

The skinny on Carbohydrate part 2: fructose

Hold your breath.  High Fructose corn syrup (HFCS) will not immediately kill you and it is not the direct cause of obesity in America.  If you read my first post the skinny on Carbohydrate Part 1 you will have learned that Carbohydrate comes in many forms.  In my initial post I covered Glucose.  This post will cover Fructose.   Glucose and fructose are very similar on the molecular level, but they are metabolized in very different ways in the body.  In this post I will describe how fructose is a uniquely metabolized and why it is very important to understand.  For those of you familiar with Fructose metabolism, you may have some surprises in store for you.  

Any person looking to get specific about nutrition needs to understand storage mechanism and storage capacity in the body for various nutrients.  The amount of storage capacity in most people for Carbohydrate is 100 grams in the liver and about 400 grams in the muscles.  If you think in terms of Calories that is 400 and 1600 respectively.   Fructose is unique in that it is processed immediately in the liver rather than going into the blood stream.  This is good in one way because it doesn’t elevate your blood sugar to illicit an insulin response, but the metabolic benefits seem to end there.

A Brief History of Fruit and metabolism.  How does this impact you today?

One issue with Fructose and all sugar for that matter, is that from an evolutionary standpoint it was not as available as it is today.  Before the agricultural revolution, fruit was the only place we could find fructose.  Fruit would have only been available during long light cycles in colder climates or seasonally in warm parts of the world.  Because of modern agricultural practices we are able to have fruit all year all over the world.  It does not take a master of logic to infer that humans would not have a massive capacity to deal with fruit and fructose in it.  The other point is that fruit has a low energy density.  Per gram by weight of fruit there is way less energy than per gram by weight of Fat.  That means you have to eat a lot of fruit to get the same amount of energy found in animal products.  Being that fruit has never been this abundant and may have been dormant in parts of the world for extended periods of time, humans did not evolve to eat a ton of fruit/fructose.  That means that the metabolic pathways to oxidize Fructose are pretty limited and specific and when active tend to hinder fat metabolism.  In the human body there are two primary energy pathways.  The Fat pathway (fatty acid oxidative pathway) and the sugar pathway (glycolytic pathway).  The easy way to think of why each of these pathways would be used, is to think of the sugar pathway as the pathway we access energy in times of short term emergency and the the fat pathway as the energy system we use for back up or long term energy.   This applies to both tapping into storage and utilizing dietary exogenous (from outside of you) fat or sugar intake.   Allow me to break it down a little further.  If I woke up 100,000 years ago, my primary objective every day would be to track down some food.   I would likely have to do so on an empty stomach because I wouldn’t have a freezer stocked up with Toasters strudels and Pizza pockets.  I would roll over in my cave thinking to myself everyday “where the heck am I going to find my food today?”  The answer is out there somewhere.  That food would likely be running around and likely be sought after by others or be in some random bush far away that I would have to walk all over to find.  To make things a little more complicated,  I would be a food source for others.  What all of this means is there would be times in which I needed to travel long distances while trying to attain food on an empty stomach. If along the way  I needed to avoid myself becoming food, I would need to be able to run away quickly.  The first energy needs comes from fat, the latter sugar.  Now ask yourself which situation was likely more common and which would I need a greater access of storage of? Humans are smart therefor we were likely hunting during the day and hiding at night from the other predators that were likely nocturnal.  It was probably relatively rare that we had to avoid being eaten.   Therefor our long term survival storage would always take priority to short term emergency avoidance.  Thus we are built to run off of fat and not sugar most of the time.  In the simplest since, that is what has shaped our metabolism over the years.   Here is the mismatch with the present energy availability and how we actually work.  Most of us eat sugar all day everyday.

Thats great, but I’m not a cave man.  Bring it out of the stone age

Sugar has never been more abundant.  The most abundant delivery mechanism of sugar today is High Fructose corn syrup.  The ratio of this syrup is 55% fructose and 45 % glucose.  Most people have 100 grams of storage capacity for Fructose, which is full most of the time in most people.  Even if it’s empty, consider if you consume 16 oz of any drink consuming high fructose corn syrup, you have taken in about 50 grams of sugar with at least 25 coming from Fructose.  For a healthy subject that could be handled well, but when you add other sugar items to the mix like a hamburger with high fructose corn syrup and natural glucose in the bun and french fries to further increase the glycemic load of the meal, you end of with a bit of a metabolic problem.  Or if you are far along in the disease spectrum this could also present a problem for you.

The nitty gritty

I did some digging on the topic of Fructose and actually came up with some surprising results.  According to a meta Analysis done by nutrition and Metabolism fructose has less of a direct impact on Triglyceride levels than one might expect.  They searched Pubmed and Scopus using 2 or more of the combination of keywords, fructose, glucose, sucrose, tracer, 13C,14C, and isotope with limitation of using human studies.  34 studies that focused on Fructose metabolism.  Two important parameters were met in order to be included in the meta-analysis.  The first parameter they narrowed the studies by was whether mixed dose Fructose testing was used.  This is important because fructose is rarely found in isolation in our diets. Typically it is either packaged with Glucose as Sucrose or it paired with glucose in high Fructose corn syrup.  If a study’s goal is to quantify the deleterious effects of anything it is pointless to conduct the study in such a way that would not typically occur in reality.  The second factor they narrowed the studies by was tracking.  It’s pretty tricky to measure where different macro nutrients end up in the body.  Because Fructose and Glucose are kind of interchangeable metabolically and they are typically paired when consumed in the real world, it is important to use specific markers on the Fructose molecules being studies to verify their eventual fate.  In the included studies they tracked where it ended up in the body using what is called a isotope tracer labeled precursors method.  In tracer methods they actually mark certain molecules so they can track the metabolic fate of certain ingested substrates.   If you like to hurt your brain check out this image comparing Fructose and Glucose metabolism

What they found was that Fructose did not lead to a significant increase in Triglyceride production, however Fructose did have a direct impact of Fatty acid oxidation decreasing endogenous (already in your body tissues) fatty acid oxidation by as high as 48% and no less that 17%.    What I found very interesting is the break down of metabolites from Fructose.   Check out this graph that compiled all of the studies results for where fructose ends up after the metabolic processes occur

Here is the breakdown: 28-54 % converted to Glucose, about 28 percent converted to Lactate an unknown value became glycogen and glycerol, and less than 1 percent ended up as lipids.

Okay nerd…Why do I care?

Basically what that means is that the fructose is not necessarily leading to increased lipid synthesis.  When combined with Glucose the mean oxidation rate from these studies is high as 66%+ or – 8.2 in exercising subjects in 2-3 hour and 45% + or – 10.7 in non-exercising subjects within 3-6 hours  However, once consumed,  fructose is prioritized metabolically to fatty acids.  Also, essentially the rest of the fructose is converted to glucose within 3-6 hours of ingestion.   Strikingly, even though there is a minor insulin response to the fructose, there is a decrease in Fatty acid oxidation.  Because of fructose you may not be gaining fat, but you certainly will not be burning less.  That renders that healthy fruit smoothie or Juice you have been instructed by some hot girl on YouTube to drink a  fat hindering mechanism.   Hmm, not a good call first thing in the morning.  Fatty acid oxidation is also decreased during exercise as a result of fructose consumption.  Here is a good idea: I’m going to go to the gym because I want to burn fat, but before I go I’m going to make sure I drink something that will prevent that from happening optimally.  Wrong!  To make matters worse, fructose also decreases muscle glycogen mobilization which will give you less free space to store future ingested sugars.   That is a bad combination is your objective is to decrease body fat.

Many nutritionist and thought leaders in the health industry are blaming Fructose for many of the problems with fat in society, but they simply have their science wrong.  It’s more likely the glucose in high fructose corn syrup and the eventual conversion of Fructose to Glucose that is the true culprit.  That coupled with full glycogen stores and a steady stream of insulin in the blood is what creates an environment for fat gain.   Again all tying back to the insulin response to sugar laden foods.  Granted the largest dose of Fructose in these studies was 90g and I am sure there are obese people out there eating much more than that in a sitting, but the fact remains that a very small proportion of fructose actually becomes lipids.  The more I research this topic, the more I start to realize that the problem people have with with weight loss or improving body composition is about the hindering of fat oxidation than the creation of fat.  The body does a good job maintaining a certain degree of homeostasis with metabolism.  This is why the population only sees and average weight gain of about 10 lb. per decade.  You would think with the way we over consume, everyone would be gaining weight at a faster rate than that.   But what happens when we try to lose weight?  Usually nothing.

The primary reason for this is the inability to extend the period of time in a given day that we are mobilizing the optimal level of fat.  There are many things you can do to speed up the process as well as increase the ratio of substrate you are using. The biggest goal is to get your body into a fasted state for as long as possible without feeling hungry and maintain muscle mass.  There is an art to that science.  One of the best places to start is actually to limit exposure to fructose especially early in the day and immediately before a workout.  Not because it increases triglyceride synthesis, but because it decreases fat oxidation.  Just because fructose does not cause an insulin response does not mean that it will not hinder your ability to burn fat.  When a human consumes sugar, priority number 1 always is to metabolize the sugar.  It just so happens this is at the expense of fat oxidation.   With glucose you get the unfortunate benefit of increase fat storage via the insulin response and with fructose you get a decreased ability to mobilize fat.  Either way you’re pretty bummed.

That is a lot of information…Just tell me what to do!

Here are 5 suggestions for navigating Fructose consumption:

1: Stop eating fruit in the morning!

It’s said that an apple a day keeps the Dr. away.  That may be true, but you need to pay attention to when you eat it.  The first thing I would suggest in limit Fructose consumption as much as possible during the day.   Especially in the morning.  The healthy trend these days is to drink a fruit smoothie with added greens first thing in the morning.  Or worse yet,  juice.  The thought behind this is it will encourage people to get a few servings of greens right away and get then some vitamins from the fruit.  Also, this is much healthier than a pastry from Starbucks or whatever garbage the majority of Americans like to eat first thing in the morning.  Of course this is the lesser of two evils, but it’s certainly not optimal.   The fact is that most people sleep in a fat burning state.  Have you ever wondered why you don’t wake up in the middle of the night starving?  Can you imagine not eating for 8 hours during your waking hours?  You would be super hungry and probably passing out or yelling at your family or coworkers.  Why is this possible at night?  Because that is the only time that most people don’t mess up their body’s ability to maximally mobilize fat.  From the time of your last ingestion to the time you go to sleep, if you are not drinking alcohol, insulin levels will finally get to basal.   At this point the pancreas can start to release Glucagon and the body’s amazing ability to balance blood sugar actually gets to work.   While sleeping, this is extended and you are actually able to dip into ketosis.  This is a big reason why many people only gain about 10 lb. per decade.  If you sleep 8 hours per night about 30 percent of the time you are burning fat at an optimal rate!  But this is how you stay even or slowly gain.  How do you actually lose weight?  The answer is to keep that window open as long as possible.  By now you should realize that Glucose and Fructose cut that process short or at least render it less effective.

2: Stop eating bread any meal that is not dinner (including gluten free bread)

Fat mobilization is also shunted by the slow release of glucose that results from eating whole grains, and other carbohydrates that you may think are healthy for you all day.  Rather than running off of fat during the day, you have to process the sugar in your blood.   Also, glucose is a much easier fuel source to run off of than fat, it just runs out quickly and you feel a need to eat more of it.   It’s a sort of glyco-dependence!  At low levels of physical activity we always prefer fat as a fuel source, but we also need to manage our serum glucose levels if they are high via oxidation and storage.  That always takes priority for two reasons.  First high blood sugar levels are toxic.  Second, it is advantageous to create fat from excess sugars from an evolutionary standpoint.  Remember the cave man needing to walk around to find food? That energy comes from fat.  If we did not have this ability we would probably not have survived as a species.  Many modern breads actually are infused with HFCS to increase shelf life and give the bread a sweeter taste.  By eating bread you not only effect your fat burning potential because of the glucose production from bread, but you are also causing all of the aforementioned negative side effect caused by fructose.  Remove the sugars from bread by not eating it and you won’t have to worry about this.  Try Almonds instead.

3: Become a nocturnal fruit eater

It is not a good idea to completely eliminate fruit from your diet because there are many benefits to eating fruit.  Two hours before bed get your fruit fix.  Its a great way to reward yourself for being deciplined about your food choices all day.  And as you desensitize yourself to a constant barrage of sweets that fruit will taste like a freakin cookie.  This also helps you mitigate the negative hastening effect fructose has on fat metabolism.  This way you can be a fat burning beast all day and still enjoy the nutritional benefits of eating fruit

4: Berry me

If you must take in fruit during the day, make it berries.  Berries are much lower in sugar than other fruits and they generally contain a greater density of phytonutrients and polyphenols.  Even better combine some berries with a small serving of fat like 15 grams or so (1/2 avocado or 25 almonds or 1-2 oz of cheese or 1 serving of nut butter).  This will slow down the digestion rate a bit because of the delayed gastric emptying associated with fat consumption.  Although I don’t really recommend snacking, some berries combined with cheese or berries or nuts might be a good option to hold you over until a more substantial meal.

5.  Stick to Fruit

This might seem obvious, but I would be remiss not to mention it: don’t get fructose from sweeteners and soft drinks!  This includes fruit juices.

If you intend to eat Fructose make sure the source is an entire piece of fruit or half a piece of fruit.  This will decrease the overall sugar and caloric impact and you will consume the fiber.  Juice is just dressed up soda without all the chemicals that will disintegrate nails and likely give you cancer.  A great rule of thumb is unless you blended it yourself or it’s pure fat, don’t drink any calories.


There you have it, fructose assessed in the long form.  I know it’s a lot of information, but this is the type of stuff that goes under the radar and undermines people efforts in weight management.  Here are a few key points:

Fructose does not become fat when over consumed

Fructose does not directly elevate insulin levels

Fructose does decrease fat oxidation rates and ratios

Consuming Fruit is better to do at night becuase it decreases fat burning.  You want fat burning high during the day

Do you need to be afraid of eating an apple or a banana? No, but you should take care to pay attention to what time of day and how much volume of those items you consume.  Berries have less negative effects when compared to other fruits.  Eating fruit or higher levels of fructose or other carbohydrates is likely better tolerated during long light cycles and at night.  If you made it this far, I admire your dedication to learning about your health.  Take control and eat smart, this way you can enjoy the foods you like and achieve your weight management and performance goals.

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