carbohydrate (CHO)

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carbohydrate (CHO)

the most abundant and economic source of food energy in the human diet, comprising 40-80% of total energy intake in different populations. The recommended ideal is at least 50-55% and a high-carbohydrate diet is defined as one providing more than 55% of energy as CHO. Contained in breads, cereals, fruits, vegetables, milk and dairy products, soft drinks, cakes, biscuits and pastry. One gram of CHO provides ∼4 kcal of energy. In Western diets about 60% of dietary CHO is in the form of polysaccharides of D-glucose, mostly starch, and about 25% 'free sugars', mainly sucrose. The quantity and quality of carbohydrate consumed have impact on energy balance, digestive function, insulin sensitivity and blood lipids. CHO is present in the blood as glucose and carbohydrate stores are in the form of glycogen in the liver and skeletal muscle and to a small extent in other tissues including, importantly, the brain. See also glycaemic index; Table 1, Table 2, Table 3, Table 4.
Table 1: Macronutrients
Energy valueReference nutrient intake (RNI)SourcesFunctionsDeficiencyExcessNotes
Carbohydrate 1 g yields 16 kJ (3.75 kcal)Minimum of 47% of total daily energy intake; not >10% should be provided by added sugars. RNI for non-starch polysaccharide: 18 g/dSugar, potatoes, rice, pasta, noodles, bread, breakfast cerealsProvides energy for metabolismWeight loss, ketosisObesity, elevated blood triglycerideDiets high in carbohydrate tend to be low in fat
Protein 1 g yields 17 kJ (4 kcal)
  • About 15% of total daily energy intake: e.g.
  • Women 45 g/d
  • Men 55 g/d
Meat, fish, eggs, nuts, pulses, dairy products, tofu, QuornComponent of all body tissues; energy source in some situationsRetarded growth; weight loss and muscle wasting; poor wound healing; impaired immune system; fat deposition in the liverPossible link with loss of minerals from bone and age-related deterioration in renal functionProtein content of Western diets is usually higher than the RNI
Fat 1 g yields 37 kJ (9 kcal)Should not exceed 35% of total daily energy intake. Of this no more than 10% should be saturated fatty acidsButter and other full-fat dairy products, margarine; cooking oils and fried food; pastry, cakes, biscuits; meat, oily fish, seeds, nuts, chocolate, crispsProvides energy for metabolism; energy stores and insulation in body fat; synthesis of steroid hormones; constituent of cell membranes, and of nerve fibresWeight loss; deficiency of essential fatty acids can lead to neurological damageObesity, with increased risk e.g. of cardiovascular disease and some cancersNormal development of the nervous system depends on the essential fatty acids, linoleic and alpha linolenic
Table 2: Micronutrients: vitamins
VitaminRNI (DoH 1991)SourcesAction/functionsDeficiencyExcessSpecial points
Water soluble
Vitamin B group
B1 Thiamin(e)0.4 mg/1000 kcalFortified breakfast cereals, yeast extract, vegetables, fruit, wholegrain cereals, milk, liver, eggs, porkCoenzyme for carbohydrate metabolismEncephalopathy can occur with alcohol excess and low food intake. Beri-beri where polished rice is stapleHeadache, insomnia, irritability, contact dermatitisRequirement related to carbohydrate intake
B2 Riboflavin
  • Female 1.1 mg/d
  • Male 1.3 mg/d
Milk, milk products, offal, yeast extract, fortified breakfast cerealsCoenzyme for the metabolism of carbohydrate, fat and proteinFissures at corners of mouth; tongue inflammation; corneal vascularizationNo toxic effects, since large quantities are not absorbedDestroyed by sunlight
B3 Niacin (nicotinic acid and nicotinamide)6.6 mg/1000 kcal as nicotinic acid equivalentsMeat, fish, yeast extract, pulses, wholegrains, fortified breakfast cerealsEnergy metabolism, as part of coenzymes NAD and NADP involved in oxidation and reduction reactionsPellagra: dermatitis, diarrhoea and dementiaLiver damage, skin irritationAlso synthesized from the amino acid tryptophan
B5 Pantothenic acidNone setWidespread in food, e.g. liver, eggs, yeast, vegetables, pulses, cerealsProtein, fat, carbohydrate and alcohol metabolismVomiting, insomniaNot reported
B6 Pyridoxine
  • Female 1.2 mg/d
  • Male 1.4 mg/d
Meat, fish, eggs, some vegetables, wholegrainsProduction of haemoglobin and of coenzymes involved in many metabolic processesRare. Metabolic and nervous system disordersPeripheral nerve damageRequirement is related to protein intake
BiotinNone setWidely distributed in many foods, e.g. offal, egg yolk, legumes, etc. Can be synthesized by intestinal bacteriaEssential in fat metabolismRare; dermatitis, hair loss, nausea, fatigue and anorexiaNone known
B12 Cobalamins15 μg/g of proteinAnimal products, meat, eggs, fish, dairy products, yeast extract
  • Essential for red blood cell formation and nerve myelination.
  • Needed for use of folate
Anaemia; irreversible spinal cord damageNot reported
  • Absorption requires 'intrinsic factor' produced by the stomach.
  • Only in foods of animal origin, so strict vegetarians and vegans require supplements
Folates (folic acid)200 μg/dGreen leaf vegetables, bread, fortified breakfast cereals, yeast extract, liverRed blood cell production; DNA synthesis
  • Anaemia; growth retardation. May contribute to Alzheimer's.
  • Fetal defects
Can mask the effects of B12 deficiencySupplements before and during pregnancy reduce the incidence of spinal cord defects
Vitamin C ascorbic acid40 mg/d
  • Citrus fruits, kiwi fruit, blackcurrants, strawberries; green peppers, green leaf vegetables, potatoes, tomatoes.
  • Content decreases with storage
Collagen synthesis, formation of bones, connective tissue, teeth. Iron absorption for red blood cell production. Acts as an antioxidantSore mouth and gums; capillary bleeding; scurvy; delayed wound healing, scar break downDiarrhoea; oxalate stones in kidneysDestroyed by cooking in the presence of air and by plant enzymes released when cutting and grating raw food
Fat soluble
Vitamin A retinol
  • Female 600 μg/d
  • Male 700 μg/d
As retinol in liver, kidney, oily fish, egg yolk, full-fat dairy produce. As the provitamin carotenes in green, yellow, orange and red fruit and vegetables, e.g. broccoli, carrots, apricots, mangoes, sweet potatoes and tomatoesVisual pigments in retina, aids night vision. Normal growth and development of tissues; essential for healthy skin and mucosae. Acts as an antioxidantPoor growth; rough dry skin and mucosae; xerophthalmia and eventual blindness; increased risk of infection; poor night visionIn pregnancy, high doses can cause fetal malformationsSynthesized in the body from carotenes present in the diet
Vitamin D cholecalciferol ergosterol10 μg/d if houseboundOily fish, egg yolk, butter, fortified margarine; action of ultraviolet rays (sunlight)Calcium and phosphorus homeostasisRickets (children); osteomalacia (adults)Rare; weight loss and diarrhoeaProduced in the body by action of sunlight on a provitamin in the skin: deficiency develops in those who are not exposed to sun
Vitamin E tocopherols tocotrienesNone setWheat germ, vegetable oils, nuts, seeds, egg yolk, cereals, dark green vegetables
  • Antioxidant.
  • Protects against cell membrane damage
Neurological abnormalities; anaemia: rare, from malnutrition or malabsorptionMuscle weakness, gastrointestinal disordersRequirement is increased with increased intake of polyunsaturated fatty acids
Vitamin K phylloquinones menaquinonesNone setGreen leafy vegetables, fruit and dairy productsNeeded for the production of prothrombin and other coagulation factorsImpaired clotting; liver damageNot so far observed from naturally occurring vitaminSynthesized by intestinal bacteria so deficiency unusual
Table 3: Micronutrients: minerals
Name and chemical symbolReference nutrient intake (adults, per day)SourcesFunctionsDeficiencyExcess
Calcium Ca700 mgMilk and milk products, green vegetables, soya beans, white bread, hard water
  • Crucial role in all cellular function, in neural transmission, muscle contraction, blood coagulation.
  • As phosphate in bones and teeth
  • Dietary deficiency not uncommon.
  • Rickets, osteomalacia from failure of Ca absorption in Vit D deficiency.
  • Low blood [Ca2+] causes tetany
Calcium deposits in soft tissue can occur, but probably not related to high intake
Chlorine Cl3.4 g (as chloride)Salt-containing foods
  • Major anion in ECF.
  • Role in maintaining electrical gradient across cell membranes
Unlikely with normal dietAs NaCl, risk factor for high blood pressure
Chromium Cr25 μgVegetables, cereals, meats, vegetable oils, whole grainsCo-factor for some enzymes involved in glucose and energy metabolism
  • Rare.
  • Impaired glucose metabolism
  • Inhibition of enzymes.
  • Occupational exposures can cause skin and kidney damage
Copper Cu900 μgMeat, drinking waterCo-factor for some enzymes; intermediate in electron transfer during oxidative phosphorylationLow activity of antioxidant enzymesVery high intake can cause liver damage
Iodine I140 μgSeafood, iodized salt, milk and milk products, meat and eggsSynthesis of thyroid hormonesThyroid swelling (goitre) with hypothyroidism: low BMR, lethargyRarely any effect; may exacerbate some skin diseases
Iron Fe
  • Women 14.8 mg
  • Men 8.7 mg
Liver, kidney, red meat, egg yolk, wholegrains, pulses, dark green vegetables, dried fruit, treacle, cocoa, molassesComponent of haemoglobin, myoglobin and many enzymes
  • Iron deficiency anaemia not uncommon.
  • In childhood, poor growth; impaired intellectual development
Can be toxic if very excessive. (from blood transfusions rather than from diet); gastrointestinal upset; may promote vascular disease
Fluoride F3-4 mgDrinking water, mostly as calcium fluoride; tea, seafoodMay be important in maintenance of bone structureIncreased risk of tooth decayUnlikely from dietary sources
Magnesium Mg
  • Women 270 mg
  • Men 300 mg
Cereals, milk, nuts, seeds, and green vegetablesCo-factor for enzymes essential in metabolism; role in calcium homeostasis; skeletal development; neuromuscular functionUncommon; can occur with malabsorption or in chronic renal failure, when it accompanies hypocalcaemiaUnlikely from dietary sources
Phosphorus-P550 mg (as phosphate)Milk, cheese, yogurt, meat, poultry, grains, fishAdenosine phosphate compounds vital in energy metabolism. With Ca in bones and teethOnly in severe malnutrition; muscle weakness, bone pain, rickets, anorexia, anaemiaIn treatment of osteoporosis or bone cancer with biphosphonates
Potassium K3.5 gFruit, vegetables, meat, wholegrains
  • Major intracellular cation; muscle contraction and nerve excitability.
  • Linked to acid-base regulation
  • Poor dietary intake rare. Can occur with prolonged use of diuretics and purgatives.
  • Muscular weakness; depression; confusion; cardiac arrhythmia
High ECF [K+] (hyperkalaemia) causes cardiac arrest
Selenium Se
  • Women 50 μg
  • Men 70 μg
Seafood, meat, grains, wheat flourKey component in the endogenous antioxidant, glutathione peroxidaseHealth implications of low intake in UK currently under DoH review. May cause abnormality of heart muscleExcessive supplements: hair loss, skin rash, neurological disorder
Sodium Na1.6 gMainly as salt: table salt, and in milk, meat, vegetables, sauces, pickles, processed foods, snacks, cheeseMajor extracellular cation; linked to ECF volume, hence to blood volume and blood pressure. Component of bone mineralLoss in sweat and diarrhoea; dilution in body fluids due to excess water intake. Weakness, cramp; faintness, confusionOedema, hypertension
Zinc Zn
  • Women 7.0 mg
  • Men 9.5 mg
Red meat, dairy products, eggs, wholegrains, peas, beans, nuts, lentils
  • Co-factor for many enzymes.
  • Synthesis of some proteins. Wound healing; immune system; physical and sexual development
  • Retarded skeletal growth; sexual immaturity.
  • Anorexia, fatigue
  • Nausea, vomiting, or anaemia with chronic excess.
  • Also decreases iron and copper bioavailability
Table 4: Ergogenic aids: supplements used by athletes
SubstanceDescriptionClaimed ergogenic effectSupporting evidence
With clear scientific evidence
CaffeineStimulant in coffee and tea
  • Benefits performance by improving alertness, concentration, reaction time.
  • Increases fat oxidation during endurance exercise.
Improves performance in most events, except very short high-intensity exercise; increases cognitive functioning during exercise.
CreatineCarrier of high-energy phosphates in muscleIncreases the energy reserve, improves strength, reduces fatigue, and increases protein synthesisIncreases intramuscular Cr and PCr; improves performance in repeated sprint bouts (and reported to do so after even a single bout); improves recovery between bouts (but response varies between individuals). Anabolic properties unclear.
  • Sodium bicarbonate
  • Sodium citrate
BuffersImproves high-intensity exercise performance by limiting decrease in pH in ECF as a whole and indirectly in muscle ICFLarge doses can improve performance
With mixed scientific evidence
Antioxidant nutrientsVitamins, especially C and EProvides protection against muscle damage by reducing oxidative stressBenefits established at cellular level; no detectable aid to performance
ArginineAmino acid in normal dietStimulates release of growth hormone, promoting gain in muscle mass and strengthSome evidence of GH promotion when combined with other amino acids (ornithine, lysine, BCAA); no conclusive evidence of effect when taken alone
Branched-chain amino acids (BCAA)Leucine, isoleucine and valine
  • Retards the development of central fatigue and so improves performance.
  • Improves efficiency of training
No good evidence of improved endurance performance. Evidence of accelerated recovery from muscle fatigue when given with other amino acids during eccentric exercise training
GlutamineAmide of amino acid glutamateMaintains a healthy immune system during training and improves muscle glycogen resynthesisDoes not affect immune function; possibly affects muscle glycogen resynthesis
GlycerolComponent of triacylglycerol moleculeInduces hyperhydration, decreases heat stress, and improves performanceDoes have the first two actions, but effects on performance are unclear
Lacking scientific support
AndrostenedioneSynthetic productIncreases testosterone and thus muscle mass and strength, and improves recoveryDoes not increase testosterone secretion; has no effect on strength
Hydroxy-methyl butyrate (HMB)Metabolite of the amino acid leucineEnhances gain in body mass and strength associated with resistance training, and improves recoveryPossible small effects only on lean body mass and strength
BoronMicronutrient present in vegetables and non-citrus fruitsIncreases testosterone levels, to improve bone density, muscle mass, and strengthImproves bone mineral density in postmenopausal women; no effect on bone density, muscle mass or strength in men
CarnitineSubstance important for fatty acid transport into mitochondriaImproves fat oxidation, helps weight lossNo supporting evidence
CholinePrecursor of acetylcholineImproves performance, decreases fatigue and enhances fat metabolismNo supporting evidence
Chromium (chromium picolinate)Micronutrient that potentiates insulin actionPromotes fat oxidation and muscle buildingNo supporting evidence
Coenzyme Q10Part of the electron transport chain in the mitochondriaImproves aerobic capacity and cardiovascular dynamicsNo supporting evidence
GinsengRoot of the Araliaceous plantImproves strength, performance, stamina, and cognitive functioning; reduces fatigueNo supporting evidence
InosineNucleoside found naturally in brewer's yeast and organ meatsIncreases ATP stores, improve strength, training quality, and performanceNo supporting evidence
Medium-chain triacylglycerols (MCT)Triglycerides containing fatty acids with a carbon chain length of 6-10Improves energy supply, reduces rate of muscle glycogen breakdown, and improves performanceNo supporting evidence
PyruvateEnd-product of aerobic glycolysisImproves endurance capacity and recovery; increases glycogen storageLimited supporting evidence
PolylactatePolymer of lactateProvides energyNo effects on performance
Wheat germ oilWheat embryo extractImproves enduranceNo supporting evidence
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