Carbohydrates

Population studies in Alaska and Greenland have revealed no apparent detrimental effects of a lifelong very low carbohydrate diets on health or longevity ^[6]. In one 1928 study, Caucasians tolerated carbohydrate-free diet for one year “quite well” ^[6].

Even if not essential nutrients, carbohydrates are an important part of healthy diets.

Functions of Carbohydrates in the Human Body

The two main functions of the dietary carbohydrates are to provide^[3,4,5]:

• Energy (about 4 kilocalories or 17 kilojoules per gram)
• Building blocks, mainly carbon atoms, for the synthesis of glycogen, fatty acids, amino acids and other substances in your body.

Chart 2. Recommended Daily Intake of Carbohydrates
Acceptable Macronutrient Distribution Range (AMDR) – the acceptable percent of calories that come from carbohydrates — is 45-65%.	Diet	Carbohydrates (grams)
	1,000 Cal diet (2-year old)	110-160 g
	1,500 Cal diet (10-year old)	170-245 g
	2,000 Cal diet (sedentary adult women)	225-325 g
	2,500 Cal diet (sedentary adult men)	280-405 g
Recommended Dietary Allowance (RDA) – the minimum daily amount of carbohydrates used by the brain	All age groups, except infants	130 g
	Pregnancy	175 g
	Lactation	210 g
Adequate Intake (AI) — the amount of carbohydrates that should suffice for the brain needs for carbohydrates for up to 98% of infants.	Infants 0-6 months	60 g
	7-12 months	95 g

Reference: IOM ^[7] NOTE: The amounts of carbohydrates presented above are not “required amounts” but the amounts which the Institute of Medicine in the U.S. recommends as part of a balanced diet. Lower or greater amounts of carbohydrates are not, by themselves, unhealthy.

Plant and Animal Carbohydrates

Plant carbohydrates include glucose, fructose, galactose, mannose,sucrose, maltose, trehalose, sugar alcohols (mannitol, sorbitol, xylitol),starch, cellulose, hemicelluloses, galactomannans, glucomannans and various other polysaccharides found in gums and seaweeds.

Animal carbohydrates include lactose and trace amounts of galactooligosaccharides in milk and whey, and glycogen in animal liver, oysters and mussels, and chitin in crabs shells.

Sugars in honey (fructose, glucose, sucrose, isomaltulose) can be considered either plant- or animal-derived.

Carbohydrates in human milk include lactose and human milk oligosaccharides (HMO).

Semi-Synthetic Carbohydrates

Carbohydrates can be semi-synthetically produced from naturally occurring carbohydrates, such as glucose, lactose or wheat or–in the United States–from cornstarch, by using enzymes and/or certain chemicals. Semi-synthetic carbohydrates may be used as artificial sweeteners, food additives, added fiber, prebiotic supplements, or binders in pills.

A. Semi-synthetic carbohydrates obtained by treating plant carbohydrates:

• Semi-synthetic sugars: altrose, arabinose, corn syrup, dextrose (D-glucose), erythrose, erythrulose, fucose, gulose, idose, high corn fructose syrup (HFCS), invert sugar, isomaltulose, lyxose, rhamnose, ribose, ribulose, sorbose, tagatose, talose, threose, trehalose, xylose (wood sugar), xylulose.
• Semi-synthetic sugar alcohols or polyols: erythritol, hydrogenated starch hydrolyzates (HSH) or polyglucitol, isomalt, lactitol, maltitol,mannitol, sorbitol, xylitol
• Semi-synthetic oligosaccharides: fructooligosaccharides (FOS), gluco-oligosaccharides, isomalto-oligosaccharides, lactosucrose, maltotriose,mannan oligosaccharides, N-acetylchitooligosaccharides, oligodextrose, oligosaccharides from melibiose, pectic oligosaccharides, xylooligosaccharides.
• Semi-synthetic polysaccharides: carboxymethycellulose, dextrin, gellan gum, inulin, maltodextrin, methylcellulose, microcrystalline cellulose,modified food starches, partially hydrolyzed guar gum (PHGG), polydextrose, pyrodextrins, xanthan gum.

B. Semi-synthetic carbohydrates obtained by treating animal carbohydrates:

• Galactooligosaccharides (GOS) obtained from lactose
• Chitosan obtained from chitin (from crab shells)

C. Glycerin(e) or glycerol can be derived either from plant oils or animal fats ad can be added to commercial animal or plant foods.

Carbohydrate Sources

• The main source of carbohydrates in the human diet are plant foods, such as cereals, root vegetables (potatoes, yams, cassava), fruits and legumes (beans, peas, lentils) and table sugar ^[1].
• Animal sources of dietary carbohydrates are milk (lactose), animal liver and seafood (glycogen).
• The main carbohydrate in human breast milk is lactose.
• Carbohydrates, mainly sugars and fibers, can be added to foods, such as fruit juices, jams, soft drinks, energy drinks, liqueurs, dairy products, sweets and sauces.

Chart 3. Foods High in Carbohydrates
FOOD (serving)	AVAILABLE CARBOHYDRATES (total carbohydrates minus fiber) (grams)
Mango chutney (1 cup, 250 g)	90
Chestnuts, European (1 cup, 143 g)	70
Apple pie (1 piece, 120 g)	55
Cookies, fortune (2 oz, 57 g)	50
Rice, white, cooked (1 cup, 158 g)	45
Raisins, seedless (2 oz, 57 g)	40
Macaroni, cooked (1 cup, 140 g)	40
Granola bar, soft, plain (2 oz, 57 g)	35
Soft drinks (12 oz, 355 mL)	Up to 35
Cake, chocolate (1/8 of 18″ cake, 64 g)	35
Doughnut, chocolate coated (large, 3-1/2″ dia, 67 g)	35
Pizza, thick crust, pepperoni (1 slice, 106 g)	30
Potato, boiled (1 cup, 156 g)	30
Figs, dried (2 oz, 57 g)	30
Fruit juice (1 cup, 240 mL)	Up to 30
Candies, hard (1 oz, 28 g)	25
Ready-to-eat cereals, dry (3/4 cup, 30 g)	25
Coffee liqueur (1 jigger, 1.5 oz, 45 mL)	25
Cornmeal, polenta, prepared (1 cup, 160 g)	20
Bread, wheat (2 slices, 50 g)	20
Apple (1 medium, 3″ dia, 182 g)	20
Ice cream, vanilla, soft (1 cone, 100 g)	20
Human breast milk (1 cup, 240 mL)	15
Honey (1 tbsp, 21 g)	15
Oatmeal, regular, prepared (1 cup, 160 g)	15
Corn syrup, dark (1 tbsp, 20 g)	15
Chocolate, milk (1 oz, 28 g)	15
Kidney beans (1/2 cup, 88 g)	15
Jam, fruit, average (1 tbsp, 20 g)	15
Wine, desert, sweet (3.5 fl oz, 100 mL)	15
Beer (12 fl. oz, 355 mL)	~13
Milk, whole, 3.2% fat (1 cup, 240 mL)	13
Pistachios, dry roasted (2 oz, 57 g)	10

Chart 3. references: USDA ^[8], Fineli.fi ^[9]

Available and Unavailable Carbohydrates

Available carbohydrates or “net carbs” are part of carbohydrates that can be completely absorbed in the small intestine and can provide about 4 Calories per gram. Available carbohydrates include:

• Sugars: glucose, fructose, galactose, mannose, sucrose, lactose, maltose, isomaltose, isomaltulose and trehalose
• Starch, dextrin and maltodextrin
• Glycogen
• Glycerol (glycerin)

To calculate available carbohydrates from the Nutrition Facts labels, distract dietary fiber from total carbohydrates.

Partially available carbohydrates, such as tagatose and sugar alcohols or polyols (sorbitol, xylitol, etc.), are partially absorbed in the small intestine and partially fermented by colonic bacteria into short-chain fatty acids and other nutrients that can be absorbed and can provide 1.5-3.5 kilocalories per gram.

Unavailable carbohydrates cannot be digested and absorbed in the small intestine. They include:

• Soluble fiber, such as fructooligosaccharides (FOS), galactosaccharides (GOS), galactomannans, glucomannans and resistant starches. Soluble fiber can be broken down (fermented) by the large intestinal bacteria into short-chain fatty acids, which can be absorbed and can provide about 2 Calories per gram.
• Insoluble fiber, such as cellulose, cannot be digested and also not fermented by the large intestinal bacteria so it does not provide calories.

Carbohydrate Digestion

Digestible Carbohydrates

Starch is partially broken down to a disaccharide maltose in the mouth by the enzyme salivary amylase, and further in the small intestine by pancreatic amylase–delivered by a pancreatic juice–into dextrins, maltotriose, maltose and isomaltose, which are further broken down by the enzymes maltase and isomaltase to glucose ^[10]. NOTE: Raw starch is digested slowly and incompletely ^[73,79].

Disaccharides are broken down to monosaccharides with the help of the enzymes in the intestinal lining: sucrose is broken down to glucose and fructose by sucrase, lactose to glucose and galactose by lactase, maltose to two glucoses by maltase, and trehalose to two glucoses by trehalase.

Monosaccharides glucose, fructose, galactose, mannose, and glycerin(e)/glycerol can be directly absorbed in the small intestine without being previously digested by enzymes. A semi-synthesized sugar tagatose, and sugar alcohols (lactitol, maltitol, mannitol, sorbitol, xylitol) can be only partly absorbed in the small intestine; the rest of them are broken down (fermented) by the large intestinal bacteria to smaller molecules that are partly absorbed.

Non-Digestible Carbohydrates

Oligosaccharides, such as fructooligosaccharides (FOS), and dietary fiber, such as cellulose, pectin and gums, cannot be digested by the enzymes in the small intestine, but can be at least partially broken down (fermented) by beneficial large intestinal bacteria to short chain fatty acids (SCFA), monosaccharides, hydrogen, methane, or carbon dioxide, which can be partially or completely absorbed.

Nondigestible carbohydrates can be a significant source of energy for individuals whose small intestine has been partly removed (short bowel syndrome or SBS) ^[11].

Prebiotics are indigestible carbohydrates that selectively promote the growth of the beneficial large intestinal bacteria. Prebiotics include fructooligosaccharides, trans-galactooligosaccharides, inulin, lactulose and resistant starches.

FODMAPs (Fermentable Oligo-, Di- and Monosaccharides And Polyols) are slowly digestible or indigestible carbohydrates that may cause abdominal bloating in some people, especially in those with irritable bowel syndrome (IBS).

Chart 4. Carbohydrate Digestive Enzymes (Glucosidases)
ENZYME	SOURCE	SITE of ACTION	FUNCTION
Salivary alpha-amylase[10]	Salivary glands, mother’s breast milk glands	Mouth	Splits cooked starch and glycogen (at alpha 1-4 bonds) to dextrins, isomaltose and maltose
Pancreatic alpha-amylase [10]	Pancreas	Duodenum	Splits cooked and uncooked starch and glycogen (at alpha 1-4 bonds) to dextrins, isomaltose and maltose
Glucoamylase [10]	Small intestinal mucosal cells	Small intestinal lining	Splits starch (at alpha 1-6 bonds) to smaller carbohydrates and cleaves glucose molecules, one by one, from starch (at alpha 1-4 bonds)
Sucrase	Small intestinal mucosal cells	Small intestinal lining	Splits sucrose to glucose and fructose
Lactase	Small intestinal mucosal cells	Small intestinal lining	Splits lactose to glucose and galactose
Maltase (alpha-glucosidase)	Small intestinal mucosal cells	Small intestinal lining	Splits maltose to two glucose molecules
Isomaltase (dextrinase)[13]	Small intestinal mucosal cells	Small intestinal lining	Splits isomaltose to two glucose molecules; splits isomaltulose and trehalulose to glucose and fructose; splits starch (at alpha 1-6 bonds) to dextrins
Trehalase [12]	Small intestinal mucosal cells	Small intestinal lining	Splits trehalose to two glucose molecules

Carbohydrate Absorption

There is an absorption limit for various carbohydrates:

• It seems that in healthy individuals only up to 60 grams of glucose per hour can be absorbed ^[34].
• Healthy individuals can absorb only about 20-50 grams of fructose from one meal, when fructose is the only carbohydrate in the meal; the presence of glucose increases the absorption rate of fructose ^[16,17].

Carbohydrate Malabsorption

Carbohydrate digestion and subsequent absorption of glucose, fructose and galactose can be impaired in viral gastroenteritis (stomach flu), food poisoning, celiac disease, Crohn’s disease, advanced chronic pancreatitis, tropical sprue ^[21], small intestinal bacterial overgrowth (SIBO)^[22], intestinal parasites, intestinal lymphoma, cystic fibrosis, after partial surgical removal of the stomach resulting in dumping syndrome or removal of the small intestine resulting in short bowel syndrome (SBS).

Glycemic Index (GI) and Glycemic Load (GL)

Glycemic response is the effect of the dietary carbohydrates on blood glucose levels ^[4]. Glucose and other carbohydrates that are broken down to only glucose during digestion (starch, maltodextrin, maltose) tend to raise blood glucose levels after meals more than fructose, sucrose, galactose and lactose ^[35]. Glycemic response depends on the glycemic index of the food, amount of ingested carbohydrates, individual differences in glucose absorption and eventual presence of diabetes mellitus ^[4].

Glycemic index (GI) is a measure of the effect of carbohydrate foods on the blood glucose levels ^[36]. Foods with a high glycemic index trigger rapid and high increase of blood glucose levels. The reference food is glucose, which has glycemic index 100 ^[36].

• Examples of foods with low GI (55 or below): most fruits and vegetables, whole-grain breads and pasta, legumes/pulses, unsweetened dairy products, products extremely low in carbohydrates (some cheeses, nuts) or high in fructose
• Grains with GI 55-70: Pasta al-dente, parboiled rice, whole-grain bread and very high-fiber cereals.
• Examples of foods with high GI (70 or above): white rice, white bread, corn flakes, baked potatoes, watermelon, croissants, extruded breakfast cereals and sugar-sweetened soft drinks ^[37,38].
• Glycemic index and glycemic load of common foods
• A search tool for glycemic index and glycemic load

Glycemic load (GL) is a product of glycemic index (expressed as a percentage) and amount of digestible carbohydrates in a meal. GL = GI/100 x available carbohydrates in grams (available carbohydrates = total carbohydrates – dietary fiber) ^[36].

GL <10 is considered low and GL>20 is considered high ^[39]. Glycemic index and glycemic load values of foods are of a limited value in predicting blood glucose spikes, since blood glucose responses to foods may vary greatly within individuals, among individuals and with food characteristics ^[39]. For example, glycemic index of a given food may increase with ripeness, food processing and cooking time and decrease with co-ingestion of fats, proteins or insoluble fiber (whole grains).

Low-Carbohydrate (Ketogenic) Diet

There is SOME EVIDENCE that a low-carbohydrate diet can be effective in:

• Better control of seizures in children with epilepsy that does not respond to regular drugs; however, the low-carb diet is often poorly tolerated long-term ^[23,24]
• Glucose transporter type 1 deficiency syndrome (GLUT1 DS) ^[30,69]

There is INSUFFICIENT EVIDENCE about the effectiveness of a low-carbohydrate diet in the prevention or treatment of diabetes mellitus type 2 ^[75,77,78], heart disease ^[60,65], high blood pressure ^[18,65], high cholesterol ^[64], or in promoting weight loss ^{[56,57,58,60,64,65]}.

Possible harms of a low-carbohydrate diet

• Individuals on a very low-carbohydrate diet who do not consume any fruits and vegetables are at risk of developing vitamin deficiencies, such as vitamin B1 (thiamin) deficiency, which can result in a damage of the optic nerve and visual loss ^[27].
• One systematic review mentions that 16 children with epilepsy died during the low-carbohydrate diet ^[26], but it is not known, if the low-carbohydrate diet itself has contributed to deaths.
• Low-carbohydrate or other unbalanced diet may trigger symptoms in individuals with variegate porphyria ^[28,29].

High-Carbohydrate Diet

There is SOME EVIDENCE that consumption of 3 or more servings ofwhole grains and/or fruits and vegetables is associated with decreased risk of diabetes type 2 ^{[34,66,67,68,72]} and stroke ^[69,70,71]. NOTE: The health benefits of whole grains, fruits and vegetables may be associated either with dietary fiber or other non-carbohydrate nutrients or both.

Total carbohydrate intake does not seem to be associated with the risk of developing diabetes 2 ^[44,72], but a diet that is high in carbohydrates and with high glycemic index may be ^[33,76].

A diet high in carbohydrates (sugars and starch, but not sugar alcohols) may increase the risk of dental caries ^[14], but there is INSUFFICIENT EVIDENCE if replacing sugars with non-sugar artificial sweeteners reduces the risk ^[15].

There is INSUFFICIENT EVIDENCE about the effectiveness of any type of carbohydrates in improving cognitive performance in older individuals ^[55]. and of diets high in whole grains in the prevention of coronary heart disease ^[72,74].

Carbohydrates and Blood Lipids (Cholesterol, Triglycerides)

There is SOME EVIDENCE that diets with low glycemic index can:

• Lower LDL cholesterol and do not affect HDL cholesterol and triglyceride levels ^[40].

There is STRONG EVIDENCE that replacing saturated fats with carbohydrates lowers blood total and LDL cholesterol but also lowers HDL cholesterol and increases triglycerides ^{[20,32,61,63]} and does not seem to decrease the risk of coronary heart disease ^[19].

Carbohydrates and Diabetes

There is SOME EVIDENCE that:

• High total carbohydrate intake is not associated with the risk of developing diabetes 2 or high blood glucose or insulin levels or HbA1c values ^[15,44].
• High consumption of sugar-sweetened beverages (soft drinks, fruit drinks, iced tea, energy drinks and vitamin water drinks) is associated with increased risk of diabetes 2 ^[51].
• Low-glycemic index diets are associated with lower HbA1c values in individuals with diabetes type 2 ^{[42,46,47,48,49,53,54]}

There is INSUFFICIENT EVIDENCE about the preventative effect of low-carbohydrate ^[50] or low-glycemic ^{[15,41,45,52]} diets on the risk of developing diabetes type 2.

Carbohydrates and Physical Performance

The type and amount of carbohydrates and their ability to be absorbed and oxidized does not seem to have any effect on the physical performance in exercises lasting less than 1 hour ^[59,62].

In exercises lasting more than 2 hours, carbohydrates intake improves physical performance mainly by increasing the carbohydrate oxidation rate ^[59].