucose can be used by any cell in the body. Additionally, under most circumstances glucose requires insulin to enter the cell, but fructose can enter the liver without the release of insulin.
  3. Digestion - Generally 100% of glucose is absorbed in your diet, while fructose can be more difficult to absorb, particularly in higher quantities. 

Is Fructose As Bad As They Say?:

Is Fructose As Bad As They Say?

To understand this let’s look at how fructose is processed by the body.

Part I Digestion - 

Fructose in nature is rarely found in isolated form, normally fructose is found bound to another glucose molecule to form sucrose (table sugar). When sucrose is consumed it’s broken down into glucose and fructose respectively, which are then absorbed through the small intestine.

Fructose is thought to be poorly absorbed in the small intestine compared to glucose, but the evidence is very inconclusive. The tests designed to show fructose intolerance administer a dietary dose of a minimum of 25 grams of pure fructose, while some studies administer as much as 75 grams! Not only is fructose never found in nature by itself, but this is also a large amount of fructose for one serving. 

25 grams of fructose is equivalent to about

  • 2 Cokes or Pepsi’s
  • 2.5 apples
  • 4 bananas
  • 300 grams of grapes

While it might be easy to pound a couple of cokes, it’s extremely unlikely to consume anywhere close to this amount of fructose from naturally occurring sugars. And evenso most coke’s have just as much glucose as they do fructose, which may help with fructose absorption. Again, I don’t think there are very many, if any people that have trouble digesting apples or grapes because of their relatively high fructose:glucose ratio. 

And even if someone does have issues digesting fruit, how do we know it’s the fructose causing the issue, not the soluble fiber or other components of these fruits?

Part II - Metabolism

Once fructose is absorbed and enters the bloodstream, it goes to the liver. This is often touted as a reason that fructose is a poisonous substance, as all toxins must go through the liver to be detoxified, but the liver doesn’t “detoxify” fructose it uses it. 

Fructose is preferentially converted into glycogen in the liver. Glycogen is our body’s sugar storage and is used to fuel our body/cells between meals and overnight, when our blood sugar (glucose) drops too low. The average adult human liver can store between 100-120 grams of glycogen, but being that the liver on average takes up 33% of glucose consumed this leaves about 75 grams of fructose to fill up your liver glycogen.

And while the liver is usually thought of as the “detoxification organ” it plays an essential role in metabolism. Particularly, it is responsible for converting T4, the inactive thyroid hormone, into T3, the active thyroid hormone, which controls your metabolic rate or how much energy your cells use vs store.

Consuming  more fructose than your liver can store as glycogen will cause it to be converted to fat through de novo lipogenesis (new fat cell production). This leads to an elevation in blood triglycerides and free fatty acids and is associated with insulin resistance and metabolic dyslipidemia or basically metabolic dysfunction.

The easiest way to think of metabolic dysfunction is like a traffic jam in your blood. In a metabolically healthy person insulin is able to shuttle excess glucose into the cell to be used as energy, as a building block for cellular structures or converted into fat or intracellular glycogen for later use. But if too much glucose is in your blood along with free fatty acids and triglycerides it creates a traffic jam. 

The increase in free fatty acids compete with glucose to enter the cell. In response, your body releases more insulin to shuttle the glucose into the cell, like a cop directing traffic. But over time more and more insulin is needed to get glucose into the cell as it competes with these fatty acids, through a process known as the Randle Cycle. This results in too much insulin, glucose and free fatty acids in your bloodstream and the metabolic traffic jam known as metabolic dysfunction.
In this metabolic traffic jam your cells become starved because they aren’t getting enough energy and as these free fatty acids sit in your bloodstream they  begin to oxidize (rust) other things in your bloodstream like cholesterol resulting in inflammation.

But here’s the thing, consuming anything in excess whether it’s fructose, glucose, fats, and even protein can cause this metabolic traffic jam to occur. None of these by themselves are inherently “bad”, but the combination of all of them at once is too much for your cells to handle. This results in inflammation occurring throughout the body. It’s not the “fault” of any one of these compounds (glucose, fructose, fat, or protein), but the combination of too many of them or overconsumption.

Read More: The Sugar Secret

Fructose Debunked:

Fructose Debunked


Fructose is said to cause a number of harmful effects including:

  • Impairing the composition of blood lipids 
  • Increasing blood levels of uric acid
  • Causing fat accumulation in the liver
  • Causing insulin resistance
  • Overeating

While all of these are true effects of fructose, they’re only true if fructose is over consumed. And again, if excess glucose, fat or protein is consumed these can cause the same thing. 

In contrast to popular belief I’d argue that consuming fructose in moderation can and actually should be part of a healthy diet. Here’s why…

Glucose is inarguably the main fuel source of your body and while certain parts of your body like the muscles and heart rely on fat for fuel at rest, other parts like your liver, thyroid and brain require glucose to function optimally.

If you don’t consume enough glucose your body will break down its fat or muscle stores (protein) to make glucose, which is an extremely inefficient process and metabolically demanding. Instead of your body using energy to build & repair itself, it must divert a portion of its energy to creating fuel (glucose) because you’re not consuming enough in your diet.

But glucose by itself has its own faults. As you now know, consuming excess glucose will result in the same metabolic traffic jam as consuming too much fructose, fat or protein. In fact, consuming fructose with glucose (like that found in sugar or sucrose) actually lessens your metabolic burden. 

Starch Vs. Sugar:

Sugar Vs. Starch


If you consume 100 grams of starch (pure glucose), only about a third of this will be processed by the liver, resulting in about 66 grams of glucose that must be processed by your other cells. The more glucose in your bloodstream, the more insulin that must be released to shuttle into your cells, creating more “traffic” in your bloodstream.

But if you consume 100 grams of sugar which is half glucose and half fructose, all of the fructose must be processed by the liver. Because the liver will naturally uptake ⅓ of the glucose and all of the fructose, this leads to about 36 grams of glucose circulating in your blood. This means that only half as much insulin needs to be released to shuttle the glucose into your cells. Additionally, the less insulin that is released decreases the likelihood that the glucose is converted into fat and is instead used as energy or as a building block to create other compounds. 

Read More: How Sugar Lowers Stress

A Balanced Diet:

The key to a healthy diet is to consume everything in balance…

  • Consuming too much protein by itself can cause a sharp increase in insulin and doesn’t supply your body with enough glucose to fuel itself, resulting in an increase in free fatty acids to be used as fuel
  • Consuming fat and fiber helps to slow digestion, causing glucose, fructose and amino acids (protein) to be released more slowly into the bloodstream. This results in less insulin needing to be released and allows your cells to burn more of the glucose & amino acids as fuel instead of storing them.
  • Consuming too much fat on the other hand will also bog down your cells, creating this “metabolic traffic jam” and hindering your cells ability to use glucose.

The best way to combat this is by consuming a balanced diet.

  1. Eat 2-4 “smaller” meals throughout the day that contain both carbs (preferably with sugar, not starches that have pure glucose), protein and fat

This will help to lessen the “metabolic traffic”, provide your cells with ample amounts of energy and help to ramp up your metabolism by providing your liver, thyroid and brain with the glucose they need to thrive. 

What About High-Fructose Corn Syrup?:


High-fructose corn syrup is defined as having greater than 55% fructose compared to glucose, so this could be anywhere from 100% fructose to 55% fructose. But generally, the research shows that most high-fructose corn syrup contains about 55% fructose.

The research on high-fructose corn syrup is rather inconclusive. On one hand it seems that it can impair liver and thyroid function, but on the other hand it seems that as long as it’s not consumed in excess it’s just like sugar. Generally, I’d advise you to do your best to get your sugar from natural sources like fruit, juices or starches, but per the current research I don’t think that high fructose corn syrup is as bad as it’s been made out to be, although I could be wrong. Additionally, the biggest detriment of high fructose corn is that it likely makes it very easy to consume more fructose than your liver can handle, as you only need a coke or two or a sizable scoop of ice.


You might make the argument that because fructose is solely processed in the liver that it would be more likely to increase the amount of fat stored in the liver, but this hasn’t played out in any of the current scientific research. In fact,  multiple studies have found that overconsumption fructose does not increase liver fat anymore than glucose (1, 2, 3)  and another study found that overconsumption of saturated fat significantly increased liver fat more than fructose. 

Moral of the story, fructose, glucose and saturated fats are all fine to consume, just don’t over consume them.

Thermo diet

Looking to boost your metabolism, improve thyroid function, and balance your hormones? The Thermo Diet is here to help! This high carb, moderate protein, and moderate fat diet is designed specifically to provide your body with easy-to-digest foods that are high in critical micronutrients. When you follow the Thermo Diet, you'll quickly enter into a state of hormonal balance - making fat loss easy while improving your skin, sleep quality, "bedroom" performance, and more!