Chapter 14 | The Deadly Effects of Fructose

Sep 06, 2019


Check out a replay of the Facebook LIVE discussion about this chapter here

You Will Learn

  • A brief review of the different types of sugars. 
  • Why fructose leads to more insulin resistance than other types of sugar. 
  • Should I be eating fruit if I’m trying to lose weight?
  • Ways too much fructose can negatively affect your health. 


About Dr. Fung, Author of The Obesity Code and The Diabetes Code

Dr. Jason Fung is a medical doctor, nephrologist by trade, who specializes in kidney disease, type 2 diabetes, and obesity. He acknowledged that traditional medicine wastes time and resources attempting to treat symptoms of disease, rather than the cause of disease. 

You can purchase The Obesity Code book HERE.


Overview of the Book

Here is the outline of the book. This post covers chapter 14 in Part 5. 

Part 1: “The Epidemic,” explores the timeline of the obesity epidemic and the contribution of the patient’s family history. It highlights the underlying causes of obesity. 

Part 2: “The Calorie Deception,” reviews the current caloric theory in depth and highlights the shortcomings of the current understanding of obesity. 

Part 3: “A New Model of Obesity,” describes how hormones are involved in the development of obesity. These chapters explain the central role of insulin in regulating body weight and describe the vitally important role of insulin resistance. 

Part 4: “The Social Phenomenon of Obesity,” dives into childhood obesity and why obesity is associated with poverty. 

Part 5: “What’s Wrong with Our Diet?,” explores the role of fat, protein, and carbohydrates, the three macronutrients, in weight gain. In addition, it examines one of the main culprits in weight gain - fructose - and the effects of artificial sweeteners. 

Part 6: “The Solution,” provides guidelines for lasting treatment of obesity by addressing the hormonal imbalance of high blood insulin through proper nutrition, sleep, and stress management. 


Sugar Review 101

The word sugar has a much broader meaning than the stuff we use to make cookies. Sugar is one of the three main categories of carbohydrates. Those are: sugar, starch, and fiber

Within the sugar category there are three basic molecules that create the different types of sugar we eat: glucose, fructose, and galactose. 

Fructose is the type of sugar found naturally in fruits. It is sweeter than glucose or galactose. 

Table sugar is also known as sucrose. Sucrose is 50% glucose, and 50% fructose, or one molecule of glucose + 1 molecule of fructose. Examples include white sugar, brown sugar, agave, honey, etc. 

Sugar found naturally in dairy is lactose, or one molecule of glucose + 1 molecule of fructose. Examples include sugar in milk, cheese, or yogurt. You can see this article for a more in-depth review of sugar. 

Starch is just many molecules of glucose. Examples include bread, potatoes, or pasta. 

Fiber is made of various other forms of complex carbohydrates and is great for your health. Check out this article to learn more about fiber. At the end are tables of which foods are good sources of fiber. 


Sucrose Metabolism 

Now that we have a basic understanding of the three main types of carbohydrates - starch, sugar, and fiber - we can better understand how they are metabolized by the body, and why fructose is worse than glucose for creating insulin resistance. I talk about how fiber is digested in this article but will not cover that here. 

Glucose Metabolism

Glucose is also known as blood sugar. Glucose circulates in your blood and is the primary source of energy for almost every cell in your body. Your body has several different ways to store excess glucose so it doesn’t build up in the bloodstream. First, excess glucose is converted to glycogen and stored in your liver and muscles. Once those glycogen stores are full, the rest is stored as fat.

When no glucose is readily available (for example overnight when you are sleeping), your body can easily convert the glycogen back into glucose to be used for energy. This mechanism helps your blood sugar levels remain fairly stable, unless you are have diabetes. 

Fructose Metabolism

Unlike glucose, fructose causes a low rise in blood sugar levels, and thus no insulin response. Therefore, some health professionals recommend fructose as a “safe” sweetener for people with type 2 diabetes. It also has a low glycemic index score compared to glucose. Further, fructose is the sugar found naturally in fruit. 

Because of those reasons, we used to think fructose wasn’t too bad for us. We failed to recognize how it is metabolized and how it can cause fatty liver and lead to insulin resistance. 

Most fructose is converted into glucose which is then converted either to very low density lipoproteins (very bad cholesterol), or just a fat droplet that is deposited into your fat cells. Unlike glucose that can be metabolized nearly anywhere in the body, fructose is almost exclusively metabolized in the liver. 

As Dr. Fung put it, fructose “acts as a missile” straight for the liver. When too much is consumed at once, it is quickly turned into liver fat.Too much fructose can lead to fatty liver and on page 164 Dr. Fung states that “fatty liver is absolutely crucial to the development of insulin resistance in the liver.” He gets into the mechanisms of why this is on page 165 if you want to learn more. 

Key words: “too much.” This only happens if there is TOO much glucose and fructose in your system where the glycogen stores are full. While fructose is the sugar found in fruit, fruit also contains fiber and other micronutrients that are good for your overall health. 

It is hard to overdo the fructose eating fruit. It is very easy to do when eating foods with added sugar. 


Is Fruit Hindering My Weight Loss?

While some health professionals, and Weight Watchers, encourage you to “eat all the fruit you want” I disagree with this statement. Most fruit (except namely avocados) are low in protein and fat. If you have been eating fruit because it is low in calories, it is also low in other nutrients besides sugar. There are other sources of fiber that have less sugar and more protein and healthy fat to keep you feeling fuller for longer. 

If you are trying to lose weight, I advocate you eat more vegetables than fruit, and when you eat fruit, don’t go crazy on portion sizes of high sugar fruits (bananas, apples, grapes, etc). I recommend biasing your fruit intake towards the higher fiber, lower sugar fruits like blackberries and raspberries. 

Remember that food isn’t the only thing we can optimize to lose weight. There is also food timing, exercise, sleep, stress, and medications. 


How Eating Too Much Fructose from Added Sugar Can Harm Your Health

  1. Increase your very low density lipoproteins (VLDL), or bad cholesterol. 
  2. Increase triglycerides. 
  3. Lower high density lipoproteins (HDL), or good cholesterol. 
  4. Increased insulin resistance which leads to obesity, type 2 diabetes, dementia, and cardiovascular diseases like strokes and heart attacks.
  5. Less appetite suppression compared to glucose. This is evidenced by the fact it is way easier to drink 1000 calories of pop or eat 1000 calories of dessert than something higher in protein, fat, or fiber.
  6. Increased leptin resistance. Leptin is your “full hormone” that tells your body to stop eating. 
  7. Increased blood levels of uric acid, leading to gout and high blood pressure. 


Bottom Line

  1. Too much fructose from added sugar can cause fatty liver, leading to insulin resistance and the host of medical conditions that follow like obesity, type 2 diabetes, cardiovascular disease, and dementia. 
  2. Eating fruit in moderation is fine for someone who is healthy. Remember fruit is not a good source of protein or fat so is not considered a balanced meal in and of itself. If you are trying to lose weight, try to bias your fruit intake to raspberries and blackberries as these are lower in sugar and higher in fiber. 
  3. The best way to avoid getting too much fructose is to limit your intake of added sugar as table sugar is 50% fructose. The American Heart Association recommends men have no more than 36 grams of added sugar per day, and women have no more than 24. The fewer the better for your health!



  1. Chapter 14. (2016). In J. Fung, The Obesity Code: Unlocking the Secrets of Weight Loss. Vancouver: Greystone Books. 
  2. George A Bray, How bad is fructose?, The American Journal of Clinical Nutrition, Volume 86, Issue 4, October 2007, Pages 895–896,
  3. 3. Gunnars, K. Is Fructose Bad for You? The Surprising Truth. Healthline. April 23, 2018. Accessed September 5, 2019.

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