What the Glut?
Fructose. Everyone knows that fructose is the enemy (or at least that guy on that forum), but is it really?
In 1957, the chemical process of the conversion of glucose to fructose was developed, and this advance led to the gradual increase in the consumption of high fructose corn syrup. Fructose is the sweetest of all natural sugars, making it popular to add in food and beverage products. It is somewhat special compared to other sugars. It has it’s own GLUT transporter, which could potentially make fructose a better option as a carb source, by lowering the total amount of insulin secreted. Fruit could be a better carbohydrate choice with humans with diabetes as well (pre,type-1, and type 2). First we need to understand the GLUT transporters.
There are 14 different GLUT proteins. Gluts 1-5 are the most studied,
Glut 1– Insulin-Independent, found on the blood, blood brain barrier, and heart. They are the major GLUT expressed in brain endothelial cells. They are also expressed in brain astrocytes, fueling neurons. It has a high affinity for glucose and is insulin-independent. GLUT 1 has a low Km. These transport glucose even when circulating levels are low.
Glut 2– Insulin-Independent, found in liver, pancreas, and small intestine. These glut 2 have a high Km and they have a low affinity for glucose. These Gluts maximally uptake glucose when glucose concentrations are high. Glut2 signals to the beat cells in the pancreas that blood glucose concentrations are high, and the beta cells respond by secreting insulin in the bloodstream. This is the second major fructose transporter.
Glut 3– Insulin-Independent, found in the brain, neurons, and sperm. Low Km, which means they have a very high affinity for glucose, so they have priority taking up glucose, compared with other gluts that have a lower affinity.
Glut 4– Insulin-dependent, found in skeletal muscle, adipose tissue, and heart. Insulin triggers the translocation of glut 4 into the cell membranes of these tissues to uptake glucose. These have a moderate Km, and moderate affinity for glucose This glut transporter is affected with people with diabetes. In type 2 diabetes, insulin cannot bind to the insulin receptors efficiently, and glut 4 will not translocate to the cell membrane, despite a large concentration of glucose in the blood. Blood glucose will remain high, and glut 4 will not translocate to cell membranes to lower blood glucose. Exercise can also stimulate GLUT4 translocation to the plasma membrane in skeletal muscle, but this pathway is not initiated by insulin.
Glut 5– Insulin-Independent, found on the Luminal side of intestinal epithelium, kidney, sperm, skeletal muscle, fat tissue, and brain. These GLUTs are the primary fructose transporters. Once GLUT5 and GLUT2 transports fructose across the apical membrane, GLUT2 helps transports the fructose across the basolateral membrane. Insulin will cause an increase in glut 5 translocation, however it is not dependent.
After studying GLUT transporters, I hypothesized that since GLUT5 is insulin-independent, and is the main carrier of fructose, followed by GLUT2 which is also independent of insulin, that eating fruit as a carbohydrate source might be beneficial for burning fat, by reducing the lower amount of the storage hormone, insulin in the blood. Once I had that thought, I started thinking about people with diseases such as diabetes type 1 and 2. I went on pubmed and found a few different articles that were interesting, and I wanted to share.
There was a group that wrote a paper on pub med that hypothesized that meals high in fructose would result in lower leptin concentrations than meals containing the same amount of glucose, since leptin secretion is regulated by insulin-mediated glucose metabolism. DING DING DING. I hit the jackpot. In the study, they collected blood samples every 30 min for 24 hours from 12 women on 2 randomized days, where the subjects consumed 3 isocaloric meals consisting of 55, 30, and 15% of total kcal from protein, carbohydrate, and fat, with 30% of kcal coming from either fructose or glucose as a carbohydrate source. Insulin responses were 65%(+/-5%) lower during the Fructose containing meal compared to the glucose containing meal. They also concluded in their study that the fructose containing meal produced a less pronounced suppression of ghrelin, compared to the glucose containing meal.
Another publication, which was a meta-analysis, identified 53 eligible randomized controlled trials with 1,126 participants, out of 3,666 citations. In diabetic participants, fructose reduced 2-hour blood glucose concentrations by 4.81 mmol/L compared to glucose.
Another publication, studied the evidence for postprandial glycemic and insulinemic responses after replacing either glucose or sucrose in foods or beverage with fructose. They found that replacing glucose and sucrose with fructose resulted in significantly lower peak postprandial blood glucose, particularly in people with prediabetes, type 1, and type 2. Similar results were obtained for insulin.
GLUT 5 and diabetes.
Rats with type 2 diabetes had a dramatic increase in GLUT5 mRNA and protein levels in the kidneys.
Rats with diabetes also had a 3-4-fold increase in GLUT 5 mRNA and protein level in the small intestine
Rats with diabetes had a 2-3 fold increase in GLUT5 mRNA and protein levels in adipose tissue.
Humans with diabetes had a dramatic increase of GLUT5 mRNA and protein levels in skeletal tissue.
Just thinking about this, it makes since to me. If you are more resistant to insulin and cannot rely on glut 4 to lower blood glucose, making more glut5’s would make since, since they are insulin-independent.
In conclusion, eating fruit/ fructose as a carbohydrate source could be a great idea for anyone trying to lower their body fat, or keep insulin levels low because of some disease, such as diabetes. Also, The article talking about how ghrelin levels were less suppressed compared to a glucose meal, which could be a useful tool hardgainers can try out. Eating fructose won’t satisfy those hunger hormones as well as say glucose, so if you struggle getting your food down, fructose could be a great option.