Friday, June 11, 2010

Fructose in fruits may be good for you, especially if you are low in glycogen

Excessive dietary fructose has been shown to cause an unhealthy elevation in serum triglycerides. This and other related factors are hypothesized to have a causative effect on the onset of the metabolic syndrome. Since fructose is found in fruits (see table below, from Wikipedia; click to enlarge), there has been some concern that eating fruit may cause the metabolic syndrome.

Vegetables also have fructose. Sweet onions, for example, have more free fructose than peaches, on a gram-adjusted basis. Sweet potatoes have more sucrose than grapes (but much less overall sugar), and sucrose is a disaccharide derived from glucose and fructose. Sucrose is broken down to fructose and glucose in the human digestive tract.

Dr. Robert Lustig has given a presentation indicting fructose as the main cause of the metabolic syndrome, obesity, and related diseases. Yet, even he pointed out that the fructose in fruits is pretty harmless. This is backed up by empirical research.

The problem is over-consumption of fructose in sodas, juices, table sugar, and other industrial foods with added sugar. Table sugar is a concentrated form of sucrose. In these foods the fructose content is unnaturally high; and it comes in an easily digestible form, without any fiber or health-promoting micronutrients (vitamins and minerals).

Dr. Lustig’s presentation is available from this post by Alan Aragon. At the time of this writing, there were over 450 comments in response to Aragon’s post. If you read the comments you will notice that they are somewhat argumentative, as if Lustig and Aragon were in deep disagreement with one other. The reality is that they agree on a number of issues, including that the fructose found in fruits is generally healthy.

Fruits are among the very few natural plant foods that have been evolved to be eaten by animals, to facilitate the dispersion of the plants’ seeds. Generally and metaphorically speaking, plants do not “want” animals to eat their leaves, seeds, or roots. But they “want” animals to eat their fruits. They do not “want” one single animal to eat all of their fruits, which would compromise seed dispersion and is probably why fruits are not as addictive as doughnuts.

From an evolutionary standpoint, the idea that fruits can be unhealthy is somewhat counterintuitive. Given that fruits are made to be eaten, and that dead animals do not eat, it is reasonable to expect that fruits must be good for something in animals, at least in one important health-related process. If yes, what is it?

Well, it turns out that fructose, combined with glucose, is a better fuel for glycogen replenishment than glucose alone; in the liver and possibly in muscle, at least according to a study by Parniak and Kalant (1988). A downside of this study is that it was conduced with isolated rat liver tissue; this is a downside in terms of the findings’ generalization to humans, but helped the researchers unveil some interesting effects. The full reference and a link to the full-text version are at the end of this post.

The Parniak and Kalant (1988) study also suggests that glycogen synthesis based on fructose takes precedence over triglyceride formation. Glycogen synthesis occurs when glycogen reserves are depleted. The liver of an adult human stores about 100 g of glycogen, and muscles store about 500 g. An intense 30-minute weight training session may use up about 63 g of glycogen, not much but enough to cause some of the responses associated with glycogen depletion, such as an acute increase in adrenaline and growth hormone secretion.

Liver glycogen is replenished in a few hours. Muscle glycogen takes days. Glycogen synthesis is discussed at some length in this excellent book by Jack H. Wilmore, David L. Costill, and W. Larry Kenney. That discussion generally assumes no blood sugar metabolism impairment (e.g., diabetes), as does this post.

If one’s liver glycogen tank is close to empty, eating a couple of apples will have little to no effect on body fat formation. This will be so even though two apples have close to 30 g of carbohydrates, more than 20 g of which being from sugars. The liver will grab everything for itself, to replenish its 100 g glycogen tank.

In the Parniak and Kalant (1988) study, when glucose and fructose were administered simultaneously, glycogen synthesis based on glucose was increased by more than 200 percent. Glycogen synthesis based on fructose was increased by about 50 percent. In fruits, fructose and glucose come together. Again, this was an in vitro study, with liver cells obtained after glycogen depletion (the rats were fasting).

What leads to glycogen depletion in humans? Exercise does, both aerobic and anaerobic. So does intermittent fasting.

What happens when we consume excessive fructose from sodas, juices, and table sugar? The extra fructose, not used for glycogen replenishment, is converted into fat by the liver. That fat is packaged in the form of triglycerides, which are then quickly secreted by the liver as small VLDL particles. The VLDL particles deliver their content to muscle and body fat tissue, contributing to body fat accumulation. After delivering their cargo, small VLDL particles eventually become small-dense LDL particles; the ones that can potentially cause atherosclerosis.


Parniak, M.A. and Kalant, N. (1988). Enhancement of glycogen concentrations in primary cultures of rat hepatocytes exposed to glucose and fructose. Biochemical Journal, 251(3), 795–802.


Kindke said...

"Fruits are among the very few natural plant foods that have been evolved to be eaten by animals" - This is my feeling exactly, and why I consider fruits to be Ok and vegtables to be unnatural.

Tubers I think probably havent had the selective pressure to evolve a strong host toxins in them, although I know there are some.

Im confused about liver glycogen VS muscle glycogen though, obviously deep ketosis depletes liver glycogen but does it also deplete muscle glycogen substantially? The thing is that 63g from a 500g muscle glycogen store doesnt sound like hardly anything, especially given that in a 30 minute resistence training you can easily extensively damage your muscles enough such that they become 'unusable' for 24-48 hours while they recover, yet we are still retaining ~85% of our muscle glycogen stores? It just doesnt sound intuitive.

Cyclical ketogenic diet is very popular amoung bodybuilders, perhaps the stimulus of going from full muscle glycogen stores to depleted in a few days due to ketosis is where the real silver bullet for fat loss is.

With myself I notice that after carb loading, in the first week of ketosis the fat loss is very accelerated.

However at the 2 week mark in ketosis fat loss really seems slow, its no wonder so many people report hitting a weight loss plateu on low-carb diets. Perhaps the real stimulus for fat loss is rapid muscle glycogen depletion?

Kindke said...

Also I wanted to add that I think your right Ned in that the fructose in fruit is generally 'neutral' to health.

Its very hard to "over-eat" 100% whole foods provided by nature. Natural food is alot of satiating than the processed stuff food companies want you to consume in endless amounts, thus somewhat limiting your fructose intake.

I couldnt see myself eating more than 3 bananna's for example, meanwhile I could easily eat a whole 6 pack of sugar doughnuts and still have room for icecream!

qualia said...

not so sure eating apples or pears (both high in free fructose) is sooo harmless with regard to fat accumulation. there's an interview with gary taubes (i believe it's on where he blames eating a few pears a day for getting fat in his past. similarly, even if i only eat one large apple, i literally can feel how my fat cells around the waist begin to tingle lol. seriously - if you know your body very well, you feel the metabolic consequences of any food after a few minutes into digestion, and apples definitely seem to "feed" my fat cells somehow. the same effect does not appear for eating kiwis for example. i say, eat 3 large apples or pears a day, and you def. will get fat rather sooner than later..

Michael Barker said...

First, let me say that I'm a diabetic and a thin one, at that. This means I don't make a lot of insulin. Fruits, in generally, cause hyperglycemia, berries, however, don't. Most modern fruits are hybrids that have been bred to yield large volumes.
They also tend to be raised in sequestered environments to protect them from the predations of animals.

Fruit would only been available in the spring or early summer and humans would have had to compete with other animals for it. It seems to me that our bodies would readily setup to take in these nutrients because of their rarity. There is a very limited capacity to store these nutrients. This suggests that this is a limited opportunistic function.
What happens when such a function gets a year round supply? A shift in metabolism.

Fruit, obviously, isn't a problem. It's endless availability probably is. Remember, we need water but with too much, we drown.

LeonRover said...

Yeah, it is a matter of amounts, is it not?

A loss of 50 gm of liver glycogen replenished by 50 gm of fructose is one situation, perhaps the equivalent of 1 lb of raw apple, or just under 200 ml of undiluted OJ.

Some studies that I have read suggest that damage MAY not begin until amounts are in excess of 150 gm fructose - 300 gm of sucrose or HFCS. In these circumstances, 100 gm of fructose is metabolised to triglycerides, with losses in production of ATP.

Drs. Cynthia and David said...

I agree with you that people greatly overestimate glycogen depletion from lifting weights or other exercising. For running at least, I know that it doesn't happen until close to 18-20 miles of race effort, but you'll certainly experience some fatigue during that time. There are plenty of mechanisms of fatigue besides glycogen depletion. I didn't realize about the fructose being used preferentially for replacing liver glycogen though- I'll have to read up on that sometime. I wonder if that is why fructose (Coke) tastes so good during long races.

What about for fruitarians, like those misguided vegans at ? I've never seen any vegan types discussing test results like lipoproteins and triglycerides and blood glucose. I think it's hard to get enough calories being raw vegan so some of the usual problems on a high fructose diet might be avoided (I hate to think of how many bathroom trips they must make though). They are pretty misguided though about the effects on teeth and bones.


Anonymous said...

Since I am concerned about total fructose consumption, I think your table needs another column. Free fructose + 1/2 sucrose = total fructose:

Apple -- 7.0
Apricot -- 1.3
Banana -- 5.4
Grapes -- 8.7
Peach -- 3.9
Pineapple -- 2.7
Pear -- 7.3
Beet -- 0.6
Carrot -- 1.1
Corn -- 1.0
Red pepper -- 2.9
Onion -- 2.5
Sweet potato -- 2.0

Byron said...

Great article like always, thanks Ned.
Fruits seems to be the pure innocent food. Sunshine-candy for animals. I would say if you really tolerate/like it, why not? I gave them up 2 years ago. Because I tucked them away kilowise. No matter which kind; bananas, apples, berries, melons aso. Teeth became very sensible and toilet paper consumption increased. I think if you´re very sugar sensible it would be very wise to stay far away from all kind of sugar. Highest I tried now is 85%-99% chocolate with just zero blood glucose effect. I was really surprised. But on the other side real cacao has besides of many others properties a bg lowering effect. Greetings.

Ned Kock said...

Hi Kindke.

Muscle glycogen depletion is very localized. You work a particular muscle, and glycogen gets depleted in that particular muscle (or muscle group) only.

The 500 g is not a pool that all muscles can draw from. The closer you can get to that is to work multiple muscle groups in one workout. The lactate generated by a workout in one muscle group goes on to feed gluconeogenesis in the liver, with the resulting glucose going back to feed muscle glycogenesis. That is the Cori cycle.

I do think that glycogen depletion is key to fat loss. The problem is that when taken too far it also leads to muscle loss. The cycling approach you mentioned may be the answer.

Ned Kock said...

Hi N95.

Without exercise, or something like intermittent fasting, too much fruit is not at good idea. If I recall it properly, Taubes is not a big fan of exercise.

Under glycogen depletion, one or two fruits per meal, even apples, may actually help you lose fat in the long term. The reason is related to Kindke's comment on cycling.

Having said that, there is indeed a lot of variation in how people respond to different types of food. The key is to pay attention to your body's responses, and tailor your diet to them. As you are doing.

Ned Kock said...

Hi Michael.

Yes, as we modify fruits we run the risk of changing them so much that at least some of them will become unhealthy. My impression is that fruits are often bred to become bigger, and weigh more, which usually means more water. The end result is often not so bad: a lower glycemic load.

I wonder if our ancestors in the African savannas experienced seasonal fruit scarcity. The areas where most of our evolution occurred seem to have been the kind of place that is very rich in both plant and animal foods.

Couple that with human intelligence, and you have to assume that they could eat a lot of fruit. What might have kept them thin was a combination of: satiety of natural foods, constant exercise (even if not too much), and a different mindset when it came to food:

Ned Kock said...

Hi Leon.

Indeed, definitely a matter of quantity, and perhaps also how it is taken. We have not evolved to drink most of our calories as adult humans.

Ned Kock said...

Hi Cynthia and David.

My own opinion is that consuming the vast majority of one's daily calories from fruits is not natural for most of us.

There is a huge amount of evidence suggesting that animal foods played a key role in the emergence of our species.

Plus, 60 percent of our brain is fat!

There are isolated populations that are healthy and that go against what I have just said above - e.g., the Kitavans. They have low HDL, relatively high trigs, and are lean and healthy.

I often wonder whether these isolated populations aren't like that because of plain evolution of food/lifestyle-related traits. Evolution is a population phenomenon, and can happen fast, even in a few hundred years (not the millions of years that many people assume):

Ned Kock said...

Thanks Byron. Yes, people should definitely adapt their eating habits to their own metabolism. We are all different, with some general similarities.

3D Face Analysis said...


I agree with your general point that we are evolved to eat fruit. If fruit is that harmful, then we would not evolve a taste for sweet things. So our taste for sweets implies that fruit has some benefit.

Sometimes people find sweets more enjoyable than meat or plants. That taste means that fruit may have a higher priority than meat or plants at certain times.


You have said:

similarly, even if i only eat one large apple, i literally can feel how my fat cells around the waist begin to tingle lol. seriously - if you know your body very well, you feel the metabolic consequences of any food after a few minutes into digestion, and apples definitely seem to "feed" my fat cells somehow. the same effect does not appear for eating kiwis for example. i say, eat 3 large apples or pears a day, and you def. will get fat rather sooner than later..

Have you checked your insulin sensitivity? Unbound glucose in fruits have a faster absorption rate than starch.

For example, a lot of people want go to the bathroom right after a few slices of watermelon. Watermelon has a lot of fast-absorbing unbound glucose, so I think it raises their blood glucose levels above their glucose renal threshold. So they excrete the extra glucose out via urine.

Another thing: Are you sure if it's fat on your waist, or is it bloating? Have you checked if you're intolerant to fructose? Do you have Candida, since fructose feeds candida which can cause a bloating effect?


You mentioned that feeding excess fructose causes metabolic problems. Are you sure that it's fructose in itself? Have you considered if it's actually Candida exacerbated by fructose? Have you considered fructose malabsorption? I don't think rats are evolved to tolerate that much fructose. In addition, fructose is found to deplete copper, zinc, and chromium. So nutrient deficiencies may be the root cause of its metabolic effects, rather than fructose.

I won't blame fructose itself for causing these effects unless the rats are assured that don't have Candida, fructose intolerance problems, or any mineral deficiency. Fructose intolerance can cause diarrhea, which can exacerbate mineral deficiency. Unless I found a study which controls these variables, I will not blame fructose. Unless I found the study, I will continue to follow our ancestral consumption of fruit and our taste for sweets.

LeonRover said...

Organism as a Whole

Just checked my earlier comment, I wrote

"Some studies that I have read suggest that damage MAY not begin until amounts are in excess of 150 gm fructose "

I was pointing out an example of the 16th century toxicologist, Paracelsus' and the 19th century French doctor Claude Bernard's Principle:

Tout est poison, rien n'est poison,tout est question de dose - everything is poisonous, nothing is poisonous, it only depends on the dose. I looked for some studies which pointed towards a POSSIBLE dangerous level.

While correlation is not causation, it does deserve to be taken into account when a biochemical mechanism, to wit, "de novo lipogenesis" - DNL - has been established as a default mechanism to process fructose when liver glycogen stores are full.

Lindeberg's analyses of diets also support the support the notion of up to 60 gm of fructose in the form of 4 to 5 Kilo of pineapple is not unsafe.

The studies suggesting a correlation of fructose with poor metabolic outcomes claim that these levels come from sucrose and/or HFCS laced soft drinks, not 12 to 15 Kilos of pineapple!

ben nguyen said...

If after fasting or working out, both the liver and muscle are glycogen depleted, would this then be the best time to consume a high fructose drink, since the liver will replenish first, and since it was empty, its less likely to convert it to TG/VLDL/LDL (dense)?

Assuming sucrose/glucose is not synthesized as efficiently by the liver, then looking at the chart, would a high fructose:glucose ratio be best?

By the way, what does the body do when sucrose is consumed in large amounts (fasting or not)?

Ned Kock said...

Hi ben.

I wouldn't recommend having a high fructose drink under any circumstance. It is not natural to drink your calories; it is natural to chew them.

Extra fructose, from sucrose or HFCS will be turned into fat by the liver, and secreted in VLDL particles that will end up as potentially atherogenic small-dense LDL particles.

ben nguyen said...

Thanks, I tend juice quite a few of my veggies/fruits, but it sounds like the liver will treat all excess sugar the same...

so not exactly sure how to interpret the chart.. is a sweet potato (sucrose) better or worse than grapes (fructose)?


Ned Kock said...

Hi ben.

Juicing the fruits and veggies yourself is a lot healthier than buying industrial juices. Still, it is more natural to chew calories than to drink them. Often what is more natural is also better in terms of health (not always though - taking antibiotics is not natural either, but can save lives).

Natural foods, be they sweet potatoes or grapes, should not be a problem for someone who has no glucose metabolism problems. For someone who does have problems, then the glucometer test mentioned in the post below (at the end) may be a good idea to see what carbohydrate-rich foods should be avoided:

Anonymous said...

The post references several interesting studies that suggest the liver won't secrete excess fructose as fat, but rather it will store it itself! (Which is the dangerous visceral fat)

This may be one of the hardest fat's to lose!

Nathaniel said...

Ned, do you feel comfortable estimating a number of grams of fructose per day that you feel is a safe upper limit for the typical person?

And what do you think of chocolate? How would you compare the impact of fructose in chocolate form compared to fruits?

Everyone has different vices; mine is chocolate. I struggle with limiting my consumption but I am truly concerned about fructose.

Ned Kock said...

Hi Nathaniel.

I don't see myself, or most people, eating more than about 2 fruits (or equivalent) per meal. If we take one apple, that's about 7 g of fructose. So 2 x 7 x 3 = 42 g of fructose. That's per day, assuming 3 meals.

I guess two cans of regular soda will get you more fructose than that.

My guess is that there are about 5 g of fructose per serving of 70 percent cocoa chocolate. So a serving doesn't add much. The rest of the 15 g or so of sugar is glucose and lactose.

But the serving is 2 squares, not the whole thing.

By the way, the combination of fat and sugar is one of the favorites of the food industry to create addictive foods. Foods that we'll crave, and eat more than we need (low satiety). So I have developed some negative feelings about chocolate, even though I like it too.

Anonymous said...

Sorry Ned, have to disagree with you on one small point. Taubes is not a fan of exercise *for net calorie reduction for weight loss" only- He agrees that it has other health benefits. He has said this many times. He ia just disagreeing with the calories in calories out theory specifically.


joeo said...

I eat about a pound of frozen blueberries a night. It sure seems excessive but I just calculated and that is about 46 grams of sugar or a little over a 12oz coke.

I bet that the sugar in fruit is just as bad for you but it is a lot easier to consume the sugar in a soda than in fruit. I used to drink 5-6 12 oz cokes a day. Fruit consumers are also going to have a raft of other good qualities like - conscientiousness, education etc. so it will be hard to see any bad quality to eating fruit in any studies.

Social Media Services said...

Fruits are the greatest gifts of nature. They keep you healthy and very very young.

All that you have to do is accept your diet mentally.

Acid Reflux Treatment said...

Fruits are the best way to treat stomach acid. Anyone suffering from ulcer, stomach acid and constipation should consume fruits daily.

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