Fruit juice can be OK… in moderation
In a nutshell
Fruit juice delivers a high dose of fructose, a sugar found naturally in fruit
Moderate consumption is tolerated by people with healthy metabolic flexibility
Fruit juice can be harmful when too much is consumed by people with poor health
This article is part of a series describing the effects of industrially- or ultra-processed food. There are three categories:
Seed oils
Sugars and starches
Refined grains
I’ve previously described the risks associated with seed oils and how too much of one sugar (glucose) can lead to things like obesity, T2D, heart disease and fatty liver disease.
Fruit juice is an industrially-processed food because it contains an unnaturally high level of fructose, the sugar naturally found in fruit.
In this article I’ll cover three things:
Fruit juice is different from whole fruit: it is a processed product
Fructose is a natural sugar but the human body handles it differently from glucose
The evolutionary significance and effects of fructose on the human body
Fruit juice is different from whole fruit: it is a processed product
Some people eat lots of different types of fruit and seem to do well. I mostly stick to berries (blueberries, raspberries, and strawberries) and apples. Irrespective of which is eaten, fruit tastes great and provides a source of sugar (fructose) wrapped up with plant fibre. This complete package is something our bodies are adapted to benefit from. For example, people like me appear to need sugar to restore muscle glycogen after strenuous exercise. All good so far.
However, it has been stated [1] that:
“…it appears that fruit itself is not likely to be harmful, but the way you eat fruit and what else you eat certainly could be.”
The fibre in fruit prevents rapid absorption of sugary fructose
Fibre slows the absorption of a fruit’s sugar from the small intestine. This gives the body more time to deal with the sugar influx and reduce potentially harmful insulin spikes [1].
When fruit is processed, the juice may be simply pressed out or the fruit may be frozen and chopped up. Irrespective, the juice is separated from the natural fruit fibre [2] and the fructose enters us more quickly. Even when chopped up fibre is not separated from the juice, absorption of fructose is still quicker than for a whole fruit. For example, it has been found that simply mixing up a portion of blueberries in a blender results in higher blood sugar than eating the berries whole [1]. Essentially the fructose is de-coupled from the natural and beneficial effects of the fruit’s fibre. When we drink fruit juice, high levels of fructose can be absorbed more quickly by our body.
Fruit juice has even been compared to sugar-sweetened beverages [2]:
“…juice is as egregious a delivery vehicle for sugar as is soda.”
A glass of pure apple juice contains around nine teaspoon equivalents of sugar, so it’s worth paying attention.
Fructose is a natural sugar, but the human body handles it differently from glucose
Natural sources of fructose
Fructose is found naturally in things like table sugar (sucrose) extracted from plants (50:50 glucose:fructose), fruit, and honey [1,2,3].
Unnatural sources of fructose
Fructose is artificially concentrated in processed products like fruit juice and high fructose corn syrup (45:55 glucose:fructose) [1,2,3].
Once processed, fructose finds its way into what we eat through all manner of industrially-processed things like baby formula, baby foods, sodas, sports drinks, energy bars, candies, coffee drinks, cakes, cookies, pastries, and bread [2,3].
There is also evidence that fructose enters the unborn and new-born from mother’s diet and breast milk, respectively [2].
The way our body and our microbiota handle fructose
Fructose from any natural and unnatural source is handled the same. Once it leaves our stomach, it is absorbed from our small intestine. In this regard, it is like glucose.
Once absorbed, however, our body handles fructose and glucose differently.
Glucose
I’ve written extensively about how our body handles glucose through the interaction of insulin and the liver. When glucose is absorbed from the small intestine, blood glucose levels rise closely followed by insulin in order to maintain the blood concentration of glucose within a safe range. This works by moving glucose out of circulation to parts of the body (e.g., muscles, liver, brain, etc.) where it is needed for energy. Once those basic needs are achieved, the liver converts excess glucose to fat which is stored until needed in the future.
Fructose
Insulin plays no role in the treatment of fructose. Instead, blood levels of fructose are kept low mostly by rapid direct metabolism in the liver and to a lesser extent the intestines, kidney, and muscle [4,5]. In this way, 30% to 50% of fructose is rapidly converted to glucose which then adds to the need for insulin regulation, energy use and storage as described above.
It should be noted that the small intestine’s capacity to absorb fructose is limited. This means that if someone is eating a lot of fruit or drinking, for example, large volumes of soda or fruit juice, all of the fructose may not be properly absorbed. Instead, it will pass on to the lower parts of the intestine including the colon where it can be fermented by the microbiota [4]. This can, in turn, cause bloating and abdominal pain such as that encountered in conditions such as irritable bowel syndrome (IBS).
The evolutionary significance and effects of fructose on the human body
Richard Feinman may be the most important nutritional scientist that ever lived, in my opinion. I always benefit from his viewpoint, and that applies to the subject of fructose. Much of what I have to say in this section is from reference 6.
Evolutionary significance
Over the time in which we evolved and became adapted to a human diet, carbohydrates, including fructose, were a sign of plenty, such as in summer and autumn when plants and their fruits were readily available. Otherwise, the availability of carbohydrates, and fructose in particular, was low and variable. Under those conditions, it seems reasonable to assume that we evolved to use carbohydrates immediately as energy and otherwise store excess as glycogen and fat for future use [6].
“Our conclusion is that fructose is best understood as part of the general pathways of carbohydrate metabolism. Any unique effects of fructose are mediated by interactions with glucose as well as by a significant conversion of fructose to glucose.”
Effects of fructose on our body
Now it is clear how fructose mostly is dealt with by the human body (I’m not going to expand on what I’ve said about the effects of excess on our gut microbiome). Fructose is a sugar. It doesn’t matter whether sugar comes from fruit, vegetables, grains, bread, cookies, soda, or fruit juice. The body cannot distinguish between natural and processed sources of sugar.
The effects of fruit juice will be dependent on two things, namely the body’s state of metabolic flexibility, and the volume of juice consumed. If our body is in a state of healthy metabolic flexibility and if we consume it sparingly, as nature intended, fructose will be used immediately for energy or stored for later use.
Conversely, if our body is metabolically inflexible, we are more likely to be harmed by consuming fruit juice.
Healthy metabolism – insulin sensitivity and metabolic flexibility
I’ve described this more extensively before.
Three major (macro) nutrients in our food are used as starting materials for fuel, namely carbohydrates, fats, and proteins.
Our bodies preferentially use carbohydrates and turn them into blood glucose
Blood glucose is kept not too high and not too low by the hormone insulin which sends glucose to the parts of the body where it is needed (brain, muscles, etc.)
When we eat more carbohydrates than we need immediately, our liver turns them into fat for storage and later use
When carbohydrates are not available, our bodies will use stored fat for fuel. In extreme conditions, stored protein (muscle) will be used for fuel
Unhealthy metabolism – long term high blood insulin and insulin resistance
When we consume too much carbohydrate, we produce too much insulin and our body (brain, muscles, fat stores, etc.) become “numb” to its messages and stop absorbing glucose from our blood. This is called insulin resistance
Insulin resistance leads to even more insulin production. In this state, blood sugar and insulin remain long-term at unhealthy levels leading to disease
Thus, if we consume too much fruit juice, we can induce an unhealthy state of long-term blood insulin levels and insulin resistance throughout our body. This unfortunately leads to ill-health with symptoms such as obesity, T2D, heart disease, high blood pressure, and fatty liver disease.
There are authors who take a more hyperbolic approach to the disease risks specific to fruit juice consumption above and beyond that of carbohydrates generally [2]. I prefer what Feinman has to say [6]:
“Fructose is a carbohydrate. It is processed by incorporation into carbohydrate metabolism. Specific effects that are brought about by fructose elevation derive from an increase in the intermediates of carbohydrate metabolism… Most important, a significant amount of fructose is converted to glucose”
Summary
Consuming too much sugar in any form is harmful but liquid calories appear particularly bad [1]. This was recently highlighted in a British newspaper article:
“Fruit juices of all types are concentrated sugar water with flavourings in reality. Their sugar content matches other sugar-sweetened beverages. So while they have a health halo because of being linked with fruit, they are … not healthy.”
I’ve avoided describing the effects of fructose on the gut microbiome. I will point out in passing, however, that therapeutic elimination of fructose from a diet can be conducted as part of the FODMAP approach to treating IBS [5].
Industrially- or ultra-processed ingredients use naturally occurring food that humans evolved to consume as a starting material. Unfortunately, processing can make them harmful when we consume too much. In a way, the unnaturally concentrated levels of fructose in fruit juice are a bit like the unnaturally high levels of linoleic acid in seed oils. Industrial processing of fruits and seeds result in potentially toxic doses of fructose and linoleic acid, respectively. I think the comparison stops there, however. Personally, I see no reason to expose myself to seed oils…ever.
For someone already consuming a high carbohydrate (sugar) diet, nine teaspoons of sugar seems like a high price to pay for a dose of vitamin C. Conversely, a glass of fruit juice occasionally is unlikely to be harmful to someone who is metabolically flexible.
References
Spector, T. (2022) Food for Life: The New Science of Eating Well. London: Penguin Random House
Lustig, R. (2021) Metabolical: The truth about processed food and how it poisons people and the planet. London: Hodder and Soughton
Gedgaudas, N (2017) Primal fat burner: Going beyond the ketogenic diet to live longer, smarter and healthier. London: Allen and Unwin
Hannou SA, Haslam DE, McKeown NM, Herman MA. Fructose metabolism and metabolic disease. J Clin Invest. 2018 Feb 1;128(2):545-555. doi: 10.1172/JCI96702. Epub 2018 Feb 1. PMID: 29388924; PMCID: PMC5785258
Noakes, T.D. et al (2023) Gastrointestinal health and therapeutic carbohydrate restriction. In Noakes, T.D., et al, editors. Ketogenic: The Science of Therapeutic Carbohydrate Restriction in Human Health. London, Academic Press, PP. 383-413
Feinman RD, Fine EJ. Fructose in perspective. Nutr Metab (Lond). 2013 Jul 1;10(1):45. doi: 10.1186/1743-7075-10-45. PMID: 23815799; PMCID: PMC3708747