Updated: May 3, 2019
Fruit juice is a junk food.
That's the way Daniel Lieberman, Professor of Human Evolutionary Biology at Harvard University, describes it. And it's not the added sugar that's the problem.
Your body is perfectly equipped to handle carbohydrates from unprocessed fruits and vegetables. It's not so happy to process concentrated, simple sugars in a liquid form. And fructose itself in small amounts isn't a problem for our bodies. It's the huge tidal wave of fructose from soft drink / soda, fruit juice, candy bars and processed baked treats that floods our liver.
In his thorough book, The Story of the Human Body, Lieberman explains that before WWI, the average person ate about 15 grams of fructose a day, mostly from fruits and vegetables. In human history, the only rapidly digestible source of fructose available was honey. And it wasn't easy to access, so our ancestors didn't eat it all the time.
Things couldn't be more different today. Fruit is a now minor source of fructose for most people. Instead, we have potent forms of industrial fructose, an entirely different beast to its natural cousin in fruit.* Thanks to high fructose corn syrup (HFCS), it's quite normal for people to eat 55 grams of fructose a day, most of it from soft drink and processed foods laden with table sugar (sucrose is half glucose and half fructose). It exists in the refined sugars used to make breakfast cereals, pastries, sodas, fruit drinks, and other sweet foods and beverages.These fake foods lie to our bodies and swiftly supply unnatural concoctions of sugar in exorbitant quantities that our digestive systems are not designed to handle.
Sugar found in nature (fructose and glucose in fruits and vegetables) comes packaged in a whole food, along with fibre that slows its absorption and makes us feel full and micronutrients that contribute to many vital functions inside our bodies. Fibre and nutrients moderates a blood sugar rise and create a steady release of energy. It's makes us feel fuller for longer and satisfies your appetite, at least for a little while.
In contrast, refined and condensed sugar supply a flood of calories that is rapidly absorbed due to the lack of fibre that used to accompany it. This causes insulin to sky rocket and then plummet. This blood sugar crash cues the body to eat again.
Let's compare a humble apple to a bottle of pure apple juice.
Should I be worried about fructose?
There’s a misplaced fear that fructose alone will cause you to put on fat. Dr Ray Peat, a hormone physiologist with more than 40 years of experience, explains that the fructose in fruit can actually help to improve your hormonal health. In contrast, too much industrial fructose (think soft drink, fruit juice, HFCS) does not have positive hormonal effects – it actually does the opposite.
Fructose in the right amount is beneficial for the liver. It stimulates the liver to activate several components of the thyroid system. (Glucose is also an important contributor to thyroid function.) The liver makes about 60 - 70 % of the active thyroid hormone, T3, from the inactive form, T4. Industrial forms of fructose can lead to a sluggish thyroid.
A healthy liver will aid metabolism. Fructose increases your body’s ability to convert carbohydrates to energy by about 20 %.
Fructose also raises your body temperature and this boosts your metabolic rate.
In the right amounts, fruit and its sugars also help to reduce cortisol. This is turn helps to balance other hormones (ie, sex hormones.)
Fructose protects against the fat deposition effect of insulin. Fructose itself doesn't restrict the secretion of insulin, but it makes insulin less harmful as it blocks insulin’s tendency to produce obesity.
The fructose in fruit occurs in small and natural amounts, far too little to cause inflammation or insulin resistance. It’s also packaged with other vitamins, minerals and fibre. Concentrated fructose can contribute to insulin resistance – the fructose in fruit juice, soft drink, junk food, often meal replacement bars and even ‘natural’ sugars like agave nectar come in doses that flood the liver and can be a disaster for blood sugar and fat gain.
What kinds of foods does Dr Peat suggest for fructose? You guessed it. Mainly fruits. Fructose as nature designed it.
Keen to learn more?
Michael Pollan, In Defense of Food. Pollan’s thorough research helps us to understand why fruit juice and HFCS is an entirely different beast compared to the fructose in whole fruit. See pages 112 - 113.
Daniel Lieberman, The Story of The Human Body. Contains an illuminating commentary on fructose metabolism and fruit from the perspective of evolution, health and disease. See pages 255 - 269.
Michael Mosley, The Clever Guts Diet. Mosley’s book includes a discussion of phytonutrients found in fruits and their impact on the microbiome and human health. See pages 22 - 24.
Dr Cordain's Table of Fruits and Sugars: a handy online table that lists the sugar content of glucose, galactose, fructose and total metabolic fructose in different kinds of fruit.
* The term 'industrial fructose' perfectly captures the unnatural, man-made state of the dominant forms of fructose today. I'm not sure of the origins of this term, but I noticed it used in a journal article on the topic: Petta S, Marchesini G, Caracausi L, Macaluso FS, Camma C, Ciminnisi S, et al. Industrial, not fruit fructose intake is associated with the severity of liver fibrosis in genotype 1 chronic hepatitis C patients. Journal of Hepatology 2013; 59: 1169 – 1176.