The Effects of Fructose

Meet fructose, glucose’s brother. Both are monosaccharides (single sugar molecules). Glucose is considered mildly sweet in comparison to fructose, which is very sweet. Sucrose (table sugar) is a disaccharide (two sugar molecules joined together). One glucose and one fructose joined together is about as sweet as sucrose.

Glucose vs. Fructose

Glucose and fructose are handled differently in your body. Glucose is essential for life—if you don’t eat it, your body will make it. When you consume foods containing glucose, it goes primarily to the muscles (75-80%), the liver (10-15%), and to other cells in the body where it’s used for energy or is stored as glycogen or fat [1].

Fructose, on the other hand, goes almost exclusively to the liver [1] . No cells can use fructose for energy, so it needs to be converted to glucose first, and not many cells have the capacity to do this. In the liver, it’s either stored as glycogen, liver fat, or converted into triglycerides (fat) where it’s then shuttled around the body. This is a very energy-intensive process, so instead of providing a rise in available energy, energy is used up.

Fructose causes the pleasure and reward centres of the brain to light up to a greater extent than glucose, leading to a greater desire for food. Studies have shown that individuals are more willing to give up long-term monetary rewards for food [3], [4]. It’s basically telling your brain to eat as much as you can right now and don’t worry about the consequences.

Now remember, although fructose is very sweet, eating it does not result in a corresponding rise in glucose and available energy—which is what your body expects. So when you eat it, your body effectively thinks it’s going into hibernation or it’s being starved.

The Effects of Fructose

Think of fructose as an energy storage molecule. It signals starvation/hibernation to the brain so that the body will go into starvation/hibernation mode, which leads to:

  • An increase in fat production and storage. It prevents fat from being burned [1], [2].
  • Fatty build up in the liver.
  • Insulin resistance in the liver and muscles so that glucose is reserved for the brain. The brain needs a constant supply of glucose because it can’t store glucose. This results in more glucose being converted to fat so that it can be stored.
  • Activation of the dopamine reward pathway, promoting hedonic eating (eating purely for pleasure).
  • Persistent levels of the hunger hormone ghrelin, telling you to keep eating.
  • Leptin resistance. Leptin is a hormone released from your fat cells when they are nearing capacity that tells your brain you are full and you don’t need to keep eating.

Think of eating lots of fruit before winter hits and they become unavailable. Doing so triggers hibernation mode, so you store fat for the winter. It ensures that incoming energy is diverted to storage.

Food manufacturers are fully aware of fructose’s effect on the brain, as well as it’s highly palatable properties, so they thought: “we need to get more fructose into our products!” They developed high fructose corn syrup (HFCS), which is VERY sweet and cheap to manufacture. HFCS is used in a lot of processed foods and soft drinks/sodas. It is a potent form of fructose, being anywhere from 60-90% fructose. It’s even hidden in the products you least suspect, like so-called ‘health’ bars and savoury snacks like pretzels. Some sports drinks also contain HFCS.

High fructose consumption has been shown to induce fatty liver, insulin resistance, and high blood pressure even in the absence of weight gain [2], [5], [6], [7], [8]. Studies in humans have shown that fructose increases energy intake (makes people eat more), reduces insulin sensitivity, increases circulating triglycerides and visceral fat stores, reduces burning of fat, and reduces energy metabolism compared with other foods such as glucose or starch.

Fructose is considered a lipogenic (fat producing) sugar. It not only increases fat storage, but it also adversely affects cholesterol (increasing blood triglyceride levels, decreasing HDL ‘good’ cholesterol levels, and/or increasing total cholesterol [5]. This sets the stage for cardiovascular disease, and when you combine fructose with a high fat diet, it’s like playing with fire.

Uric acid is a by-product of fructose metabolism. Uric acid can increase blood pressure, gout, and inflammation [8]. Uric acid can also crystallise in plaques in the arteries, contributing to cardiovascular disease pathogenesis. High uric acid levels increase your risk of type 2 diabetes by 70-90%. If your uric acid levels are elevated, I would suggest limiting your fructose consumption.

What’s striking with fructose is that these negative metabolic effects occur independently of energy intake, meaning it can happen even in the setting of calorie restriction [2].

High fructose consumption has also been found to cause leaky gut and endotoxemia, where toxins from the gut make their way into the blood because the gut wall barrier is compromised [5]. This increases whole-body inflammation, and both are strongly linked to non-alcoholic fatty liver disease (NAFLD).

Fructose in Fruit

While fruit contains fructose, it is less likely to induce metabolic syndrome due to the lower fructose content per fruit when compared to a soft drink. Fruit also contains other molecules (fibre, phytonutrients, vitamin C, and other antioxidants) that may help to combat the effects of fructose [2].

Low doses (like in a piece of fruit) are also partially metabolised by the gut into glucose, so less fructose makes it to the liver [8]. Also, the fibre in a piece of fruit is beneficial to the gut microbiome and may protect you from leaky gut.

I still suggest limiting your fruit intake to two servings per day and limiting or avoiding dried fruit, fruit juice, and honey. As a rule, the sweeter the fruit, the higher the fructose content. I generally recommend having your piece or serving of fruit straight after a meal. This will give the gut more time to break down the fructose. The acid and vitamin C in the fruit also improves metabolism and can lead to more stable blood sugars.

Fructose in Processed Foods 

Fructose in HFCS and processed foods is formulated in such a way that it saturates the receptors in your gut so that more of it ends up getting to your liver and disrupting your gut microbiome. This is intentional so it can stimulate the starvation/hibernation signal, driving you to eat and want more. Steer clear of processed foods!

If you’d like to learn more about the dangers of processed foods and what you can eat instead, please watch my video on Whole vs. Processed Foods. 

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