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In this paper of the WHO, it is claimed that we should limit our consumption of free sugars:

WHO recommends a reduced intake of free sugars throughout the lifecourse. [...]
Free sugars include monosaccharides and disaccharides added to foods and beverages by the manufacturer, cook or consumer, and sugars naturally present in honey, syrups, fruit juices and fruit juice concentrates.

At first, I was wondering if the sugars in a fruit can be different from sugars in a juice. According to the answer to my question here, the sugar molecules are exactly the same in both (in the case of a 100% pure juice).

So, why this recommendation? Why doesn't our body digest sugars from fresh fruit the same way as sugars from juice? To make it simple, my question only concerns 100% pure juice with no added sugars.

I tried to find an answer and, according to the answers of this question, because our body is slow to digest sugars from a fruit, it assimilates it better. Is it right? And, if it is the case, if we eat fiber and starch with our fruit juice, it should be the same as a fresh fruit, right?

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Sugars in 100% natural fruit juices are chemically the same as in whole fruits. They mainly include glucose, fructose and sucrose:

  • Apple nutrition data (expand the carbohydrate section)
  • Apple juice nutrition data

Sugars in whole fruits are "incorporated" into the fruit, which means the digestive system first needs to physically decompose the fruit and then extract and absorb sugars, which takes some time. In fruit juices, sugars are "free," so they are absorbed quicker than from whole fruits, which results in higher blood glucose levels, which is a risk factor for diabetes type 2 (Defeatdiabetes, Diabetes.co.uk).

Also, fruit juices are liquid, so they pass through the stomach quicker than whole fruits, so they fill the stomach for a shorter time and may be therefore less satiating. This can make you drink more juice than you intended, which can result in unwanted weight gain.

If you eat foods high in fiber along with juice, the fiber will slow down the absorption of sugars from the juice (Nutrients). If you eat foods that contain mainly plain starch (white bread, cookies or rice, or potatoes) along with juice, the starch will be quickly digested and absorbed as glucose and will raise the blood glucose even quicker than the sugar from the juice.

The effect of nutrients from foods on blood glucose level after meals is expressed as glycemic index (GI): the higher the GI, the higher blood glucose (Harvard):

  • Glucose = 100
  • Cornflakes (mainly plain starch) = 81
  • Potato, boiled (mainly plain starch) = 78
  • White bread (mainly plain starch) = 75
  • White rice (mainly plain starch) = 73
  • Sucrose = 65
  • Honey = 61
  • Apple juice = 41
  • Apple: = 36
  • Kidney beans (high in fiber) = 24
  • Fructose = 15

In conclusion, natural sugars from whole fruits, fruit juices and artificially added sugars are all digested in the same way, but sugars from juices are absorbed quicker, which can result in higher blood glucose levels. Foods that contain mainly plain starch can raise the blood glucose levels much more than fruits and fruit juices.

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    $\begingroup$ It's interesting that Fructose has such a low GI... considering that honey is basically made of fructose (low GI) and glucose (high GI) it falls roughly in the middle of the two. And for reference HFCS which is always mentioned as bad maxes out at 68 which is still lower than four of the foods on that list. $\endgroup$ – Michael Aug 5 at 2:24
  • $\begingroup$ It's also interesting the apple juice is only about 10% higher than apple - probably not far off the difference between apples. $\endgroup$ – Chris H Aug 5 at 7:35
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    $\begingroup$ @ChrisH Yes, we've bred apples for a long time to make them the easily digestable sugars they are today (though evolution did it's fair bit even before us - fruits are carriages for seeds, "designed" as easy sugar) :) It's funny how thousands of years of breeding are now essentially being reversed, because we're no longer energy deficient by default. But there's still that one difference between juice and an apple - you usually eat one or two apples, but drinking a bottle of juice is something you easily do without even noticing (though I know people who eat apples the same way :D ). $\endgroup$ – Luaan Aug 5 at 8:02
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Drinking the juice without the fruit can easily lead to over-consumption. It is after all harder to eat four apples than drinking 500ml apple juice. As the liver breaks the fructose through lipogenesis it creates fat and can cause non-alcoholic fatty liver and obesity.

Since sugar (fructose/glucose/sucrose et al.) can be absorbed already through the mucus membrane, it also causes a rapid elevation in insulin levels leading to insulin-resistance. Insulin-resistance has an impact on depositing white adipose fat which in turn has several harmful effects on the hormone production of the body. Leptin resistance from insulin-resistance alone will cause you to be unable to regulate hunger properly, even further elevating excess fat storage in adipose fat.

You may notice you don't feel full when drinking sugars, but you do much more readily when eating them. WHO is recomending against all sugar beverages and juice is just as unhealthy as soda in this regard.

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  • $\begingroup$ "You may notice you don't feel full when drinking sugars" I'd be quite interested in some numbers about this; I certainly do feel just as full when drinking sugars as when eating the corresponding amount of fruit (which is to say, "not much"). The main difference is that you're more likely to drink juice at the same time as eating your main meal, while you'd probably eat the apples as either a snack or a dessert after a meal. Insulin resistance is a real risk, but rather than sugar from apples and juice being different, I'd expect frequency has more blame - you can sip juice for hours. $\endgroup$ – Luaan Aug 30 at 11:43
  • $\begingroup$ @Luaan Liquids are readily absorbed by the body. "Sip" the same volume of carrots or similar fibers and you certainly will feel full. Sipping a liter of carrots will certainly not result in the same sensation as sipping a liter of juice. $\endgroup$ – Pebermynte Lars Sep 11 at 8:58
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Jan's answer is great, but I'm just going to quote the paper you reference:

The systematic review of the effect of intake of free sugars on body weight included 30 of the 7895 RCTs and 38 of the 9445 cohort studies initially identified as meeting the inclusion criteria. Meta-analysis of the five trials in adults with ad libitum diets (i.e. no strict control on food intake) found that reduced intake of free sugars was associated with a decrease in body weight (–0.80 kg; 95% confidence interval [CI]: –1.21, –0.39). Meta-analysis of the 10 trials that involved increasing sugars intake (mostly sugar-sweetened beverages) suggested a comparable weight increase (0.75 kg; 95% CI: 0.30, 1.19). Meta-analysis of the 11 trials that examined isoenergetic exchanges of free sugars with other carbohydrates showed no change in body weight (0.04 kg; 95% CI: –0.04, 0.13).

TL; DR:

  • If you keep the same energy intake, free sugars have no effect on body weight
  • If you eat without energy controls, reducing free sugar intake has a large effect on body weight

So in other words, it's not really that free sugars are easier to digest or anything; it's just that if your food has more free sugars, you're more likely to increase your overall energy consumption. This might have relation to feelings of satiation, blood sugar levels etc., but in the end, it's that simple - if you don't watch your energy intake, limiting free sugars in your diet will likely decrease your daily energy intake.

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  • $\begingroup$ I guess this is the link to the paper ncbi.nlm.nih.gov/pubmed/23321486 ; would you include it in your answer? There are several other systematic reviews that conclude pretty much the same: nutrients (sugars or other carbs, fats) are not fattening by themselves but only when they contribute to excessive calorie intake. $\endgroup$ – Jan Aug 5 at 10:09
  • $\begingroup$ I'd challenge that "If you keep the same energy intake, free sugars have no effect on body weight". The liver has specific response to fructose through lipogenesis, so even if keeping the same energy intake, fructose has an impact on body weight. In fact, you have to have an energy depleted liver for fructose in isolation to have the same characteristics as other carbohydrates. $\endgroup$ – Pebermynte Lars Aug 29 at 15:39
  • $\begingroup$ @PebermynteLars You can challenge that, but it's not my claim. I'm just quoting the WHO meta-analysis that the OP has a misunderstanding about. $\endgroup$ – Luaan Aug 29 at 17:12
  • $\begingroup$ @Luaan Have a read at the Harward Health letter quoted in my answer for an updated understanding of lipogenesis. The section "From fructose to fat" is quite clear on the mechanics. Not all sugars are equal. $\endgroup$ – Pebermynte Lars Aug 30 at 7:59
  • $\begingroup$ @PebermynteLars I didn't claim all sugars are equal. The case the OP is asking about is about the same sugars, and regardless, my answer doesn't touch on that subject at all, because irrespective of the various mechanisms at play, the quoted meta-analysis found just one thing of statistical significance - free sugars make you more likely to increase your total energy intake. In the end, your answer says pretty much the same thing, just with an explanation of one of the mechanisms involved. $\endgroup$ – Luaan Aug 30 at 11:38

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