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What is glucose?
- Calories per gram = 3.8
- Glycemic index = 100
- Sweetness, relative to sucrose = 70-80%
- Net carbs = 100%
Glucose Chemical Formula and Picture
Glucose formula is C6H12O6 .
Picture 1. Glucose structure
Glucose Functions in the Human Body
Glucose is a source of energy. Glucose from foods can provide 3.8 Calories per gram [2,3]; glucose powder and glucose for intravenous infusion, available as dextrose monohydrate, can provide 3.4 Calories per gram .
Glucose is a precursor for fructose, galactose, ribose and deoxyribose (in RNA and DNA in chromosomes), lipids, proteins, glucuronic acid, glucosamine and galactosamine (in the cartilage) and other substances required for proper body functioning [5,7].
Is glucose an essential nutrient?
Glucose deficiency syndrome has not been observed so far, so glucose, like other carbohydrates, is not considered an essential nutrient .
What is the recommended glucose intake?
You do not need to consume any glucose or other glucose-containing carbohydrates, such as starch or sucrose, to survive and maintain normal blood glucose levels, as long as you consume enough other nutrients from which glucose can be produced in your body: proteins , fats , fructose or galactose [23,24] or sugar alcohols, such as sorbitol or xylitol  . The production of glucose from non-carbohydrate sources (dietary or body fat and protein) is called gluconeogenesis .
During a usual, high-carbohydrate diet, your brain can use only glucose as a fuel . The brain burns about 20% of total calories you get (2,000-2,500 Cal/day in a sedentary person’s diet), so about 400-500 Calories per day, which you can get from 100-130 grams of glucose [5,27,28].
Low-carbohydrate diet. If you start to consume less than 100 g of glucose per day, the first day your body will produce glucose from glycogen stores in your liver and muscles and later from the protein and fat you eat. If you stop to eat, (fasting, starvation), some fats from your body stores will be converted to glucose and ketones and both will be used as a fuel for your body, including the brain. Some of the ketones will appear in your blood, which is known as ketosis, and, when excreted, they will give a fruity or acetone-like odor to your breath and urine. If you starve for a prolonged time, when your body fat stores are exhausted, proteins from your muscles will start to break down and will be partly converted to glucose and used as a fuel for your brain and rest of the body.
Any lack of glucose in a diet by itself should not be harmful for a healthy person with appropriate calorie intake . In order to prevent otherwise harmless ketosis and fruity breath, you need to consume the amount of carbohydrates that yield about 100 grams of glucose [29,30-pp.42,44], by consuming, for example, 200 grams of bread  or 2.3 cups (363 g) of rice  per day. There is also 50% glucose in sucrose and lactose.
There are some reasons not to consume excessive amounts of glucose (see Can glucose be bad for you?).
Foods High in Glucose
- Fruits and honey  can contain significant amounts of naturally occurring free glucose (as a simple sugar). Dried fruits contain more glucose per gram than the fresh ones.
- Glucose added to foods as a sweetener, which is labeled either as “glucose” or “dextrose,” can be found certain fruit juices, canned fruits and other canned foods, soft drinks, jams, dairy products, pastries, ice cream, sweets, snacks, sauces, salad dressings, liqueurs, infant formulas and other foods, and also to certain medicinal syrups and pills.
- “Energy tablets” can contain glucose (dextrose).
- Syrups high in glucose include liquid glucose or liquid dextrose , invert sugar, high fructose corn syrup (HFCS), date palm syrup, corn syrup (glucose syrup) and agave nectar  (Chart 1).
- Glucose powder is commercially available as “dextrose,” “grape sugar” or “corn sugar.”
- Corn syrup solids or glucose syrup solids or dried glucose syrup is a powder obtained by drying glucose syrup.
- Cereals, vegetables, legumes, nuts and seeds usually contain less than 2 grams of naturally occurring free glucose .
Chart 1: List of Foods High in Free Glucose
|FOOD (serving size)||GLUCOSE (grams)|
|Soya drink, sweetened (1 cup, 237 mL)||20|
|Dates, medjool (2 oz, 57 g)||19|
|Apricots, dried (2 oz, 57 g)||19|
|Pineapple, canned solids and liquids (1 cup, 250 g)||19|
|Dried glucose syrup or glucose syrup solids (1 tbsp, 20 g) ||Up to 18|
|Grape juice, unsweetened (1 cup, 237 mL)||18|
|Jackfruit, raw (1 cup, sliced, 165 g)||16|
|Carbonated beverages (12 oz, 355 mL)||Up to 16|
|Raisins, seedless (2 oz, 57 g)||15|
|Chocolate (1 oz, 28 g)||Up to 15|
|Figs, dried (2 oz, 57 g)||14|
|Prunes, pitted (2 oz, 57 g)||14|
|Liquid dextrose corn syrup (1 tbsp, 20 g) ||3-14|
|Pickles, cucumber, sweet (1 cup, 153 g)||14|
|Caramel sauce (150 g) ||9|
|Cherries, sweet (1 cup,138 g)||~9|
|Sport drink (12 oz, 355 mL)||~9|
|High fructose corn syrup HFCS-42 (1 tbsp, 20 g) ||7.5|
|Invert sugar syrup (1 tbsp, 20 g) (info from a producer)||~7|
|Honey (1 tbsp, 21 g)||~7|
|Date palm syrup (1 tbsp, 20 g) ||~7|
|Blueberries (1 cup, 148 g)||~7|
|Banana (8″ long, 136 g)||~7|
|Fruit juice, average (1 cup, 237 mL)||~7|
|Honey (1 tbsp, 21 g) ||4-7|
|Grapes (1 cup, 92 g)||~6|
|Cured pork for hamburger (1 slice, 138 g)||~6|
|Tomato puree (1 cup, 250 g)||6|
|High fructose corn syrup HFCS-55 (1 tbsp, 20 g) ||6|
|Barbecue (BBQ) sauce (2 tbsp, 34 g)||~6|
|Cider, sweet, 4.7 Vol% alcohol (12 oz, 355 mL) ||5|
|Agave nectar (1 tbsp, 20 g) ||4|
|Pancake syrup (1 tbsp, 20 g)||4|
|Light corn syrup (1 tbsp, 20 g)||4|
|Wine, white, sweet (5 fl. oz, 150 mL) ||4|
|Kiwi (1 fruit, 90 g)||~4|
|Oral rehydration solution, WHO-ORS (250 mL) ||3.5|
|Nectarine (2″ dia, 150 g)||3|
|Strawberries (1 cup, 152 g)||3|
|Dark corn syrup (1 tbsp, 20 g)||3|
|American cheese, pasteurized process (1 cup, 113 g)||3|
|Wine, dessert (3.5 fl. oz, 100 mL)||3|
|Watermelon, pieces (1 cup, 150 g) ||2|
|Onion, raw (medium, 2″, 110 g)||2|
|Molasses, blackstrap (1 tbsp, 20 g)||2|
Chart 1. sources: [34,35]
Other Dietary Sources of Glucose
During digestion, glucose is released from foods containing starch, dextrin, maltodextrin, sucrose, lactose, maltose, trehalose and glycogen.
Chart 2. Examples of Foods that Yield Glucose During Digestion
|FOOD||Amount of glucose (grams) after digestion|
|Chestnuts, European, roasted (1 cup, 143 g)||68|
|Barley, pearled, cooked (1 cup, 157 g)||38|
|Spaghetti (1 cup, 140 g)||37|
|Rice, white, cooked (1 cup, 174 g)||35|
|Cake, chocolate (1 piece, 64 g)||33|
|Potato, boiled (1 cup, 156 g)||28|
|Oatmeal (1 cup, 234 g)||23|
|Bread, white (1 big slice, 2 oz, 56 g)||23|
|Beer, strong ale, 8 vol% alcohol (12 oz, 355 mL)||22|
|Barley malt syrup (1 tbsp, 20 g)||12-15|
|Beer, 3.7 vol% alcohol (12 oz, 355 mL)||13|
|Beans, kidney, canned (1/2 cup, 128 g)||13|
|Lentils, boiled (1/2 cup, 100 g)||12|
|Chickpeas, boiled (1/2 cup, 82 g)||9|
|Peanuts, roasted (2 oz, 57 g)||8|
|Peas, green, boiled (1/2 cup, 80 g)||6|
|Milk, 3.2% fat (1 cup, 244 g)||6|
|Yogurt, plain, regular (6 oz, 180 mL)||5|
|Sunflower seeds, dry roasted (1 oz, 28 g)||4|
|Wine, table (5 oz, 150 mL)||4|
|Table sugar (1 tsp, 4 g)||2|
Chart 2 sources: [34,42,producers]
Glucose Absorption and Tolerance
Glucose is absorbed in the small intestine, mainly in the jejunum  but also in the duodenum [44,45] and ileum . The rate of glucose absorption partly depends on the rate of glucose gastric emptying. In various studies, glucose from 5%, 12.5% and 25% solutions emptied from the stomach at the same rate: about 30 grams per hour, which is about 2 Calories per minute [51,52,53].
Glucose absorption limit (or laxation threshold) might be as low as 60 grams per hour [47,48,49] or, according to another source, 120 grams per hour ; higher rates of glucose ingestion may trigger diarrhea, especially in small children.
Glucose is absorbed in the jejunum (the middle part of the small intestine) by the help of transport molecules SGLT1 (the sodium-dependent glucose-galactose transporter)  and GLUT2 transporters  and in the ileum (the last part of the small intestine) and colon by the help of GLUT7 transporters .
Chart 3. Glucose Absorption Rates from Different Carbohydrates
|GLUCOSE ABSORPTION||GLUCOSE SOURCES and their GLYCEMIC INDEX [GI]|
|QUICK AND COMPLETE glucose absorption||
|SLOW BUT COMPLETE glucose absorption||
|POOR (INCOMPLETE) glucose absorption||
|Glucose NOT ABSORBED. Glucose from indigestible carbohydrates cannot be absorbed as such but can be broken down (fermented) in the large intestine by beneficial bacteria into short-chain fatty acids (SCFAs), which can be absorbed and provide some energy.||
Factors that Affect the Rate of Glucose Absorption
- The type of carbohydrate ingested (see Chart 3). The effect of various carbohydrates on the rise of blood glucose levels in healthy individuals (in decreasing order): glucose > sucrose > amylopectin > amylose . High-amylose products have lower glycemic index (GI) than low-amylose ones [57,71]. In one 1987 study in individuals with diabetes 2, blood glucose levels were markedly lower after ingesting kidney beans, lentils and high-amylose muffins than after glucose solution, whole-grain bread, potatoes, rolled oats and rice, all containing the same amount of carbohydrates .
- Gastric emptying rate [73,74]:
- Factors that STIMULATE gastric emptying and thus glucose absorption:
- Liquid meal, greater meal volume and carbohydrate concentration, regular high-carbohydrate diet, caffeine , Indian spice mixture Garam Masala, excessive eating
- Low blood glucose levels (hypoglycemia), obesity, diabetes mellitus type 1 and 2
- Rapid gastric emptying: achalasia, autonomic neuropathy in alcoholism or diabetes, hyperthyroidism, irritable bowel syndrome, duodenal ulcers, stomach surgery, thyrotoxicosis.
- Drugs (long-term use): azithromycin, cannabis, cimetidine, clarithromycin, erythromycin, orlistat, thyroid hormones
- Gastric emptying is faster in men than in women.
- Factors that SLOW gastric emptying and thus glucose absorption:
- Fats, proteins, viscous soluble fiber (beta-glucan in oats, or guar gum), long-term low-carbohydrate diet, decaffeinated coffee
- Anti-diabetic drugs (acarbose, metformin, exenatide, pramlintide),
- Nausea, pain (including migraine), hypothermia, fever, functional dyspepsia, infections, severe trauma, head or neck spinal cord injury, stomach distension after binge eating,
- Delayed gastric emptying or gastroparesis: adrenal insufficiency, critical illness, diabetes mellitus, gastric ulcer and other stomach or abdominal conditions (cancer, gallbladder disease, pancreatitis, liver cirrhosis, chronic kidney disease), hyperglycemia, immobilization, neurological conditions (autonomic neuropathy, multiple sclerosis, stroke, Parkinson’s disease), stomach surgery
- Factors that STIMULATE gastric emptying and thus glucose absorption:
- Small intestinal disorders:
- Impaired carbohydrate digestion due to lack of digestive enzymes: pancreatic insufficiency due to chronic pancreatitis, pancreatic cancer, resection of pancreas, cystic fibrosis, glucose-galactose malabsorption, sucrase-isomaltase deficiency
- Impaired glucose absorption due to celiac disease, cystic fibrosis, Crohn’s disease, Glucose Transporter 1 Deficiency Syndrome (GLUT1 DS), hardening of arteries (atherosclerosis) that supply the bowel, lymphoma, small intestinal bacterial overgrowth (SIBO), surgical removal of part of small intestine (short bowel syndrome), severe chronic diarrhea
- Substances and drugs that affect glucose absorption: analgesics (acetaminophen), antacids (aluminium hydroxide), anticholinergics (atropine), meclozine), antidepressants (amitriptyline, duloxetine), antipsychotics (haloperidol, lithium), cannabis, laxatives, L-Dopa, melatonin, nicotine, nonsteroidal anti-inflammatory drugs (ibuprofen, indomethacin), opioids (codeine, heroin, morphine), serotonin, sucralfate
Glucose-Galactose Malabsorption or Intolerance
Individuals with a rare genetic disorder glucose-galactose malabsorption have an impairment of the SGLT1 transporters, molecules in the intestinal lining that enable glucose and galactose absorption . The main symptom, appearing in newborns, is severe diarrhea after consuming glucose, galactose, sucrose (glucose + fructose), lactose (glucose + galactose), isomaltulose, trehalose, starch and other glucose or galactose-containing carbohydrates . The disease cannot be cured, but symptoms can be prevented by avoiding glucose and galactose-containing carbohydrates. Affected individuals can survive into adulthood and some of them can start to tolerate glucose better with time .
Glucose Transporter 1 Deficiency Syndrome (GLUT1 DS)
GLUT1 DS is a rare genetic disorder with a lack of transporter needed to transport glucose from the blood to the brain, which means the brain of the affected individuals cannot use glucose as a fuel . Symptoms can include seizures (usually before age of two), small head size (not present at birth), movement disorders, speech and learning impairment . There is no cure for GLUT1 DS; symptoms can be partly prevented by a low-carbohydrate or ketogenic diet [77,78].
When glucose is absorbed from the intestine into the blood, it triggers the release of the hormone insulin from the pancreas. Insulin enables glucose to enter the muscle and fatty cells . Glucose can enter the brain, liver, skin, internal organs and blood cells without the help of insulin. Once inside the cells, glucose can be broken down to release energy by two ways:
- During rest and moderate exercise, glucose is broken down in the presence of oxygen into carbon dioxide (CO2) and water. The process yields energy in the form of 30-38 ATP molecules .
- During intense exercise, such as sprint, when oxygen cannot be delivered quickly enough to the muscles for the aerobic metabolism, glucose is broken down into lactate. The process yields energy in the form of 2 ATP molecules .
When the amount of glucose from the diet exceeds your current body energy needs, some glucose–up to about 500 grams in an average person, and up to about 1,000 grams in a trained athlete–can be stored as glycogen (a polysaccharide composed of glucose) in the liver and muscles [23,80,81,82]. During exercise, up to about 60 grams glucose per hour can be used for energy (oxidized) [83-p.3]. When the glycogen stores are full, any additional dietary glucose can be converted to body fats (triglycerides) or, during training, to muscle proteins [23,58].
Vitamins and minerals involved in carbohydrate metabolism [84,85,86]:
- Vitamin B1 (thiamin)
- Vitamin B2 (riboflavin)
- Vitamin B3 (niacin)
- Vitamin B5 (pantoothenic acid)
- Vitamin B7 (biotin)
Normal Blood Glucose Levels
- Normal fasting (morning) blood glucose levels: 70-99 mg/100 mL or 3.9-5.5 mMol/L [90,91]
- Normal blood glucose levels 2 hours after ingesting 75 grams glucose (Oral Glucose Tolerance Test or OGTT): less than 140 mg/100 mL or 3.9-8.1 mmol/L 
Effect of Dietary Glucose on Blood Glucose Levels
- There are about four grams of glucose in the blood of a 154 lbs (70 kg) person .
- You do not need to consume carbohydrate-containing foods or, for some time, any food at all to maintain normal blood glucose levels, since your body can synthesize all glucose it needs from dietary or body fats or proteins .
- Dietary glucose has a high glycemic index (GI = 100), which means it can considerably increase blood glucose levels . Glucose from meals with low glycemic index causes lower blood glucose levels after meals than glucose from foods with high glycemic index .
- In individuals with diabetes mellitus type 1 or 2, meals high in glucose can cause transient hyperglycemia .
- In sensitive people, meals high in glucose can trigger postprandial (reactive) hypoglycemia .
- In one study, during exercise at 70% intensity only 34% of ingested glucose appeared in the blood and during exercise at 83% intensity only 25% .
Possible Health Benefits of Glucose
Glucose from food can provide 3.8 Calories of energy per gram.
Glucose by mouth or by injection can treat low blood glucose levels (hypoglycemia).
Glucose ingested along with fructose promotes fructose absorption and thus help prevent bloating or loose stools after fructose ingestion in individuals with fructose malabsorption.
Glucose and Exercise Performance
Glucose ingestion can improve performance during moderate but not intense, exercise.
- In one 1998 study, ingestion of 250 mL of 8% glucose solution every 15 min by trained cyclists improved time to exhaustion during cycling at 70% intensity by 30% ; in another study, ingestion of 100 mL of 1.6% glucose solution every 10 minutes by active but untrained men, prolonged time to exhaustion in moderate exercise by about 20% .
- In one 2000 study, ingestion of 84 grams glucose in a 6% solution by trained men had no effect on time to exhaustion during exercise at 83% intensity . In one 1992 study, ingestion of solution with either 4.5% or 17% glucose did not significantly affect the rate of glucose use (oxidation) by the body during 80 minutes of exercise at 70% intensity .
In many studies, glucose in amounts up to 50 grams taken before memory tasks improved short-term memory or attention in children , healthy young and old adults [9,10,11,12,13,14] and in older persons with senile dementia [15,16], long-term memory [17,18], but in one study it did not affect memory .
Can glucose be bad for you?
Glucose and Dental Caries
Glucose can promote dental caries, because bacteria in the mouth break it down into acids, which can demineralize tooth enamel .
Diabetes or Glucose Intolerance
In diabetes mellitus type 1, the pancreas does not produce enough hormone insulin so glucose cannot enter the muscle and fat cells, so it accumulates in the blood and causes hyperglycemia . In diabetes type 2, the body cells are partially resistant to insulin, so they have reduced ability to take up glucose, so, again, glucose accumulates in the blood .
Risk factors for diabetes mellitus type 2:
- High-carbohydrate diet
- Antidepressants (medications) 
- Non-alcoholic fatty liver 
- Binge alcohol drinking was associated with the increased risk of diabetes 2 in women .
- 45-70% chronic alcoholics with alcoholic liver disease (hepatitis or cirrhosis) have glucose intolerance or diabetes type 2 .
Causes of hyperglycemia in individuals with diabetes [60,111]:
- Excessive eating of carbohydrate foods, especially those with high glycemic index, such as table sugar, white bread, cornflakes, potatoes
- Insufficient dose of insulin or anti-diabetic pills
- Sedentary lifestyle
- Emotional stress
- Infection, such as common cold or flu, injury, heavy illness, surgery
- Certain drugs, such as steroids 
Hyperglycemia by itself does not necessarily cause any symptoms, but when blood glucose exceeds a certain level, glucose starts to be excreted in the urine and drags water with it what may result in dehydration with unusual thirst and hunger, excessive drinking, excessive urination, urination during the night, itchy skin, fatigue, unexplained weight loss . Hyperglycemia in diabetes is treated with insulin, oral anti-diabetic drugs and diet .
Glucose and a Drop of Blood Pressure After Meals (Postprandial Hypotension)
High-carbohydrate meals containing glucose, sucrose or starch can trigger a drop of blood pressure after meals by more than 20 mm Hg (postprandial hypotension) in sensitive individuals: in elderly, those with autonomic neuropathy due to diabetes mellitus, Parkinson’s disease, paraplegia, individuals with kidney failure on hemodialysis  or multiple system atrophy . The drop of blood pressure after carbohydrate meals is probably caused by the rise of blood levels of insulin, which dilates vessels and due to blood shift to the intestinal arteries . Smaller meals with less glucose can probably help to prevent a drop of blood pressure .
The drop of blood pressure tends to be greater with increasing glucose content of meals, meal volume and temperature and the rate of gastric emptying . The drop of blood pressure can be inhibited by guar gum, which slows gastric emptying  or acarbose, which inhibits carbohydrate digestion .
Glucose Interactions With Other Nutrients and Drugs
- Glucose enhances sodium absorption in the small intestine and this in turn enhances water absorption, so glucose enhances water absorption . This is why glucose is used in sport drinks and oral rehydration solutions [113,114]. Glucose (or other sugars) added to beverages in concentrations greater than 6-8% may slow gastric emptying of fluids and therefore slow water absorption .
- Glucose enhances fructose absorption [116,117].
- Glucose in large amounts, especially when given as an intravenous injection, may lower blood levels of vitamin B1 (thiamin), phosphate, magnesium and potassium. This is known as refeeding syndrome, which is especially dangerous in alcoholics, who often already have several vitamin and mineral deficiencies .
- A combination of alcohol and carbohydrates (sweet liqueurs, vodka and soft drinks, rum and cola, gin tonic, or alcohol with carbohydrate snacks) may trigger reactive hypoglycemia with hunger, shakiness, dizziness and weakness within 1-3 hours after consumption . Mechanism: carbohydrates stimulate insulin secretion and alcohol enhances its effect what results in an excessive drop of blood glucose [119,120].
- Proteins (amino acids) trigger the release of insulin, and they reduce the increase of blood glucose levels after meals when added to a carbohydrate meal: slightly in healthy individuals and markedly in those with diabetes 2 . In one 2007 study in healthy individuals, 3-7 grams of amino acids leucine, isoleucine, valine, lysine and threonine in various combinations added to glucose drink reduced the rise of blood glucose levels after meals for more than 40% related to meals containing glucose alone; this effect was probably due to increased release of insulin triggered by amino acids . In the same study, 18 grams whey protein added to a glucose drink reduced blood glucose levels after the meals by more than 50%.
Glucose as a sweetener is produced from starch, in the United States usually from corn (maize) starch  and in other countries also from wheat , barley, sorghum, rice or potato, tapioca (cassava) and sago palm starch.
Glucose (Dextrose) and Cooking
- Glucose syrup is available as “liquid glucose” or “liquid dextrose.”
- Glucose powder is commercially available as “dextrose,” “grape sugar,” or “corn sugar”. Glucose powder is a white or colorless, crystalline substance without odor, about 74-82% as sweet as sucrose ; it is available either as dextrose anhydrous or dextrose monohydrate.
|Chart 4. Dextrose Properties|
|PROPERTIES||Dextrose anhydrous||Dextrose monohydrate|
|Water content||<1%||Up to 8.5%|
|Solubility at 77 °F (25° C)||90 g/100 mL water||90 g/100 mL water|
|Melting point||235-329° F (135-165° C) The melting point rises with the rate of heating.||83° C (181° F)|
|Hygroscopic (attracts moisture)||Yes||Yes|
|Caloric value||3.8 kcal/g||3.4 kcal/g|
Chart 4. source: 
Caramelization of glucose starts at 160° C (320° F) . Glucose is a reducing sugar  so it undergoes the Maillard browning reaction.