Glucose

Glucose Chemical Formula and Picture

Glucose formula is C₆H₁₂O₆ ^[1].

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 ^[4].

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 ^[21].

Foods High in Glucose

• Fruits and honey ^[33] 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 ^[6], invert sugar, high fructose corn syrup (HFCS), date palm syrup, corn syrup (glucose syrup) and agave nectar ^[36] (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 ^[34].

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) [36]	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) [36]	3-14
Pickles, cucumber, sweet (1 cup, 153 g)	14
Caramel sauce (150 g) [37]	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) [38]	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) [39]	~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) [33]	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) [38]	6
Barbecue (BBQ) sauce (2 tbsp, 34 g)	~6
Cider, sweet, 4.7 Vol% alcohol (12 oz, 355 mL) [37]	5
Agave nectar (1 tbsp, 20 g) [40]	4
Pancake syrup (1 tbsp, 20 g)	4
Light corn syrup (1 tbsp, 20 g)	4
Wine, white, sweet (5 fl. oz, 150 mL) [37]	4
Kiwi (1 fruit, 90 g)	~4
Oral rehydration solution, WHO-ORS (250 mL) [41]	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) [37]	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 ^[43] but also in the duodenum ^[44,45] and ileum ^[46]. 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 ^[50]; 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) ^[23] and GLUT2 transporters ^[23] and in the ileum (the last part of the small intestine) and colon by the help of GLUT7 transporters ^[63].

Chart 3. Glucose Absorption Rates from Different Carbohydrates
GLUCOSE ABSORPTION	GLUCOSE SOURCES and their GLYCEMIC INDEX [GI]
QUICK AND COMPLETE glucose absorption	Maltotriose from liquid glucose and brown rice syrup [55,56] Rapidly digestible starches from freshly cooked potatoes, white bread, white rice [GI = up to 110] [57,58,59] Maltose from starch and as free maltose from spelt, kamut and sweet potatoes [GI = 105] [55,57,60] Free glucose from sugar-containing beverages [GI = 100] [23,57] Maltodextrin, a thickener [GI = 91] [59,61-p.9] Trehalose, a sweetener [GI = 72] [61-p.9;62] Sucrose, a sweetener [GI = 68] [60] Lactose from milk [GI = 46] [60] Trehalulose, a sweetener [65] Slowly digestible starches from legumes and whole-grain bread [GI = about 40] [57,58,59]
SLOW BUT COMPLETE glucose absorption	Isomaltulose [GI = 32] [57,66]
POOR (INCOMPLETE) glucose absorption	Isomalto-oligosaccharides (IOS): a mixture of maltose, isomaltose, panose, isomaltotriose, isomaltopentose [67]
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.	Raffinose and stachyose from legumes Human milk oligosaccharides (HMO), galactooligosaccharides (GOS) (from milk), fructooligosaccharides (FOS) and inulin (from wheat, onions, Jerusalem artichokes, chicory roots, leeks and certain commercial products) Polydextrose, added to certain commercial products [69] Resistant starches from whole-grain products, high-amylose products, legumes, unripe bananas, cooked and cooled pasta  and resistant maltodextrins in certain commercial products Glucomannan or konjac gum Xanthan gum (artificial gum) Gellan gum (artificial gum) Cellulose from apples, certain vegetables, whole wheat products, legumes, seed husks

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 ^[70]. 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 ^[72].
• 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 ^[23], 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
• 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 ^[75]. 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 ^[75]. 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 ^[76].

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 ^[77]. Symptoms can include seizures (usually before age of two), small head size (not present at birth), movement disorders, speech and learning impairment ^[77]. There is no cure for GLUT1 DS; symptoms can be partly prevented by a low-carbohydrate or ketogenic diet ^[77,78].

Glucose Metabolism

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 ^[23]. 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 ^[79].
• 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 ^[79].

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)
• Chromium
• Iron
• Magnesium
• Manganese

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 ^[91]

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 ^[81].
• 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 ^[5].
• Dietary glucose has a high glycemic index (GI = 100), which means it can considerably increase blood glucose levels ^[60]. Glucose from meals with low glycemic index causes lower blood glucose levels after meals than glucose from foods with high glycemic index ^[92].
• In individuals with diabetes mellitus type 1 or 2, meals high in glucose can cause transient hyperglycemia ^[91].
• In sensitive people, meals high in glucose can trigger postprandial (reactive) hypoglycemia ^[93].
• 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% ^[94].

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 ^[113]. 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 ^[115].
• 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 ^[118].
• 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 ^[119]. 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 ^[121]. 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 ^[54]. 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 Production

Glucose as a sweetener is produced from starch, in the United States usually from corn (maize) starch ^[87] and in other countries also from wheat ^[88], barley, sorghum, rice or potato, tapioca (cassava) and sago palm starch.

Related Nutrients

• Fructose
• Galactose
• Sucrose
• Monosaccharides
• Sugars
• Carbohydrates