Why Fruit Sugar Causes Fatty Liver and Weight Gain in Adults and Children

Fruit sugar (fructose) occurs naturally in honey, fruits, berries, flowers, and many root vegetables. It is a simple sugar, much sweeter than another familiar simple sugar, glucose.

Unlike glucose though, our cells are not able to use fructose to make energy. Instead, the fructose we eat or drink goes to the liver for processing.

In theory, the liver can convert fructose to glucose, which can then be added to the body’s glucose pool for energy production. This doesn’t happen often because usually glucose is present in the food or drink, so there’s more than enough glucose already available, and the liver doesn’t waste energy converting fructose to add to an already-high glucose level.

However, the liver does convert some of the glucose to fructose (via the polyol pathway), adding to the fructose already available from the digested fruit and drinks. If you’re interested in a little biochemistry, there’s a good description of the polyol pathway at this Wikipedia page.

The fructose is converted to a type of fat called triglyceride, and to a substance called uric acid (more details of each in the sections below).

Fatty human liver (NAFLD)

The triglyceride made from fructose is stored in the liver at first. At this early stage, routine blood tests may show liver enzymes increasing, but still within the ‘normal’ ranges, so your doctor may suggest ‘keeping an eye on it’.

Over time, as more and more fructose is consumed, more and more is converted to fat. The liver enlarges, and the fat hinders normal liver functions. Now ALT, AST and GGT, three of many enzymes made in the liver, spill out into the bloodstream and show up, elevated, in your next blood test. Raised ALT , raised AST, and raised GGT are signs of an unhealthy, fatty liver.

Lab Test Online is a useful website where you can find out the meaning of ALT, AST, and GGT, as well as your other blood tests and their abbreviations.

Along with the enzyme elevations, if palpation of the liver, or an ultrasound scan, confirms an enlarged liver, a diagnosis of fatty liver disease (also known as non-alcoholic fatty liver disease – NAFLD for short) is made.

Progression of NAFLD leads to insulin resistance, then metabolic syndrome, and eventually to an alarming array of debilitating lifestyle diseases such as obesity, type2 diabetes, and cardiovascular disease.

Fatty goose liver

Fattening of the liver is well known to French farmers. Geese or ducks are fed copious amounts of corn, which the birds digest to sugars.

The sugars cause their livers to become greatly enlarged and engorged with fat by the same mechanism human livers become enlarged and engorged with fat… a build-up of triglycerides.

After the birds are harvested, their triglyceride-laden livers are processed into the delicacy, foie gras (foie is French for liver; gras is French for fat).

Fatty livestock

Apart from humans and geese, starch fattens other animals, too. Farmers use a variety of carbohydrate fodders to fatten their livestock before taking them to market.

In several countries, even though ruminants spend the first six months or so of their lives eating grass and other plants on the pastures like wild flowers, many spend the last months being fattened in confined feeding operations (CAFO).

There, the animals are given dry feeds, mainly wastes from the human food processing, seed oil, and alcohol brewing industries. For example, crumbs from the biscuit and cereal manufacturers, a feed known as BCM.

If you’re interested, the Agrifeeds New Zealand website has pictures and short descriptions of the various fattening carbohydrates, many of which fatten humans, too.

Fatty blood (high triglycerides)

The triglycerides building up in the liver (described in the NAFLD section above) eventually spill out into the blood stream.

Elevated triglycerides in the blood are usually accompanied by a low level of the so-called ‘good cholesterol’ (HDLc), both of which your routine blood tests will show.

The ratio TG-to-HDLc (the amount of triglycerides compared with the amount of HDLc) is now considered an important indicator of cardiovascular disease risk.

What we want is low triglycerides and high HDLc. If your triglyceride level is rising, it’s not worth ignoring it.

This article from the Mayo Clinic explains the import of high triglycerides, and what you can do to lower them.

Uric acid

In many animals the uric acid made in the liver from fructose can be converted by an enzyme called uricase, to a substance called allantoin, which is then excreted. This conversion process is a natural mechanism to keep uric acid levels low.

Ancient genetic mutation

Some 15 million years ago, during the Miocene Epoch, a genetic mutation occurred in humans (and great apes and some lesser apes) that removed our ability to make the conversion enzyme, uricase.

As a result of the mutation, we can’t convert uric acid to allantoin to get rid of it, and therefore have a continually higher level of uric acid in our blood compared with other mammals.

Some researchers think the loss of the ability to make uricase has been advantageous, because uric acid is a powerful antioxidant and scavenger of free radicals. In other words, uric acid protects us from free radicals, thereby increasing lifespan and decreasing the chances of developing cancer in later life.

However, like many compounds in the body, there’s a ‘Goldilocks’ amount. A little uric acid is beneficial, but high uric acid raises blood pressure, leads to gout, kidney stones, and contributes to insulin resistance, weight gain, diabetes, and the rest of the long list of lifestyle diseases.

Fat switch

Back in the Miocene, insulin resistance and resulting weight gain, caused by the increase in uric acid, may have been beneficial survival mechanisms.

As the Ice Age approached and the European forests declined, fruit that was once abundant became scarce. The increase in uric acid acted a bit like a fat switch, allowing our ancestors to become insulin resistant and store as fat, whatever fruit they could find.

Then, in times of famine, the fat was used to make energy, enabling survival until more food became available.


A similar mechanism of fat storage is programmed in hibernating mammals. Copious amounts of very sweet fruits are eaten in late Autumn, greatly increasing the animals’ fat deposits before the cold Winter arrives.

Fruit in late Autumn is at its sweetest, because the fructose content is at its highest. The fructose and increased uric acid production cause the animals to develop fatty liver, become insulin resistant, and obese.

Then, while they shelter in their dens from the Winter snows and ice, fasting until Spring arrives, they slowly mobilise their body fat and use it to produce life-giving energy.

When they emerge from hibernation in the Spring, the animals are slim, and no longer have fatty liver or insulin resistance. The yearly cycle starts anew, but they must wait until late Autumn before they can gorge on sweet fruit again.

Fructose available every day

Thanks to modern agricultural technologies, food processing, world-wide logistic supply chains, and supermarkets, humans have access to fructose-stuffed foods, fruits, and drinks all year round.

For many, that means a fat switch permanently stuck in the ‘on’ position. The result is an epidemic of people with fatty liver disease, high uric acid levels, high triglycerides, low HDLc, insulin resistance, and weight gain.

Fatty children

Even more alarming is the increasing number of children with fatty livers.

Well-meaning parents ‘treat’ their young ones to fruit juices, colas, sports drinks, cordials, squashes, and flavoured milk drinks, more or less on demand.

Even dairy-free ‘milk’, like almond ‘milk’, soy ‘milk’, and oat ‘milk’ are packed with added sugar, as a look at the ingredients label will confirm.

All that fructose causes many youngsters to develop fatty liver, setting them on the road to debilitating diseases and contributing to the expectation they’ll be the first generation to live shorter lives than their parents.

Sugar addiction

The nucleus accumbens is where humans experience the ‘feel-good’ sensations after using alcohol, tobacco, other drugs, socialising, listening to music, and a variety of other pleasurable activities.

Fructose has been shown to light up that ‘feel-good’ centre, too.

Clinical research by pediatric endocrinologist at the University of California San Francisco, Professor Robert Lustig, has clearly demonstrated fructose acts on the nucleus accumbens in the brains of adults and children.

The pleasure response is similar to any other substance of addiction. And like the other addictive substances, adults and children want to experience the pleasure of fructose again and again.

Added sugar and fructose

Manufacturers of foods and drinks are well aware of the nucleus accumbens and what it means for profit margins.

They pay consultants to find the ‘bliss point‘ by testing on volunteers before they launch new products.

For a food, the bliss point is the combination of sugar, salt, fat, and a host of other additives that volunteers find the most appealing to their senses of sight, touch, smell, mouth-feel, and taste.

For a drink, it’s the combination of sugar, fructose, glucose, acidity regulators, colourants, and flavourants that give the drink the greatest ‘come-back-for-more’ appearance and flavour.

Have a look at this blueberry juice carton available in a local supermarket.

blueberry juice drink ingredients list at thrivelowcarb.com

In spite of the misleading picture on the front of the box depicting healthy blueberries, only 10% is actual blueberry, and even that small amount is derived from processed concentrate.

The sugar, glucose-fructose-syrup, and the other ingredients have been added in various proportions to stimulate the drinker’s nucleus accumbens in just the right way to have children and their parents wanting more, ensuring repeat purchases.

What to do?

If you don’t want to be manipulated by food manufacturers, ignore the pretty picture on the front of containers and carefully read the ingredients lists.

If you can’t resist juice, make the juice yourself at home, from real whole fruit.

Even better than the home-made juice, is to eat the whole fruit and not juice the fruit at all.


If you try juicing at home, you’ll see the amazing quantity of fruit you and your children are actually drinking, when you knock back a glass of juice in a restaurant.

Also, you’ll see you’re throwing away the fibre beneficial for gut health. In addition, fibre has many nutrients attached to it, so you throw them away, too.

Apart from the attached nutrients, another benefit of fibre is that it slows the rate of digestion of the fruit, meaning the fruit sugars are slowly released and are much less likely to cause harmful sugar and insulin spikes.

In contrast, it’s very easy for thirsty children to polish off a big glass of juice in just a minute or less. Their blood glucose is rocketed sky high, giving their young pancreas a big kick into insulin-producing action, and sending their sensitive livers into fructose-disposal overdrive.

The best drink to quench thirst is water. It’s cheap, has zero calories, no effect on the addiction centre, and it won’t make your liver, or your children’s young livers, fat.

Fruit marketing

Fruit has been very successfully marketed as a necessary component of a so-called ‘balanced’ diet. As a modern convenience, fruit juice has been promoted as an acceptable substitute for whole fruit, forming part of the ‘5-a-day’ recommendation.

Unfortunately, the slogans like ‘5-a-day’ were invented to increase sales and have nothing to do with nutritional science.

Do we need fruit?

Truth be told: there is no clinical evidence to show fruit is necessary for survival. In fact, we don’t need to eat any carbohydrate at all. That’s because human livers are equipped to make glucose from fat or protein (in a process called gluconeogenesis).

Another healthy advantage of gluconeogenesis is that the liver makes just enough glucose to satisfy our needs: no more, no less. There are no glucose lows, no glucose highs, and no insulin spikes.

Sucrose (table sugar)

Even though you may avoid fruit juices, sugary drinks, and fruit, it’s still possible to get doses of fructose.

Table sugar is, in fact, two simple sugars joined together. Every teaspoon of sucrose is actually half-a-teaspoon of glucose joined to half-a-teaspoon of fructose.

As soon as the sugar gets in your mouth, enzymes in your saliva quickly start breaking the sucrose into its glucose and fructose halves.

The glucose half rapidly enters your bloodstream, raising your blood sugar level, and the fructose portion rapidly lights up your brain’s pleasure centre on its way to your liver.

Sugar everywhere

Although sugar is sometimes added to food prepared at home, at least you can control how much sugar goes into the mixing bowl.

Not so with foods ordered in a restaurant. There are no ingredients labels attached, and frankly, you have no idea what’s in the food you’re eating.

Fortunately, factory-made foods do have labels.

Check the labels on the packaged products you’ve got in your fridge and cupboards and you’ll be surprised how much sugar many of them contain.

These are two examples from our local shops…

My wife eats a very small quantity of bread, which she bakes herself. Her basic whole-wheat bread recipe has four ingredients: organic whole-wheat flour, water, yeast, pinch of salt.

Compare her home recipe with this ‘healthy’ whole-wheat bread at a local supermarket. It has a long list of 20 ingredients, 5 of which are sugar sources: sugar, milk, milk derivatives, malt extract, raisin juice concentrate.

supermarket bread ingredients sugar highlighted seen at thrivelowcarb.comAnd this frozen meal of hake in Thai sauce contains a long list of 23 ingredients, including 5 sources of sugar: maltodextrin, dextrose, milk solids, sugar, orange powder.

hake fillets sugar highlighted seen at thrivelowcarb.com

Instead of the processed fish dish above, try this recipe for making hake in Thai sauce at home, using healthy whole-food ingredients.

In summary, for a healthy liver…

You probably don’t need to make any changes to your current food choices and lifestyle if you are…

  • fit and healthy 
  • full of energy the whole day
  • sleep well
  • happy and mentally well-balanced, not depressed
  • the results of your annual blood tests are good
  • your blood pressure is a normal 120/80 or less
  • you don’t need any medications
  • you are pleased with how you look and feel

However, like many other people, if some of the following apply to you, you need to make different food and lifestyle choices now, to halt the slow downhill slide to senility…

  • your waistline has expanded
  • some of your blood test results have elevated values
  • your blood pressure is creeping up
  • your blood sugar and insulin are elevated
  • you have bouts of tiredness during the day, mental fog, or depression
  • you are not happy with what you see in the mirror

Here are a few tips…

Drinks Tips

  • Why risk ruining your liver, and your children’s livers, with sugar-laden fruit juice, colas, sports drinks, cordials, squashes, and flavoured milks?
  • Healthy alternatives are fresh water, and freshly-brewed teas without sugar or milk. For example, here are 10 health benefits of green tea.

Fruit Tips

  • If you can’t abstain from fruit, limit it to one or two small servings a day as part of a meal. Eating fruit as a desert after a meal slows the release of sugar and fructose and avoids detrimental sugar and insulin spikes.
  • Enjoy the whole fruit. Don’t ruin it by juicing.
  • Choose fruit like berries, nectarines, apricots, plums instead of grapes and bananas. Most tropical fruits and grapes are high in very sweet fructose. The other fruits are packed with vitamins, minerals, colourful antioxidants, and fibre, giving you a healthy bang for your buck, without the glucose and insulin spikes of sweet tropical fruits.

Food Tips

  • Whenever you can, buy in-season produce from your own country… meats, fish, eggs, cheese, fresh vegetables, salads, fruits, spices, nuts… and make your own meals at home. You’ll know exactly what you’re eating and get the most nutrition for your money.
  • Unlike preparing your food at home, if you eat food in a restaurant, or order from a fast-food outlet, you have no idea what’s actually in the food you’re eating, because the foods don’t come with ingredients labels.
  • If you can afford it, buy organic foods. They have not been sprayed with pesticides, insecticides, or fungicides and their growth has not been encouraged with synthetic fertilisers. Take a look at the Environmental Working Group’s list of pesticide-laden fruits and vegetables in their ‘Shoppers’ Guide to Pesticides in Produce’.
  • Encourage your children to have fun helping you in the kitchen. They’ll grow up knowing how to choose and cook real food – the knowledge they need to bring their own healthy children into the world.
  • Starches are composed of small clumps or granules of glucose molecules. Our digestive processes separate the granules into glucose molecules again, which enter the blood stream and cause a rapid rise in blood sugar and insulin. Some of the glucose is used for energy. Some is converted to fat (triglycerides) and stored in the liver, waist, and hips. Some is converted to fructose. Some of that fructose is converted to fat (triglycerides), and some fructose is converted to uric acid. If you eat starches (pasta, rice, corn, bread and other baked goods), it’s wise to limit them to very small amounts.
  • How much starch are you actually eating? To find out, use a food tracker for a few days. A good tracker is Cron-O-Meter, which you can download for free at cronometer.com and use it on your cellphone, tablet, or pc.
  • If you can’t avoid factory-made foods from a supermarket, read the labels before you buy. Choose ones with short lists of ingredients you recognise, without added sugars, colourants, flavourants, or other chemical additives.



Professor Richard Johnson has been researching the ‘fat switch’ in hibernating animals, and in humans, for years, has a wonderful grasp of fructose and uric acid metabolism, and a very enjoyable way of imparting his knowledge.

His talk, ‘Sugar and its Role in Driving Obesity and Fatty Liver‘, is a very interesting one, which he gave to a young audience in New Zealand at a FIZZ – Fighting Sugar in Drinks – workshop, explaining what happens when they have sugar-and-fructose-laden foods and drinks.

I’ve added the 30-minute video to our Thrive Low Carb collection on YouTube. If you’d like to watch it, please scroll down the list and you’ll see it at number 17 in the collection.

There are two more of Professor Johnson’s videos in the list (numbers 18 and 19).


This short (14 minute) video, from a series by the ‘What I’ve Learned’ YouTube channel, explains how fructose ruins liver health. It starts by talking about alcohol, but stick with it to hear about starch, sugar, and fructose.


Professor Johnson is not only a leading researcher of the effects of fructose and uric acid in humans, he is a consultant nephrologist in active hospital practice. He was co-author of a text-book on nephrology, and has written books about sugar, fructose, and uric acid. If you like a good read, there are more details at the books page of his website.

Dr David Perlmutter is board-certified in neurology and nutrition. He has written several best-sellers with an emphasis on maintaining a healthy brain and avoiding dementia. To browse his books, visit the book page of his website here.

Suggestions? Questions? Comments?

Please use the comment box to send me any suggestions, questions or comments you may have. Thanks for reading. Hope it helped you [and your liver :-)].

Updated 04 February, 2022

5 thoughts on “Why Fruit Sugar Causes Fatty Liver and Weight Gain in Adults and Children

  1. Pingback: How added sugar harms us and what to do about it | Thrive Low Carb

Leave a Reply

Your email address will not be published. Required fields are marked *