Unless you've been living under a rock for the past few years, you should be familiar with Dr. Robert H. Lustig.

He is the doctor who gave one of the most famous lectures in the history of nutrition called Sugar: The Bitter Truth, where he reviewed the science behind the harmful effects of sugar.

It has been viewed almost 4 million times. If you haven't already, then you can watch and read more about it here.

It's been 4 years since then and a lot of new research has come out about how sugar (because of the high amount of fructose) can cause harm to the body.

In the video above, Lustig revisits the topic of fructose and its effects on obesity and metabolic disease. Please skip to 4:30, as the intro is long and boring.

Excess fructose causes insulin resistance

Added sugar (sucrose) and high fructose corn syrup are about half fructose, half glucose.

Sugar is broken down into these two simple sugars in the digestive tract, before it hits the bloodstream.

Whereas glucose is found in every cell on the planet and is a necessary part of life, the same can not be said about fructose. The liver is the only organ that can metabolize fructose in any significant amount (1).

When we eat small amounts of it, or we've just finished a workout, the fructose will be used to make glycogen (stored glucose) in the liver (2).

However... if we didn't just finish a workout, or we are eating large amounts at a time (like a bottle of soda or a few scoops of ice cream) then the fructose will be turned into fat (3).

When this happens, some of the fat can lodge in the liver, leading to non-alcoholic fatty liver disease (4, 5, 6, 7).

This process also leads to something called insulin resistance in the liver, meaning that the liver stops "seeing" the insulin in the blood. When that happens, the body starts producing even more insulin (8, 9).

It has been shown in controlled metabolic studies in humans, that consuming large amounts of fructose can lead to insulin resistance and chronically elevated insulin levels, as well as problems with cholesterol, triglycerides and blood sugars (10).

Insulin resistance is a key feature of the metabolic syndrome, a cluster of symptoms believed to be cause type II diabetes, obesity and heart disease.

This alone can explain how fructose, more than any other nutrient, may be a leading cause of some of the biggest health problems in the world.

High insulin blocks the action of leptin in the brain

Leptin is a hormone that is secreted by fat cells.

The bigger the fat cells, the more leptin they secrete into the bloodstream.

When this happens, the leptin is supposed to function as a signal, telling the brain that the fat cells have plenty of energy.

When leptin works as it is supposed to, it tells the brain that we don't need to eat and that we can burn energy at a normal rate.

Leptin is a hormone that is crucial for the regulation of long-term energy balance.

One of the problems in obesity, is that the fat cells are packed with fat... and they do in fact secrete a whole bunch of leptin into the bloodstream.

However, the brain doesn't "see" the leptin, a condition known as leptin resistance. This is common in obese individuals and is central to the study of obesity.

According to Dr. Lustig, high insulin levels can block the action of leptin in the brain and there are some studies to support this (11, 12).

If this is true, then it could explain how insulin resistance and elevated insulin levels lead to leptin resistance and therefore obesity.

Studies also show that having elevated triglycerides in the blood can block leptin from getting into the brain. Fructose is one of the key drivers of elevated triglycerides (13).

Of course, the way the brain regulates energy balance is extremely complicated and not all scientists agree that this is what happens. But it is an interesting hypothesis that seems to make sense on the surface.

Added sugar is bad, fruit is not

I totally agree that fructose from added sugars is harmful.

However, the same does NOT apply to the fructose from fruit.

Comment: Here, the author is clearly wrong. Sugar is sugar is sugar and the human being did not evolve on, nor is s/he designed to live on, fruit.

As with everything else in nutrition, this depends completely on the dosage and context.

When people who don't exercise and eat a Western diet (the majority of people) consume large amounts of fructose from added sugar, this can be a complete disaster... leading to metabolic syndrome and all sorts of horrible diseases.

If anything, excessive amounts of fructose from added sugars may be the single most damaging aspect of the modern diet.

However... fruits (that also contain fructose) will NOT have the same effect.

Comment: This is not correct. The author obviously has an agenda and it is not good health for the reader.

Fruits have lots of water, fiber and a low energy density. They take a while to chew and make you feel full very fast. It is almost impossible to overeat fructose by eating fruit.

In fact, most people would probably see major health benefits by eating some fruit instead of some of the other crap in the Western diet.

The only reason I can think of to avoid fruit, is if you have some sort of intolerance, or are on a very low-carb, ketogenic diet.

Comment: A low-carbohydrate or ketogenic diet has been shown to be the healthiest for nearly everyone as well as having the capacity to heal multiple conditions and possibly even the property of rejuvenation.

Lustig agrees that fruit is healthy despite the small amounts of fructose, but the problem is that he doesn't always make it clear, which detracts from his message and makes people defensive.

Comment: It seems that this author is quite defensive about the fruit issue probably because she is addicted to sugar.

So... fructose from added sugars is bad, but it does NOT have the same effect if you're getting it from real foods like fruits. Period.