Fructose Question

[Brodie Butland]

Jordan,

I have a question along the lines of your meat+veggies. How well can fruits be used instead of veggies? For example, each morning I have a smoothie made with coconut milk and a mixture of blackberries, blueberries, strawberries, and/or grapes with a banana. If I were trying to do a carb reduction like you describe above to lose body fat, would eating a lot of fruits as opposed to vegetables be counterproductive because of the higher sugar?

Sorry if this is a vague/idiotic question...just something I've always been a little curious about, and the low-carbers tend to have conflicting views.

By the way, thank you very much for devoting a lot of your limited free time to this subforum. I was crunched for spare time when I was going through law school, and that wasn't even a legitimate doctorate program--can't imagine what it would be like for a program where you, you know, actually have to do a residency and prove you're worthy of the degree. Props to you!

BB

[Jordan Feigenbaum]

Thanks for the kind words, Brodie.

I don't think there's anything wrong with a little bit of fruit, but I don't like it as the main carbohydrate source very much. In your situation, you might be getting a bunch in the smoothie depending on the volume and type of fruits you end up using. If leaning out is a bigger concern, I'd probably avoid a smoothie with a bunch of fruit and here's why:

The metabolism of fructose tends to be orexigenic (appetite stimulating) via how it does not cause an elevation in insulin or leptin. Also, the way it is metabolized in the hypothalamus tends to end up causing the appetite to be stimulated rather than inhibited after a meal. Additionally, fructose metabolism primarily takes place in the liver with only small amounts of it being broken down by skeletal muscle (muscles like glucose). If the liver's glycogen stores are nearly topped off, the fructose is ultimately gives rise to a triaglycerol (fat) or fatty acid (palmitic acid IIRC which last I read, is associated with blocking leptin from its receptor in the hypothalamus). Fat can accumulate in the liver and progress to non alcoholic fatty liver disease, although this probably isn't linked to high consumption of fruit, but HFCS in sodas and processed foods. Still, there has not been any published data about what the safe upper limit for fructose is. Further, it's not essential and has potential to adversely affect metabolism, satiety, and lipid parameters.

I'm not using any of this to scare you because I honestly think a little bit of fruit is okay. I also think that fruit is a pretty weak source for dietary carbohydrates overall, and that they should be limited, as they do have a decent micronutrient/vitamin/phytonutrient profile. If I were doing low carb, I wouldn't eat any fruit FYI.

[Joel Chapman]

Thanks for the kind words, Brodie.

I don't think there's anything wrong with a little bit of fruit, but I don't like it as the main carbohydrate source very much. In your situation, you might be getting a bunch in the smoothie depending on the volume and type of fruits you end up using. If leaning out is a bigger concern, I'd probably avoid a smoothie with a bunch of fruit and here's why:

The metabolism of fruit tends to be orexigenic (appetite stimulating) via how it does not cause an elevation in insulin or leptin. Also, the way it is metabolized in the hypothalamus tends to end up causing the appetite to be stimulated rather than inhibited after a meal. Additionally, fructose metabolism primarily takes place in the liver with only small amounts of it being broken down by skeletal muscle (muscles like glucose). If the liver's glycogen stores are nearly topped off, the fructose is ultimately gives rise to a triaglycerol (fat) or fatty acid (palmitic acid IIRC which last I read, is associated with blocking leptin from its receptor in the hypothalamus). Fat can accumulate in the liver and progress to non alcoholic fatty liver disease, although this probably isn't linked to high consumption of fruit, but HFCS in sodas and processed foods. Still, there has not been any published data about what the safe upper limit for fructose is. Further, it's not essential and has potential to adversely affect metabolism, satiety, and lipid parameters.

I'm not using any of this to scare you because I honestly think a little bit of fruit is okay. I also think that fruit is a pretty weak source for dietary carbohydrates overall, and that they should be limited, as they do have a decent micronutrient/vitamin/phytonutrient profile. If I were doing low carb, I wouldn't eat any fruit FYI.

Can you describe how fruit metabolism (in a non-diabetic) does not cause an elevation in insulin? Are you saying there is no release of insulin? Or are you saying something else, like insulin is released and rapidly cleared from the blood leaving blood glucose levels relatively stable...?

[Paul1]

Both fruits and vegetables are full of complex carbohydrates, and the glycemic index of fruit is very low. Also, the fructose in fruit doesn't present nearly the same biological problem as high fructose corn syrup, or plain old table sugar (sucrose) which is simply metabolized to glucose and fructose before absorption. The reason is that the concentration / abundance of fructose within fruit is WAY less than in foods with concentrated sweets.

I don't agree with the above post -- there is plenty of clinical literature showing that MORE fruit consumption produces better outcomes for a variety of vascular, metabolic, and body composition measures. Also, nothing is "metabolized in the hypothalamus" -- that doesn't even make any sense.

[Jordan Feigenbaum]

I inadvertently said fruit while referring to fructose, they aren't the same thing and I apologize. Yes fructose is A sugar in fruit, but there are also other carbohydrates in the fruit as well. Fruit may or may not cause insulin to be released depending on all sorts of things like fructose content, glycemic load, meal components, etc. but fructose in and of itself is not insulinogenic.

Paul1, the concentration of fructose in a serving size of fruit is less than can of coke (~35g in a 20oz soda). To get this amount of fructose in fruit that's about 5 bananas, 3 apples, or 9 cups of strawberries, which is...ummm ALOT. However, if someone were to be making a smoothie with lots of fruit and concentrating this source to get more fruit in the blender, then it's not unreasonable that this amount of fructose could be had in a "fruit" based shake, which is potentially what the OP was doing and why I made the comment.

Yes there is clinical literature on both sides of the fence. I don't think there's anything magical about fruit, but I wouldn't abolish it from anyone's diet unless we were trying to do a low carb diet. If you want to eat fruit knock yourself out, but it's not causing any better outcomes for vascular, metabolic/body comp, or other metrics in and of itself, that is, it may be correlated but it's certainly not causative. Also, fructose, glucose, lactate, ketone bodies, etc. are metabolized in neural tissue like the hypothalamus. How else would the cells get ATP to sustain themselves? This is straight out of medical biochemistry textbooks. A quick google search will elucidate the pathway, although as you can imagine, there's still a lot of bickering going on about what exactly fructose and fruit does in the body and brain on the whole.

Here's a quote from the PNAS

"The American diet, especially that of adolescents, contains highly palatable foods of high-energy content and large amounts of high-fructose sweeteners. These factors are believed to contribute to the obesity epidemic and insulin resistance. Previous investigations revealed that the central metabolism of glucose suppresses food intake mediated by the hypothalamic AMP-kinase/malonyl–CoA signaling system. Unlike glucose, centrally administered fructose increases food intake. Evidence presented herein indicates that the more rapid initial steps of central fructose metabolism deplete hypothalamic ATP level, whereas the slower regulated steps of glucose metabolism elevate hypothalamic ATP level. Consistent with effects on the [ATP]/[AMP] ratio, fructose increases phosphorylation/activation of hypothalamic AMP kinase causing phosphorylation/inactivation of acetyl–CoA carboxylase, whereas glucose has the inverse effects. The changes provoked by central fructose administration reduce hypothalamic malonyl–CoA level and thereby increase food intake. These findings explain the paradoxical fructose effect on food intake and lend credence to the malonyl–CoA hypothesis."

And from the Diabetes Journal showing the evidence of a Glut5 (fructose receptor) on the brain:

"A recent study by C.F. Burant et al. (13) demonstrates that GLUT5 is a high-affinity fructose transporter with a much lower capacity to transport glucose. To characterize the potential role of GLUT5 in fructose and glucose transport in insulin-sensitive tissues, we investigated the distribution and insulin-stimulated translocation of the GLUTS protein in human tissues by immunoblotting with an antibody to the COOH-terminus of the human GLUTS sequence. GLUTS was detected in postnuclear membranes from the small intestine, kidney, heart, four different skeletal muscle groups, and the brain, and in plasma membranes from adipocytes."

[Joel Chapman]

Under what conditions would eating fruit not cause a release of insulin, and how common would that be?

[danradin]

Sounds like you suggest avoiding/limiting fructose during carb refeeds?

[Brodie Butland]

Paul1, the concentration of fructose in a serving size of fruit is less than can of coke (~35g in a 20oz soda). To get this amount of fructose in fruit that's about 5 bananas, 3 apples, or 9 cups of strawberries, which is...ummm ALOT. However, if someone were to be making a smoothie with lots of fruit and concentrating this source to get more fruit in the blender, then it's not unreasonable that this amount of fructose could be had in a "fruit" based shake, which is potentially what the OP was doing and why I made the comment.

Just to give a little context: the total mixture would be something like one banana, five strawberries, five blackberries, and ten grapes. Or maybe a small handful of blueberries will replace the strawberries, etc. We're only talking about 12-16 fluid ounces when the thing is blended, depending on how well I eyeball the mixture. Would this be more consistent with a "limited" fruit intake as you suggest in post 2?

[Jordan Feigenbaum]

Under what conditions would eating fruit not cause a release of insulin, and how common would that be?

Perhaps I should be more clear, any sort of meal you eat will result in a slight uptick in insulin at the very least due to the cephalic insulin response (this may have recently been renamed, but I'm not aware of it). Cherries, grapefruit, plums, peaches, etc. all have low glycemic loads, with cherries and grapefruit also having low glycemic indexes. So the foods providing small amounts of glucose to metabolize per serving (low glycemic load) and also not breaking down into sugar very quickly (glycemic index) would be less likely to raise insulin. Additionally, small amounts of low GI/low GL fruits probably don't represent a big insulin bolus. I generally don't think about fruit in terms of insulin release, but rather overall carbohydrate intake and fructose consumption.

Sounds like you suggest avoiding/limiting fructose during carb refeeds?

Totally. Probably not a total deal-breaker, but optimally I wouldn't do a significant amount of fructose in a carb-refeed.

Just to give a little context: the total mixture would be something like one banana, five strawberries, five blackberries, and ten grapes. Or maybe a small handful of blueberries will replace the strawberries, etc. We're only talking about 12-16 fluid ounces when the thing is blended, depending on how well I eyeball the mixture. Would this be more consistent with a "limited" fruit intake as you suggest in post 2?

Yea this is fine, unless you're doing a low-carb diet.

[thefinalsql]

If you are trying to lose weight via carb reduction stay away from fruit, at least in the beginning. Definitely for the first two weeks.

I have done a lot of low carb dieting with varying degrees of success. Earlier this year I lost 30+ lbs on a so called body builder diet, small meals, with carbs, every three waking hours.

It is my conclusion that a low carb diet works by naturally limiting the number of calories you eat. While I low carb dieted I did not count calories, but ate whatever I wanted that contained either no/minuscule carbs, or limiting items with 2g/carb or less per serving and eating nothing with more than 2g/carb per serving. On the body builder diet you have to count carbs because you are eating 6 times a day. If you eat a normal size meal at each interval you will never make it through six meals or if you do, you will be gaining weight, not losing.

Diets are different ways of achieving the same goals. If you are eating fruit or other appetite inducing(insulin spiking) foods, it is better to count calories rather than rely on the satiety of the low carb diet naturally keeping you from eating too much.