Apples: systems that thrive on fructose

Health, Molecules

Three unexpected systems that thrive on fructose

By Juman Hijab

Reading time: minutes

Original date: April 11, 2023  

Updated: July 13, 2023

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a. Red apples.., Dec 4, 2017.

If fructose is so bad, why is it in apples?

Have you seen the headlines?

It seems as if the whole planet is anti-fructose. Julie Andrews should be singing this! “A spoonful of sugar makes the liver go bad…….the liver go bad..... the liver go bad....."

Something does not make sense!!  If fructose is so bad, why is it in apples? Isn’t there a saying, an apple a day….? Apples and pears are two of the fruits with high ratios of fructose to glucose. Bees use nectar that has fructose in it to produce honey (1).

Nobody seems to have told apples and pears to avoid high fructose sugar! I haven’t seen a honey bee drop dead from a heart attack, have you?

Let's try and see what's good about fructose. In this article, I discuss three cellular systems that thrive on fructose.

Systems that thrive on fructose: The brain

The thing is that fructose is an essential sugar that is used by many cells. Let's start with the brain.

Did you know that the brain produces its own fructose? It takes the glucose from the blood and converts it to fructose (2). This allows certain cells - Glia cells - to serve as a backup pantry in the brain. They take up the fructose and generate glycogen stores. Then, when glucose levels are low, glial cells break down the glucose to produce lactate and pyruvate. Neurons use those molecules as an alternate fuel during periods of fasting or starvation (3, 4, 5, 6).  

Unfortunately, a diet high in calories, salt, and concentrated sugars revs up the conversion of glucose into fructose in the brain. This leads to too much of a good thing. In fact, the brains of patients with Alzheimer's dementia seem to have 3-5 x the levels of fructose in their brain compared to normal (7).

Systems that thrive on fructose: Sperm cells

Another group of cells that thrives on fructose are sperm cells.

Sperm cells need fructose like we need coffee: To wake up and get moving!! Semen is high in fructose. The sperm cells use fructose as a source of energy. Without fructose, the sperm cells are sluggish. In fact, men with abnormal levels of fructose in their semen are known to have problems with fertility (8, 9). 

Interestingly, studies have supported having shorter periods of ejaculatory abstinence for men with infertility. For example, ejaculatory abstinence of days led to an expected higher sperm count and a higher semen volume. However, a shorter period of only 4 hours of ejaculatory abstinence showed that the sperm - even though there were less of them to go around - had improved motility. In support of this, the relative fructose levels in the semen were decreased with the 4 hour ejaculatory abstinence period supporting the fact that the spermatozoa were actively using fructose as an energy source for motility (10).

It could be that a fresh ejaculate from the prostate gland and seminal vesicles has less inflammatory and cellular by-products, allowing the sperm to function better.

Systems that thrive on fructose: Fetuses of many mammals

And do you know what else is dependent on fructose? Pregnant mammals (like whales and dolphins) have their unborn babies swimming in a fluid that is high in fructose. The unborn offspring are enclosed in an environment that is warm, nurturing and sweet (11, 12). 

Yes, whale and dolphin fetuses are awash in fructose. Both their fetal blood and the amniotic fluid has fructose. Other mammals where this is typical include pigs, sheep, and cows (11, 12, 13). For the unborn piglets, the fructose-containing liquids are like gold! They give energy to the fetus, provide nutrients for new cell growth, and help stimulate the development of supportive cells. It is only when the piglets are close to delivery that the sugars switch roles and glucose replaces fructose as the number 1 sugar in the blood. 

It turns out that pigs, sheep, and cows have a specialized type of placenta that is different than that of humans, dogs, cats, and rodents.  And it is the placenta that creates the switch from glucose to fructose (14).

Conclusion (and a question)

In this article, I highlight the fact that fructose - as a molecule - is not intrinsically toxic. In fact, it is essential in many biologic systems and is commonly found in natural products (fruits, honey). I described three systems that thrive on fructose: our brain, sperm cells, and several mammalian fetuses.

Interestingly, there may be a common theme to the use of fructose as an intermediary molecule. Fructose shifts the metabolism to the production of fuel (fat and glycogen). Large fat deposits are not only a superb storage place for energy but also store an incredible amount of water. Thus, animals that are exposed to droughts (camels) or salinity (whales) or prolonged hibernation (bears) accumulate large fat reserves (15).

MSG: Another molecule that is not intrinsically toxic

Here’s a question I want to leave you with: If MSG is so bad, why is it found in breast milk? (you can read more, here and here).


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dementia, diabetes, fat, fetus, fructose

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