Carbohydrates, Fats and Proteins: Functions, Sources and Deficiencies

What to remember

• Carbohydrates are the main and cheapest source of energy (4 kcal/g); fats give 9 kcal/g and carry fat-soluble vitamins and essential fatty acids; proteins (4 kcal/g) build and repair the body.
• Protein = nitrogen × 6.25; biological value measures protein quality; kwashiorkor is protein deficiency with oedema, marasmus is deficiency of energy and protein with severe wasting.
• Essential fatty acids (linoleic and alpha-linolenic) and nine essential amino acids must come from food.

Carbohydrates

Carbohydrates contain carbon, hydrogen and oxygen, with hydrogen and oxygen usually in the ratio 2:1 as in water.

Classification

Digestion: starch is acted on by salivary and pancreatic amylase to maltose; the enzymes maltase, sucrase and lactase in the small intestine produce glucose, fructose and galactose, which are absorbed.

Functions

• Main source of energy: 4 kcal per gram. The brain and red blood cells depend on glucose.
• Protein-sparing action: with enough carbohydrate, protein is used for growth and not burned for energy.
• Prevent ketosis: without enough carbohydrate, fats are burned incompletely and ketone bodies build up. At least about 50-100 g carbohydrate a day is needed to avoid this.
• Dietary fibre: adds bulk, prevents constipation, lowers blood cholesterol and glucose rise, and helps prevent colon problems.
• Lactose helps calcium absorption.

Sources: cereals, millets, roots and tubers, sugar, jaggery, honey, fruits (fructose), milk (lactose). Fibre: whole grains, pulses, vegetables, fruits with skin.

Glycemic index (GI) shows how fast a carbohydrate food raises blood glucose compared with glucose. Whole grains, pulses and most vegetables have a low GI; white rice, white bread and glucose have a high GI.

Deficiency and excess: a low carbohydrate diet leads to weight loss, weakness, ketosis and loss of body protein. Too much (with too little exercise) leads to obesity, diabetes risk and dental caries. A low-fibre diet causes constipation and piles.

Recommended: about 55-60% of total energy from carbohydrate; added sugars should be small.

Fats and lipids

Lipids include fats (solid at room temperature) and oils (liquid), phospholipids (lecithin) and sterols (cholesterol). A triglyceride is one glycerol with three fatty acids.

Classification of fatty acids

Essential fatty acids (EFA): linoleic acid and alpha-linolenic acid; the body cannot make them. Arachidonic acid can be made from linoleic acid. EPA and DHA are important for brain and eye development.

Trans fats form during partial hydrogenation (vanaspati, bakery fats). They raise LDL ("bad") cholesterol and lower HDL ("good"), so intake should be as low as possible. Cholesterol is found only in animal foods (egg yolk, organ meats, butter, ghee); vegetable oils have none.

Functions

• Concentrated energy: 9 kcal per gram.
• Carry and help absorb fat-soluble vitamins A, D, E and K.
• Supply essential fatty acids.
• Form cell membranes and hormones (cholesterol is needed to make steroid hormones and vitamin D).
• Insulation and protection of organs; give satiety and flavour.

Sources: visible fats such as oils, ghee, butter, vanaspati; invisible fats in nuts, oilseeds, milk, egg, meat, fish.

Deficiency: a diet low in fat leads to loss of weight, poor absorption of fat-soluble vitamins, and EFA deficiency (dry, scaly skin, poor growth and poor wound healing). Excess: obesity, raised blood cholesterol, atherosclerosis and heart disease.

Guide: about 20-30% of energy from fat; saturated fat should be a small part; use a mix of different oils for a good balance of fatty acids.

Rancidity: spoilage of fats by oxidation (oxidative) or by lipase enzymes (hydrolytic); antioxidants such as vitamin E slow it. Useful constants: iodine value shows degree of unsaturation (higher iodine value, more double bonds); saponification value shows the average chain length of fatty acids.

Proteins

Proteins are chains of amino acids and contain nitrogen (about 16%). Therefore protein = nitrogen × 6.25 (100 ÷ 16).

Amino acids: about 20 amino acids make proteins. Nine are essential for adults: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. (Infants also need arginine in the early period.) Non-essential amino acids (alanine, glutamic acid, etc.) can be made by the body.

Classification of proteins by quality

• Complete (first-class): contain all essential amino acids in good proportion; e.g. milk, egg, meat, fish (casein, albumin, myosin).
• Partially incomplete: support life but not growth; e.g. most cereals and pulses (gliadin in wheat).
• Incomplete: lack one or more essential amino acids; e.g. zein in maize (lacks tryptophan and lysine) and gelatin (lacks tryptophan).

Measures of protein quality (worked formulas)

• Biological value (BV) = (nitrogen retained ÷ nitrogen absorbed) × 100.
• Digestibility coefficient (true digestibility) = (nitrogen absorbed ÷ nitrogen intake) × 100.
• Net protein utilisation (NPU) = BV × digestibility coefficient ÷ 100 = nitrogen retained ÷ nitrogen intake × 100.
• Protein efficiency ratio (PER) = gain in weight (g) ÷ protein eaten (g) in growing young rats.
• Chemical score = amino acid in test protein ÷ same amino acid in reference protein × 100 (lowest one is the limiting amino acid).
• Egg and human milk proteins have the highest BV.

Worked example 1: 3 g of a food protein contains 0.48 g nitrogen. Protein = 0.48 × 6.25 = 3 g.

Worked example 2: Nitrogen intake 10 g, nitrogen absorbed 9 g, nitrogen retained 7.2 g. BV = 7.2/9 × 100 = 80. Digestibility = 9/10 × 100 = 90%. NPU = 80 × 90/100 = 72.

Nitrogen balance: positive in growth, pregnancy and recovery; negative in fever, injury and starvation.

Functions

• Growth, maintenance and repair of tissues (body-building).
• Enzymes, hormones (insulin), antibodies, haemoglobin and transport proteins.
• Maintain fluid balance and acid-base balance (plasma proteins; albumin holds water in blood).
• Provide energy (4 kcal per gram) when carbohydrates and fats are short.
• Blood clotting and muscle contraction.

Sources: milk and milk products, egg, meat, fish, pulses, soybean, nuts, oilseeds, cereals (smaller amount but large because of the quantity eaten). Planning for children: use mixtures such as cereal-pulse or cereal-milk.

Recommended intake for a healthy adult is about 1 g per kg body weight per day (a little less in the latest ICMR figures); it is higher in infants, children, pregnancy and lactation. About 10-15% of energy should come from protein.

Protein-energy malnutrition (PEM)

Marasmic kwashiorkor shows features of both. Treatment: gradual refeeding in a hospital or nutrition rehabilitation centre, treating infection and dehydration, and micronutrient supplementation; prevention by breastfeeding, timely complementary feeding and immunisation.

Gomez classification (weight for age against a standard): Normal above 90%; Grade I (mild) 75-90%; Grade II (moderate) 60-75%; Grade III (severe) below 60%. Weight for height and height for age give wasting and stunting respectively (Waterlow). In community programmes, mid-upper arm circumference (MUAC) is a quick check: below 11.5 cm is severe acute malnutrition in children aged 6 months to 5 years.

Comparison table: three macronutrients

Exam traps

• Glycogen is the animal storage carbohydrate; starch is the plant storage carbohydrate.
• Fat gives 9 kcal/g; carbohydrate and protein 4 kcal/g.
• Cholesterol occurs only in animal foods; vegetable oils are cholesterol-free.
• Linoleic acid is omega-6; alpha-linolenic acid is omega-3.
• Kwashiorkor has oedema; marasmus does not.
• Marasmus is a lack of total energy and protein; kwashiorkor is mainly protein lack.
• Zein of maize lacks tryptophan and lysine.
• NPU = BV × digestibility; PER is based on weight gain per gram of protein eaten.

One-liners

• 1. Protein contains about 16% nitrogen.
• 2. Nitrogen × 6.25 gives the protein value.
• 3. Lysine is the limiting amino acid in cereals.
• 4. Methionine is limiting in pulses.
• 5. Lactose is made of glucose and galactose.
• 6. Sucrose is made of glucose and fructose.
• 7. Cellulose is dietary fibre that is not digested by humans.
• 8. Linoleic acid is an essential fatty acid.
• 9. Trans fat raises LDL cholesterol.
• 10. Flaky-paint dermatosis is a sign of kwashiorkor.
• 11. Marasmus gives an "old man" look.
• 12. Nine amino acids are essential for adults.

Practice questions

1. The storage form of carbohydrate in the human body is

   1. Starch
   2. Cellulose
   3. Sucrose
   4. Glycogen
   Answer

   D. Glycogen

   Glycogen is stored in liver and muscle.

2. Lactose is made of

   1. Two glucose units
   2. Fructose and galactose
   3. Glucose and fructose
   4. Glucose and galactose
   Answer

   D. Glucose and galactose

   Sucrose is glucose + fructose; maltose is glucose + glucose.

3. The sweetest of the natural sugars is

   1. Maltose
   2. Lactose
   3. Glucose
   4. Fructose
   Answer

   D. Fructose

   Fructose is found in fruits and honey.

4. Which carbohydrate is not digested by human enzymes and acts as dietary fibre?

   1. Starch
   2. Maltose
   3. Cellulose
   4. Glycogen
   Answer

   C. Cellulose

   Humans lack cellulase.

5. The protein-sparing action of carbohydrate means

   1. Protein is not needed
   2. Fat is made from protein
   3. Protein is saved for growth and repair
   4. Protein is turned into fat
   Answer

   C. Protein is saved for growth and repair

   With enough energy from carbohydrate, protein is not burned for energy.

6. Ketosis occurs when the diet is lacking in

   1. Vitamin C
   2. Calcium
   3. Fibre
   4. Carbohydrate
   Answer

   D. Carbohydrate

   Fat is burned incompletely and ketone bodies accumulate.

7. Foods with a high glycemic index

   1. Contain no carbohydrate
   2. Lower blood glucose at once
   3. Cannot be digested
   4. Raise blood glucose quickly
   Answer

   D. Raise blood glucose quickly

   GI compares the blood-glucose rise with that of glucose.

8. The essential fatty acids are

   1. Oleic and palmitic acids
   2. Butyric and lauric acids
   3. Linoleic and alpha-linolenic acids
   4. Stearic and palmitic acids
   Answer

   C. Linoleic and alpha-linolenic acids

   The body cannot make them.

9. Linoleic acid belongs to the family of

   1. Omega-6 fatty acids
   2. Omega-3 fatty acids
   3. Saturated fatty acids
   4. Trans fatty acids
   Answer

   A. Omega-6 fatty acids

   Alpha-linolenic acid is omega-3.

10. Cholesterol is present only in

    1. Cereals
    2. Vegetable oils
    3. Fruits
    4. Foods of animal origin
    Answer

    D. Foods of animal origin

    Plant foods contain no cholesterol.

11. Trans fatty acids are formed mainly during

    1. Sprouting of grains
    2. Boiling of rice
    3. Partial hydrogenation of oils
    4. Fermentation of milk
    Answer

    C. Partial hydrogenation of oils

    They raise LDL and lower HDL cholesterol.

12. A fat with a high iodine value has

    1. Less energy per gram
    2. More unsaturation
    3. More cholesterol
    4. More saturation
    Answer

    B. More unsaturation

    Iodine value measures double bonds.

13. Which is an essential amino acid?

    1. Lysine
    2. Glutamic acid
    3. Alanine
    4. Serine
    Answer

    A. Lysine

    Lysine is one of the nine essential amino acids.

14. The number of amino acids essential for adults is

    1. Nine
    2. Twelve
    3. Five
    4. Twenty
    Answer

    A. Nine

    Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

15. The protein zein of maize lacks

    1. Methionine only
    2. Tryptophan and lysine
    3. Leucine and valine
    4. Histidine only
    Answer

    B. Tryptophan and lysine

    Zein is an incomplete protein.

16. A first-class (complete) protein is found in

    1. Maize
    2. Wheat
    3. Milk and egg
    4. Gelatin
    Answer

    C. Milk and egg

    They contain all essential amino acids in good proportion.

17. Protein-energy malnutrition with oedema, moon face and flaky-paint skin is

    1. Rickets
    2. Marasmus
    3. Scurvy
    4. Kwashiorkor
    Answer

    D. Kwashiorkor

    Oedema is the key feature of kwashiorkor.

18. A severely wasted infant with an 'old man' face and no oedema has

    1. Beriberi
    2. Pellagra
    3. Marasmus
    4. Kwashiorkor
    Answer

    C. Marasmus

    Marasmus is energy and protein deficiency.

19. Kwashiorkor is typically seen in children aged about

    1. Under 3 months
    2. 1 to 3 years
    3. 8 to 10 years
    4. Adolescents only
    Answer

    B. 1 to 3 years

    It often follows weaning or the birth of a new baby.

20. Which classification of malnutrition uses weight-for-age against a standard?

    1. Bitot classification
    2. Waterlow only for height
    3. Gomez classification
    4. Apgar score
    Answer

    C. Gomez classification

    Gomez grades by percent of standard weight for age.

21. Mid-upper arm circumference below which value signals severe acute malnutrition in children of 6 months to 5 years?

    1. 11.5 cm
    2. 18 cm
    3. 13.5 cm
    4. 15 cm
    Answer

    A. 11.5 cm

    MUAC below 11.5 cm is severe; 11.5 to 12.5 cm is moderate.

22. Nitrogen in a protein weighs 2.4 g. The protein present is

    1. 24 g
    2. 6.25 g
    3. 9.6 g
    4. 15 g
    Answer

    D. 15 g

    2.4 × 6.25 = 15 g.

23. A food contains 50 g protein. Its nitrogen is about

    1. 8 g
    2. 12.5 g
    3. 3.1 g
    4. 16 g
    Answer

    A. 8 g

    50 ÷ 6.25 = 8 g (16% of 50).

24. A food has 1.6 g nitrogen per 100 g. The protein content per 100 g is

    1. 25.6 g
    2. 10 g
    3. 1.6 g
    4. 6.25 g
    Answer

    B. 10 g

    1.6 × 6.25 = 10 g.

25. Nitrogen absorbed is 10 g and nitrogen retained is 8 g. The biological value is

    1. 18
    2. 20
    3. 125
    4. 80
    Answer

    D. 80

    BV = 8/10 × 100 = 80.

26. Nitrogen intake is 12 g and nitrogen absorbed is 9.6 g. True digestibility is

    1. 125%
    2. 80%
    3. 96%
    4. 20%
    Answer

    B. 80%

    9.6/12 × 100 = 80%.

27. A protein has a BV of 70 and digestibility of 90%. Its NPU is

    1. 63
    2. 7.0
    3. 80
    4. 160
    Answer

    A. 63

    NPU = 70 × 90/100 = 63.

28. Young rats gained 45 g while eating 15 g of a test protein. The PER is

    1. 60
    2. 0.33
    3. 3.0
    4. 30
    Answer

    C. 3.0

    PER = weight gain ÷ protein eaten = 45/15 = 3.0.

29. A child weighs 6 kg; the standard weight for age is 12 kg. By the Gomez classification the grade is

    1. Normal
    2. Grade I
    3. Grade II
    4. Grade III
    Answer

    D. Grade III

    6/12 = 50%, below 60% = Grade III (severe).

30. A diet of 2400 kcal gives 30% of energy from fat. The fat in grams is

    1. 100 g
    2. 80 g
    3. 72 g
    4. 60 g
    Answer

    B. 80 g

    30% of 2400 = 720 kcal; 720/9 = 80 g.

31. A diet has 300 g carbohydrate, 40 g protein and 50 g fat. Its energy is

    1. 1810 kcal
    2. 2050 kcal
    3. 1620 kcal
    4. 1500 kcal
    Answer

    A. 1810 kcal

    1200 + 160 + 450 = 1810 kcal.

32. An adult weighing 60 kg takes protein at 1 g per kg. The daily protein is

    1. 120 g
    2. 30 g
    3. 60 g
    4. 6 g
    Answer

    C. 60 g

    1 × 60 = 60 g.

33. The test protein has 2.5 g lysine per 100 g protein; the reference has 5.0 g. The chemical score for lysine is

    1. 25
    2. 200
    3. 75
    4. 50
    Answer

    D. 50

    2.5/5.0 × 100 = 50.

34. Which of the following statements is/are correct? 1. Glycogen is the storage carbohydrate in animals. 2. Starch is the storage carbohydrate in animals.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Starch is the plant storage carbohydrate.

35. Which of the following statements is/are correct? 1. Marasmus shows oedema. 2. Kwashiorkor shows oedema.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Oedema is absent in marasmus.

36. Which of the following statements is/are correct? 1. Vegetable oils contain no cholesterol. 2. Ghee contains no cholesterol.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Ghee, an animal fat, contains cholesterol.

37. Which of the following statements is/are correct? 1. Dietary fibre is digested to give 9 kcal/g. 2. Dietary fibre can help to prevent constipation.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Fibre is not digested by humans.

38. Which of the following statements is/are correct? 1. Biological value = nitrogen retained ÷ nitrogen absorbed × 100. 2. NPU = BV × digestibility ÷ 100.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both formulas are right.

39. Which of the following statements is/are correct? 1. Zein is a complete protein. 2. Egg protein is of very high biological value.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Zein is incomplete.

40. Which of the following statements is/are correct? 1. Fats help in absorption of vitamins A, D, E and K. 2. Fats carry water-soluble vitamins.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Fat carries the fat-soluble vitamins.

41. Which of the following statements is/are correct? 1. Palmitic acid is an essential fatty acid. 2. Linoleic acid is an essential fatty acid.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Palmitic acid can be made by the body.

42. Which of the following statements is/are correct? 1. Trans fats raise LDL cholesterol. 2. Trans fats raise HDL cholesterol.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    They lower HDL.

43. Which of the following statements is/are correct? 1. Protein gives 9 kcal per gram. 2. Albumin helps to hold water in the blood.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Protein gives 4 kcal/g.

44. Which of the following statements is/are correct? 1. Sucrose is a disaccharide. 2. Lactose is a monosaccharide.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Lactose is a disaccharide.

45. Which of the following statements is/are correct? 1. Low carbohydrate intake can cause ketosis. 2. A diet very low in fat can cause essential fatty acid deficiency.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.