After watching Sugar: The Bitter Truth multiple times, I keep wondering about why fruit would contain fructose if it can only be bad for us. Of course, like Dr. Lustig said so well, fructose in fruit is minimal compared to today and often comes with the cure, fiber, but still, why would it be there in the first place?
Vegetables often contain toxins and unwanted chemicals, but those are often to protect themselves, while fruits are produced to attract animals in eating them and spreading the seeds.
I keep on thinking that a certain amount of fructose had an important function in evolution. Perhaps getting fatter before winter was one of them. After all fruits are mostly available before the winter season comes. The fact that it messes with the hunger signals would make us eat more than we really needed so we could get even fatter.
If course, I'm not suggesting that one should strive to get at least some fructose because I think that once you become metabolically deranged you've gone too far and minimizing would be a good idea because of that.
I guess what I'd like is to clarify the place of fruit in our diets and put perspective where some people have come demonized fruit completely.
Also, I see a lot of people recommending roots and tubers over fruits and it makes sense that our ancestors had access to more tubers than fruits, but then again, doesn't tubers have a very similar sugar profile than most fruits? In that sense, wouldn't tubers and fruits have basically the same metabolic effect on us? Correct me if I'm wrong though.
asked byPaleo_Seb (3690)
Get FREE instant access to our Paleo For Beginners Guide & 15 FREE Recipes!
on August 02, 2010
at 06:02 AM
From the plant's perspective, the fructose is there to attract creatures to eat it and thereby spread and fertilize the seeds. So fructose definitely has a purpose to the plant. Perhaps at first, the addition of fructose by the plant made a nice sugary flavor that was even sweeter than straight glucose and this enhanced the chances of creatures eating the fruit. Over time, maybe those of us who could best utilize the fructose as well as the glucose had an advantage and so humans slowly evolved so that we could utilize the fructose. And of course, fruits have glucose as well so they were a relatively easy source of calories all along.
Of course, fruits now are much sweeter than natural ones and you don't even have to wait for a season or climb a tree to get them. And now you can even get the glucose and fructose already squeezed out in a jar for you with even more sugar added in. How convenient! At some point, I think the accessability of fructose has far outstripped our bodies' ability to deal with it. In moderate quantities consumed in a natural way, I suspect it was quite good for us, helping to replenish glycogen and provide an easy source of calories and even water. Few things taste more delicious than fruit right after a very hard sweaty workout.
I think it's just another case of something that is useful for most in moderation but quite damaging when taken to the extreme and then added in with wheat and all kinds of other garbage ta boot! -Eva
on April 12, 2011
at 05:52 PM
You should track down the studies Dr. Lustig cites. Most of the don't say what he says they say and it's quite funny because often the fact that they don't is really easy to find (in the conclusion). For example there is a study he cites that said that rats gained twice as much weight on a high-fructose diet as on a control diet. The "twice as much" was 4 g vs. 2 g, which was statistically insignificant. Chris Masterjohn does research on fructose and his fructose-fed rats are leaner and he was unable to find any liver damage. It's hard to get negative results like that published unfortunately. Chris did question Lustig's findings in Jimmy Moore's podcast and Lustig basically refused to answer his question and said he'd send him the studies. The studies did not show anything wrong with fructose. No wonder he wouldn't talk about them on the air.
Lustig is just another person who has a narrative about something and cherry picks studies to support that narrative. There is absolutely NO evidence that fructose in fruit is harmful and the evidence to support the idea that even purified fructose is harmful is tenuous. There is strong evidence that conditions blamed on fructose like fatty liver disease require something else to activate it. I'm voting for the combo of fructose + high PUFA being the culprit.
There has never been a study showing eating fruit causes AGE formation.
It's also not true that wild fruits are less sweet than domesticated fruits. There are many wild fruits that are substantially sweeter than domesticated fruits. A good example would be something like the pawpaw, though there are many more.
I eat fruit ad libetum. My triglycerides are 32 and I am very lean. I think it is worth restricting if you are damaged by a bad diet, but otherwise it should not be blamed.
on August 02, 2010
at 06:12 AM
Why would you ask "why fruit would contain fructose if it can only be bad for us"? That's basically a human-centric view of plant metabolism and a wrong-sided view of evolution and natural selection.
Animals have evolved to make use of the energy that plants create for their own use from photosynthesis. That's sugars and starches. Humans have evolved to utilize glucose and glucose starch pretty efficiently, with minimal damage to the system from metabolization. We have also evolved to make use of fructose, but it might cause some damage to the system in the process of being broken down in the liver. And we can't make use of fructose starch.
Most plants have evolved to rely about equally on glucose and fructose, and most also store some starch (they also create fibers, which are indigestible or semi-digestible carbohydrates).
More evidence that we prefer glucose to fructose: we have ample digestive enzymes for breaking down polysaccarides made of glucose, such as common plant starch (amylose, dissolved by the enzyme amylase). But we are less able to break down saccarides made of fructose (i.e. "Fructans" e.g. fructoligosaccarides (FOS), inulin, etc.) because we are lacking any enzymes for breaking them dwn. Instead, beneficial gut bacteria feed on the fructose starches in plants and convert them to beneficial fatty acids in the gut. ETC.
on October 01, 2010
at 01:52 PM
This is a not particularly relevant but little known fact.
Human seminal fluid contains fructose that seems to act as the source of energy for sperm cells.
In fact the GLUT5 protein that is now known to absorb fructose into the body in the intestine was first discovered in the testes and sperm cells.
on March 24, 2011
at 07:13 AM
Only simple sugars are absorbed into the blood stream: glucose, galactose and fructose. All are hexoses. Glucose is a universal sugar. Galactose is an animal sugar. Fructose is a plant sugar and we are poorly adapted to eat it as Dr Lustig points out in his video. Fructose, uniquely, has only 5 carbons in its inner ring structure while the other 2 have six carbons. That means fructose must be handled by different enzymes than glucose, and it turns out they are much the same ones used for alcohols, which we are also poorly adapted to eat. The adaptations we do have are probably the result of avoiding the relatively short term toxic effects we would get from eating spoiled fruit (alcohol) and the fruit sugars themselves. Fruits were, after all, a source of energy in the energy limited paleo world. They tended to come on mostly in the Fall when we needed to fatten for the rigors of winter and early spring. It was adaptive to fatten then. Now, when we have huge amounts of fructose all year round, it is causing serious diseases of civilization.
Dr Lustig also failed to mention other bad effects of fructose. It is roughly 10X more active than glucose at glycation formation, similar in glycation activity to galactose. Mother Nature seems to have selected the least damaging sugar for the basic fuel. Glycations, as they transform to AGEs (advanced glycation end products, produce ROS (reactive oxygen species) and many other nasties such as formaldehyde and beta-amyloid proteins (implicated in Alzheimer's). In fact, AGEs appear to be primary causes of autoimmune disorders including neuropathies, arthritis, macular degeneration and more. AGEs are directly involved in cataracts and other diseases. Fructose, because of its high glycation potential, can bond to DNA and can even split DNA thus it may be a teratogen and, through blockage of gene functionality, may be a cancer promoter.
Fructose, in the long run, is nasty stuff. It is a non-essential molecule, and it doesn't matter what source it comes from, it is detrimental. It could be from a nice crisp Fuji apple or classic Coke; once absorbed into the blood stream it doesn't have a label such as "organic apple, I'm okay" it will just do its damage.
Another thing that Dr Lustig contends is very suspect in my mind. He claims that fiber, such as in whole fruits and vegetables, by slowing down absorption of sugars, makes them okay. A few minutes delay would be expected to make little difference in the number of molecules the body must deal with. And, even when phosphorylated on the first pass through the liver, fructose has become an active triose. There's a slight beneficial effect but it's unlikely to be major protective effect.
Fructose should really be eliminated from the diet to the greatest extent possible. I have done so for 8 years now, and the health benefits are amazing. I was, at age 60, in metabolic syndrome. On the very low fructose diet I lost 60 lbs, blood pressure went from 145/95 to 115/70, fasting blood glucose went from 95-65 mg/dl, LDL cholesterol dropped 80 mg/dl while HDL went up and triglycerides went down, too. All from simply keeping fructose below 4 g/day. These benefits happened in less than a year and I have maintained them for 8 years, "effortlessly" with no hunger, loss of energy or headaches; just better health. The mentioned effects are very well spelled out in Dr Lustig's lecture on hepatic fructose metabolism. Had that lecture been around I would have expected those effects, but my research was still in its beginning stages then and much of the information he presents has been discovered in the mean time. I was surprised at how it worked on so many criteria at the same time.
I finished transitioning onto a very low carb, limited protein, high fat diet the first of March and the benefits of that: a wonderful feeling of well-being and an increased level of steady energy are great.
on August 02, 2010
at 04:33 AM
Fructose is also sweeter than glucose, so that may be the reason it evolved as the primary sugar in most fruits, because of it's increased ability to attract animals.
on August 02, 2010
at 05:11 AM
I think fructose gets such a bad reputation because of high-fructose corn syrup, a sweetener made from corn. But I have to wonder if HFCS is just a scapegoat and is not really worse than sucrose. Sucrose is 50% fructose and 50% glucose. HFCS is 55% fructose and 45% glucose. There is only 5% more fructose in HFCS and that is apparently enough to make it "high".
If it is worse than sucrose, it might not be the fructose, but the fact that it is made from a grain. Of course the idea that HFCS could be causing problems because of being a grain product is not really going to go down well. The corn lobby is powerful, last thing they want is people thinking corn is unhealthy and that it isn't a vegetable.
on February 26, 2013
at 07:03 PM
I believe that plants evolved fructose as an artificial sweetener. It tricks animals into thinking that a fruit contains more sugar than it actually does since fructose is sweeter than glucose. Animals have in turn evolved to be able to digest fructose but they are not very good at doing so.
on October 30, 2010
at 10:39 PM
First off, I believe in eating fruit. I am coming to question eating sugar and sweet liquids.
I've been very curious about fructose since viewing the video from Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology. Some of the takeaways from this video are that Fructose is bad. Fructose is digested mostly in the liver. Somewhere around 30% of the food energy from fructose goes right into fat stores from the liver. It also results in increased uric acid output, which makes you more likely to get gout and hypertension. Increased uric acid output increases blood pressure. Eating fructose also suppresses Ghrelin, which causes you to be hungry.
Here are some articles about fructose increasing blood pressure.??The mechanism is that fructose is processed in the liver. The liver emits more uric acid as a result. Increased uric acid??output raises blood pressure. They have also shown that lowering uric acid levels also lowers blood pressure
Fructose consumption increases appetite. As Dr. Robert H. Lustig says in the video above, Fructose suppresses Ghrelin. Ghrelin is the signal that is supposed to tell your brain that you are full. Suppressing that signal increases your appetite.
This article from The Journal of Clinical Endocrinology & Metabolism says some of the same things: "Dietary Fructose Reduces Circulating Insulin and Leptin, Attenuates Postprandial Suppression of Ghrelin, and Increases Triglycerides in Women"
This appears to be another way that Fructose increases your appetite: "Fructose Metabolism By The Brain Increases Food Intake And Obesity, Review Suggests" I'm puzzled by this article, because previously, we've read that normal cells in your body don't metabolize fructose directly, the liver has to do it.
I don't need my appetite increased, it is already too large already!
Even though normal cells in your body can not metabolize fructose directly, it appears that cancer cells can metabolize fructose to speed cancer growth!
"Pancreatic tumor cells use fructose to divide and proliferate, U.S. researchers said on Monday in a study that challenges the common wisdom that all sugars are the same." ... "They said their finding, published in the journal Cancer Research, may help explain other studies that have linked fructose intake with pancreatic cancer, one of the deadliest cancer types."
"While it's widely known that cancer cells use glucose to fuel their growth, last week's findings were the first to link fructose to cancer growth. The cancer cells grew at fructose concentrations easily attainable in our North American diet and they did so at a similar rate to glucose.
However, the data showed that cancer cells used fructose and glucose in very different ways, even though they are structurally similar. Compared to glucose, fructose was a potent activator of a key cellular pathway that drives cancer cell division. Not only did cancer cells prefer fructose, the sugar also triggered cellular activities that enabled malignant cells to use both glucose and fructose more rapidly."
The Wiki on Fructose and sucrose are interesting reading, too:
The Entries for "Liver Disease', 'Digestive Problems", "Metabolic syndromes", and "Gout" are food for thought. What it does to your liver sounds nasty.??
on August 02, 2010
at 12:29 PM
Dose response: to an extent it's excellent for restoring liver glycogen in excess: metabolic syndrome
on August 02, 2010
at 04:03 AM
It is true that before winter comes and our plant gathering food source is no longer available, the need to 'fatten up' is important. Same with the timing of nuts to increase our Omega 6 fat source ( spring/summer brings Omega 3 sources). Fruit may be a role in this but who is to say the fructose in fruit was meant for us.
on July 30, 2015
at 07:48 PM
Why does only the liver feed fructose straight into fat? I think it’s quite clear why this happens. We have a symbiotic relationship with plants. Plants want to spread their seeds around, so they surround them with fructose. High-fructose material surrounding the seeds gets us and other animals to eat them and this craving of fructose makes us eat them a lot and we end up carrying their seeds around and spreading them. But at the same time, it gives us an advantage because those fruits ripen just at the end of the growing season, which generally means, in almost all environments, that you’re not going to have much to eat over the next few months. So the best way to survive is to convert everything you eat at that time into fat. That is the long-term storage mechanism that allows you to survive until the next growing season.
on December 17, 2012
at 08:35 PM
It may be cheaper for plants to feed the animals with fructose, and in the same way it may be cheaper for the sugar industry to feed their customers.
on April 12, 2011
at 01:57 PM
Carbs are not bad. Refined processed carbs are the culprits. Our ancestors ate local fruits and berries in season and when available. A small, but real, part of their total dietary picture. What modern man has done with sugar and fructose is contrary to nature but appeals to our evolutionary taste for things sweet. I don't see how eating an organic apple or a handful of berries can be anything but positive.