Dr. Robert H. Lustig is a pediatric endocrinologist, an expert on obesity in children and a very talented speaker. He's also the man who brought the dangers of fructose into mainstream attention.
His presentation above, Sugar: The Bitter Truth, is a 90 minute lecture that delves deep into the science and biochemistry behind fructose consumption, with over 6 million views.
This video is a must watch. I've seen it 3 times already and it is about as entertaining as a movie in my opinion. He also published a bestselling book on sugar called Fat Chance.
Why Is Fructose Bad For You? Some Important Points
Sugar (sucrose) and high fructose corn syrup both supply a significant portion of the total calories in a standard western diet.
They both consist of two simple sugars: glucose and fructose.
Glucose also comes from starches like potatoes, our bodies produce it and every cell on earth has glucose in it. Glucose is a molecule absolutely vital to life.
Fructose, however, is not. Humans don't produce fructose and throughout evolutionary history have never consumed it except seasonally when fruit were ripe.
Glucose and fructose are metabolized very differently by the body.
The key thing to realize, is that while every cell in the body can use glucose, the liver is the only organ that can metabolize fructose in significant amounts.
When people eat a diet that is high in calories and high in fructose, the liver gets overloaded and starts turning the fructose into fat.
Lustig and other scientists believe that excess fructose consumption may be a key driver of many of the most serious diseases of today. These include obesity, type II diabetes, heart disease and even cancer.
The Harmful Effects of Excess Fructose
Eating a lot of fructose in the form of added sugars may:
- Make your liver synthesize fats, which are exported as VLDL cholesterol, which leads to dyslipidemia (high blood triglycerides and cholesterol), fat around the organs and ultimately, heart disease (1, 2).
- Increase blood levels of uric acid, leading to gout and high blood pressure (3, 4).
- Cause deposition of fat in the liver, potentially leading to non-alcoholic fatty liver disease (5, 6).
- Cause insulin resistance, which can lead to obesity and type II diabetes (7, 8).
- Insulin resistance leads to elevated insulin and insulin like growth factor (IGF-1) in the entire body, which may ultimately cause cancer (9, 10).
- Fructose doesn't affect satiety in the same way as glucose, making you eat more total calories automatically if your fructose intake is high (11).
- Excess fructose consumption may cause leptin resistance, disturbing body fat regulation and contributing to obesity (12, 13).
- Sugar may be addictive (14).
Leptin resistance, elevated insulin and addictive cycles of cravings and binge eating are a recipe for fat gain.
If potentially leading to obesity, cancer, heart disease and diabetes isn't reason enough to stop eating added sugars, I don't know what is.
Be aware that not all of this has been proven beyond a shadow of a doubt in controlled trials, but the evidence is still very strong and more studies will paint a clearer picture in the coming years and decades.
Fructose From Added Sugars Is Bad For You, Fruit Is Not
It's important to realize that all of this does not apply to whole fruit.
Fruits aren't just watery bags of fructose, they are real foods with a low energy density and lots of fiber.
They're hard to overeat on and you would have to eat very large amounts to reach harmful levels of fructose. In general, fruit is a minor source of fructose in the diet compared to added sugars.
The harmful effects of fructose apply to a western diet supplying excess calories and added sugars. It does not apply to the natural sugars found in fruits and vegetables.