Fruit is generally a healthy, nutritious food. Fruit is high in fiber, low in fat and calories and is a good source of vitamins and minerals. If you want something sweet for dessert, fruit is a good choice.
However, there are two groups of people for whom fruit is not the best food choice: bodybuilders and and anyone trying to maximize fat loss. 80-90 percent of the calories in fruit are supplied by simple sugars, fructose and glucose.
Some fruits, such as grapes and oranges, contain a lot of glucose, but most fruits supply the bulk of their calories as fructose, which is also known as fruit sugar. The bottom line is that fructose is rapidly converted to fat by the liver.
Whereas most other carbohydrate sources are preferentially stored as glycogen, fructose is preferentially converted to fat and stored in adipose tissue. This is a consequence of the molecular structure of fructose, which allows it to skip a key regulatory point in carbohydrate metabolism.
This regulatory point is a step in the glucolytic pathway catalyzed by the enzyme phosphofructokinase-I (PFK-I). As you know, from previous articles about carbohydrate metabolism and thermogenesis, the dietary energy (calories) supplied by carbs is used for several purposes.
Some of it is simply lost as heat during its digestion and metabolism in a process we know as diet-induce thermogenesis. You can loosely think of this as ï¿½??frictionï¿½?? in the metabolic pathway, and this energy loss contributes to the generation of body heat.
Most of the dietary energy is used to maintain the basal metabolic rate (BMR) ï¿½?? the energy cost of keeping your body alive. Some of the energy is used to perform work, like exercise and activities of daily life.
After that, any energy left is stored as glycogen in muscles and in the liver. If you consume too many calories from carbohydrate after glycogen stores are full, the rest will be converted to fat (triglycerides) in the liver, and transported by the blood to fat cells (adipose tissue) for storage.
So excess calories from any carbohydrate source can be converted to fat. The enzyme that regulates whether dietary energy supplied by carbohydrate is stored as glycogen or fat is PFK-I. It shuttles carbs into glycogen stores until full , then it switches the flow of carbohydrates from glycogen synthesis to fat synthesis.
Glycogen is the storage form of carbohydrate in animals, and the amount of glycogen you can store is quite limited. the upper limit is generally believed to be between 250-400 grams, depending on the amount of skeletal muscle mass you have.
This amounts to only 1000-1600 calories ï¿½?? not even enough energy to fuel your body for one day. The deal with fructose is that it totally skips the enzyme PFK-I, which is the regulatory step responsible for making sure glycogen stores are full before fat synthesis is switched on.
Instead of being stored as glycogen, fructose gets directly converted to fat by the liver. Now I think you can see why I have a problem with recommending fruit for bodybuilders. to get a detailed understanding of fructose metabolism, we should start at the beginning.
Fructose is absorbed from the small intestine and directly transported to the liver by the portal vein. The first enzyme to act on fructose is fructokinase, which adds a phosphate group to the sugar to form fructose-1-phosphate (F1P).
Glucose is similarly phosphorylated at the six position of the hexokinase, forming G6P. All cells have hexokinase and thus have the ability to phosphorylate glucose. This means that all cells can metabolize glucose for energy.
On the other hand, fructokinase is virtually confined to the liver. So while glucose is a general substrate for all body tissues, fructose represents a carbohydrate load targeted for the liver. The next thing that happens is F1P is split by the enzyme aldolase to form glyceraldehyde (GA) and dihydroxyacetone phosphate (DHAP).
This means that products of fructose metabolism enter the glycolytic pathway at the triose phosphate level (i.e., as three carbon sugars). Glucose, on the other hand, is phosphorylated to yield G6P, which may proceed directly to glycogen synthesis.
To be broken down for energy, glucose must first pass through the rate-limiting PFK-I step. Fructose metabolites enter below this step, and thus bypass an important point of regulation. Fructose therefore is more prone to be converted to fat, while glucose is more prone to be converted to glycogen.
The biochemistry is much more complex than is appropriate for this post, but I have pointed out the salient features of the pathway to explain why glucose-based carbohydrate sources are better than fructose, especially for people trying to minimize body fat stores.
Scientific studies have proven that while fructose is effective at replenishing liver glycogen stores, starch (glucose polymers) is much more efficient at replenishing skeletal muscle glycogen stores.
This makes sense because complex carbs are released into the bloodstream slowly whereas simple sugars are released very rapidly, potentially overwhelming the glycogen synthesis pathway and ï¿½??spilling overï¿½?? into fat stores.
Furthermore, the increased insulin release resulting from simple sugars causes more of the sugar to be converted to fat. Youï¿½??ve probably heard that fructose is low on glycemic index, which means it raises blood sugar very slowly and elicits only a small insulin release.
You know that a slow, steady insulin response is good. Since insulin is a potent stimulus for fat storage, we want to keep insulin levels fairly low, so be this reasoning it seems like fructose would be good.
The problem is that the REASON fructose has a low glycemic index and results in a small insulin release is that it is converted to fat in the liver. It doesnï¿½??t raise blood sugar very much because it is released from the liver as fat instead of sugar.
Fructose has a MUCH greater tendency to be converted to fat than other carbohydrate sources, so why use it? Now you understand the biochemistry behind my controversial stance on fruit. It isn’t that I NEVER advocate fruit, it is that I would use it sparingly for bodybuilding purposes.