Similarities of glucose and fructose
Before going through the differences, it is important to note that there are some common characteristics of glucose and fructose, exerting varied effects on the body, and undergoing different metabolic processes.
Both fructose and glucose are monosaccharides, have the same caloric values (roughly 4Cal/g), and the same molecular formula of CC6H1206.
Both fructose and glucose have a sweet taste (fructose is slightly sweeter than glucose).
Differences between glucose and fructose
- Absorption (1, 2, 3)
Fructose: is poorly absorbed and can cause fructose intolerance. It is moved to the colon, where it undergoes fermentation by bacteria producing gas, bloating and diarrhea.
Glucose: is well absorbed.
- Brain interaction (4)
Fructose: is slowly up taken through the brain barrier, causing the reduction of neural activity.
Glucose: is up taken quickly to the brain barrier where it is metabolised using most of the oxygen available in the brain. Glucose raises the level of neural activity for about 20 minutes after its consumption.
- Cell entrance (5, 6)
Fructose: is mainly (but not exclusively) metabolised by the liver. Other cells that accept fructose (although to a much lesser extent) are testicles, kidneys, skeletal muscles, fat tissue and brain. Fructose is up taken to the cells via a transporter called GLUT5 and also via GLUT2, to a smaller extent, which accepts fructose to the liver cells.
Glucose: is up taken to most of the cells in the body for metabolism. It uses the following transporters to distribute glucose to corresponding cells: GLUT1 (to all cells), GLUT2 (to gut, liver and pancreas), GLUT3 (to central nervous system and brain), GLUT4 (to cells that need insulin to be up-taken such as skeletal muscles, fatty tissue and heart).
- Diabetics risk (7)
Fructose: presents a long term risk for diabetics. Fructose only slightly increases blood glucose levels and insulin production after a meal. It does not cause sudden spikes in insulin levels and doesn’t present an immediate danger if over-consumed. A chronic excess of fructose, however, leads to increased lipid production, obesity, insulin resistance and metabolic syndrome, all of which are associated with increased long-term risk of diabetes.
Glucose: a high intake presents an immediate danger for diabetics, since it has a high glycemic index and load. This means that an overdose of glucose causes sudden spikes of insulin which can be dangerous and even fatal for diabetics.
- Energy source (8)
Fructose: is a non-essential energy source that has a similar action in the body to ethanol (with the exception of the effects on the brain).
Glucose: is a primary and preferred energy source and required by the brain cells. Note: ketones can serve as a substitute but a high ketogenic diet may not be sustainable in the long term.
- Energy storage as glycogen (9)
Fructose: needs to be converted into glucose first in order to be stored in glycogen stores.
Glucose: gets stored as glycogen in the liver and muscle cells.
- Food sources (10)
Fructose: not present in starch.
Glucose: present in starch.
- Gout (11, 12)
Fructose: a high intake produces uric acid, which increases the risk of gout.
Glucose: hasn’t been associated with an increased risk of gout.
- Insulin response and glycemic index (13)
Fructose stimulates insulin in a much lesser degree than glucose. The glycemic index and load are much higher for glucose than fructose.
Carbs Glycemic Index Serve (g) Glycemic Load Glucose (1) 100 10 10 Fructose (2) 23 10 2
- Leptin resistance (7, 14)
Fructose: has a minor impact on insulin release after a meal. Insulin increases the release of leptin, a hormone that inhibits appetite. If there is less insulin in circulation, leptin levels are not as high and appetite is not suppressed as effectively as after glucose intake. As a result, a high fructose diet leads to overeating.
Glucose: has a more significant effect on leptin. Glucose increases insulin production which increases the release of leptin, sending us a signal that we should stop eating, therefore, suppressing our appetite.
- Metabolism (15)
Assuming the same amount of fructose and glucose is ingested, the summary of the products of metabolism is:
Fructose: almost 100% of the ingested fructose goes to the liver cells for metabolism. Because of a much higher amount of fructose entering the liver cells, more VLDL particles are generated, causing dyslipidaemia. The excessive fat production will also generate free fatty acids that enter the muscle cells, causing insulin resistance. Lipid droplets that will remain in the liver, can lead to fatty liver disease. A by-product of fructose metabolism, JNK1, will cause inflammation and decrease liver insulin sensitivity. (read more..)
Glucose: 20% of the glucose ingested gets to the liver. Some is stored as glycogen and some enters mitochondria, where is utilised for cellular energy. A small amount of VLDL particles is produced from unused mitochondria energy. High amounts of VLDLs are associated with increased risk of heart disease. (read more..) The rest is absorbed and metabolised by most (but not all e.g. sperm) cells in the body. Glucose is especially important in the brain, where is metabolised as a primary fuel source.
- Molecular shape (16)
Both fructose and glucose are monosaccharides, called hexose, containing six carbon atoms and have a formula C6H12O6. However, they differ in their ring structures. The differences in shape mean that some enzymes, with specific shapes of active sites (a place where substance fits into the enzyme), will accept one but not the other.
- Sperm (17, 18)
Fructose: sperm cells use fructose as their energy source.
Glucose: needs to be converted to fructose in order to be used by the sperm as an energy source.
- Sweetness ranking (19)
Fructose: Fructose tastes 1.17 times sweeter than sucrose (table sugar). Fructose is 1.74 sweeter than glucose.
Glucose: Sucrose (table sugar) tastes 1.49 times sweeter than glucose.
- Triglycerides (7, 20)
High fructose amounts produce much higher levels of triglycerides (fats) during its metabolism in the liver than glucose. Studies show that there are approximately 30% more triglycerides in the blood on a high fructose diet than on high glucose diet. Please note, however, that the triglyceride production also depends on the level of physical activity and glycogen stores. For instance, triglyceride production from fructose does not have much impact in elite athletes.