This document provides an overview of key nutrients including carbohydrates, proteins, fats, vitamins, and minerals. It discusses the classification, functions, daily requirements, deficiency and excess of each nutrient. Carbohydrates, proteins and fats are the three macronutrients that provide calories and energy. Carbohydrates include starch, sugars and fiber. Proteins are made of amino acids and are essential for growth and repair. Fats provide calories and help absorb fat-soluble vitamins. Vitamins and minerals are micronutrients that enable various chemical reactions in the body. A balanced diet should contain the appropriate amounts of each nutrient to maintain good health.

1. NUTRIENTS
(CLASSIFICATION,FUNCTION,EXCESS & DEFICIENCYDAILYREQUIREMENT)
RANJANA CHATAUT

2. Introduction
• Nutrients are substances derived from food during the process of
• digestion. Nutrients are chemical substances your body uses to build, maintain,
and repair tissues. They also empower cells to send messages back and forth to
conduct essential chemical reactions, such as the ones that make it possible for
you to
o Breathe
o See
o Move
o Hear
o Eliminate waste
o Smell
o Think Taste
• . . . and do everything else natural to a living body

3. • Our diet should provide adequate amount of all nutrients to
maintain good health and physical efficiency.
• Daily food intake should be such that of the total consumption,
about 15 to 20% is protein, 40 to 50% is complex
Carbohydrates and 20 to 30% is fat.

4. • Healthy Diets Require
• Six categories of nutrients: (Macro& Micro)
• 1. Water
• 2. Carbohydrate- sugars, starch, cellulose
• 3. Protein
• 4. Fats
• 5. Vitamins
• 6. Minerals

5. CARBOHYDRATE, PROTEIN, FATS AND
MINERALS

6. CARBOHYDRATE
• Carbohydrates are the polyhydroxy organic compounds
• Made up of carbon, hydrogen and oxygen
• Major source of energy

7. FUNCTION
• Energy: supply energy for immediate age by the body
• Liver: glycogen protects the liver against bacterial toxins and poisonous substances.
• CNS: glucose is the food required by the brain for its metabolism
• Heart: the efficiency of the heart as a pump depends upon carbohydrates as fuel

8. CONTD…..
General
• Glycogen acts as important storage of food material of the
organism.
• metabolism of aminoacids and fatty acids
• Protective function ( protective coat such as glycoclayx that covers
intestinal epithelium& mucous)
• Act as intermediates in respiration and carbohydrates metabolism
e.g., (trioses).
• Participate in lipid synthesis.

9. Sources
 There are mainly three source of carbohydrate
– Starch : Complex carbohydrate found in cereals , roots ,and tubers
– Sugar : Simple Carbohydrates
• Glucose: Blood
• Fructose: Fruit
• Galactose: Milk
• Sucrose: Table sugar
 Monosaccharides: fruits, berries, vegetables and honey.
 Disaccharides: table sugar, sugar beet, sugar cane and fruits.
– Cellulose( contribute to fibers) : indigested component of carbohydrate with scarcely
any nutritive value , contributes to dietary fiber.
– Insoluble fiber: cellulose in vegetables; provides roughage
– Soluble fiber: pectin in apples and citrus fruits; reduces blood cholesterol
levels

10. Other sources of dietary carbohydrates include:
• Oligosaccharides: grains and vegetables
• Starch polysaccharides: cereals, whole grains, rice, pasta,
potatoes, peas, corn and legumes.
• Non-starch polysaccharides: dietary fiber such as
cellulose(main part of cell wall in plantseg.bark,leaves,)
hemicelluloses(cellular fibres of plant cell), pectins and gums.

11. Daily Requirements
Age Amount
0-6 months 108cal/kg
7-12 months 98 cal/kg
1-3 years 1240cal/day
4-6 years 1690 cal/day
7-9 years 1950 cal/day
10-14 years 3100cal/day
For man 25 yrs of 65 kg 3200 cal/day
For woman 25 yrs of 55 kg 2300 cal/day
For pregnancy Daily increase of about 300 calories during second and third
trimester
For lactation Daily 600 calories increased from the normal requirement

12. Dietary requirements
• –Minimum 100 g/day to maintain adequate blood glucose
levels
• RDA:400g
–Recommended minimum 130 g/day
–Recommended intake: 45–65% of total calorie intake; mostly
complex carbohydrates

13. Deficiency Diseases
• Acidosis
• Ketosis
• Hypoglycemia
• Fatigue & decreased energy levels
• Unhealthy weight loss
• Loss of sodium
• Glycosuria (excess of sugar in urine)
• Galactosemia(inability to metabolize galactose )
• Diarrhoea and flatulence
• Under weight.

14. Excess
• Hyperglycemia
• Diabetes
• Obesity

15. Diabetes
• Type I Diabetes
• 1. juvenile onset(before
• 40)
• hyposecretion of insulin
• body does not produce insulin (insulin dependant)
• (IDDM)
• • Type II Diabetes
• 1.late onset (adult)
• Insulin produced is less Efficient & managed by exercise &diet

16. • sugar in blood and urine
• urinate too often and produce too much urine
• Too thirsty
• Too hungry
Symptoms

17. • Arteriosclerosis
• Cardiovascular problems
- Heart disease
- Stroke
- High blood pressure
• Gangrene
• Blindness
• Kidney damage
Complications

18. Obesity &
overweight
Overweight and obesity are defined as abnormal or excessive fat accumulation that may impair health.
Worldwide obesity has more than doubled since 1980.
In 2014, more than 1.9 billion adults, 18 years and older, were overweight. Of these over 600 million were obese.
39% of adults aged 18 years and over were overweight in 2014, and 13% were obese.
Most of the world's population live in countries where overweight and obesity kills more people than underweight.
41 million children under the age of 5 were overweight or obese in 2014.
Obesity is preventable.

19. PROTIENS
• Proteins are substances found in every cell. The body needs proteins to
build and repair all body tissues. Protein is an important part of blood
cells. Proteins are made up of carbon, hydrogen, oxygen, and nitrogen
atoms that are formed into basic units called amino acids.
• There are 20 different amino acids. Nine of them are essential, and the
other eleven amino acids can be produced by the body.
Complete proteins/Animal Incomplete proteins/Plant

20. Functions
 Proteins as enzymes: Accelerate the rate of metabolic reactions.
 As structural cables: Provide mechanical support both within cells
and outside.
 As hormones, growth factors: Perform regulatory functions and
gene activators.
 As hormone receptors and transporters: Determine what a cell
reacts to and what types of substance enter or leave the cell.
 As contract element: Form the machinery for biological movements.

21. CONTD….
Others:
• Act as the defense against infections by protein
antibodies
• Service as toxins
• Form blood clots through thrombin

22. Sources
• Animal source : poultry, milk, cheese, eggs, meat, wet and dry fishes .
These protein contain all the amino acid in adequate amount.
• Plant source : Peas, beans, cereals, lentils, pulses, and nuts.
They are poor in EAA.

23. Daily requirements
• Babies need about 10 grams a day.
• Teenage boys need up to 52 grams a day.
• Teenage girls need 46 grams a day.
• Adult men need about 56 grams a day.
• Adult women need about 46 grams a day (71 grams if pregnant or
breastfeeding)
• Daily intake: 1g/kg body weight per day
• 4 kcal/g energy
• RDA:56g

24. Deficiency and Diseases
• Abdominal enlargement,
• excessive loss in urine and disease to lower urinary tracts
• Vomiting , Diarrhea
• Nephrosis , Lassitude , Oedema
• Marasmus
• Kwashiorkor (Protein malnutrition)
• Marasmic – Kwashiorkor
• Negative nitrogen balance.

25. FATS

26. INTRODUCTION
• Fats are organic compounds that are made up of carbon, hydrogen,
oxygen and are a source of energy in foods.
• Fats belong to a group of substances called lipids, and come in liquid or
solid form.
• All fats are combinations of saturated and unsaturated fatty acids.

27. FUNCTION
• Fat is one of the 3 nutrients along with protein and carbohydrates that supply calories to the body.
• Fat provides 9 calories per gram, more than twice the number provided by carbohydrates or
protein.
• Fat serves as the storage substance for the body's extra calories. It fills the fat cells (adipose tissue)
that help insulate the body.
• They are an important energy source, when the body has used up the calories from carbohydrates,
it depends on the calories from fat.
• The essential fatty acids are linoleum and linolenic acid and are important for controlling
inflammation, blood clotting, and brain development

28. SOURCES
• Dietary sources
– Triglycerides
• Saturated fats in meat, dairy foods, and tropical oils
• Unsaturated fats in seeds, nuts, olive oil, and most
vegetable oils
– Cholesterol in egg yolk, meats, organ meats,
shellfish, and milk products
• Saturated fats are found in animal products and vegetable oils but most other
vegetable oils contain unsaturated fat and are healthy.
• Unsaturated fats:
Monounsaturated fats include olive and canola oils.
Polyunsaturated fats include fish, safflower, sunflower, corn, and soybean oils

29. CONTD…
• Trans fatty acids are found in fried foods, commercial baked goods (donuts, cookies, crackers),
processed foods, and margarines.
• Hydrogenated and partially hydrogenated fats refers to oils that have become hardened (such as
hard butter and margarine). Partially hydrogenated means the oils are only partly hardened.

30. DEFICIENCY
• Deficiency of fats causes dry skin, hair loss, scaly skin, loss of menstruation, cold intolerance, poor
resistance to infection, and bruising
• Low fat in the diet can also affect brain function and lead to metabolic syndrome also increases the
rates of depression and suicide.

31. Contd…
• The significance of these deficiencies goes far beyond the presence of these obvious
deficiency symptoms and relates to a large variety of:
• medical conditions
• including cardiovascular disease
• inflammatory/auto-immune conditions
• mental health conditions, etc.

32. Vitamins
• Vitamins are essential organic nutrients, required in small amounts.
• They cannot be synthesized by the body. Must be obtained by outside
sources like diet, rumen bacteria & sun.
• Required for growth, maintenance, reproduction and lactation.
• Fat-soluble vitamins: carried in fatty parts of foods and dissolve in fats
(body stores them in fat... build up can be dangerous)
• Water-soluble vitamins: dissolve in water (body does not store them)

33. VITvitamins
sfvvvvSwsasasvvvvvbxzahx
Fat-soluble Watersoluble
Vitamin A Non B- Complex B-Complex
Vitamin D Vitamin C Thiamine (B1)
Vitamin E Riboflavin(B2)
Vitamin K Niacin (B3)
Pantothenic acid(B5)
Pyridoxine(B6)
Biotin(B7)
Folic acid (B9)
Vitamin B12
33

34. • Vitamins are organic (carbon) compounds needed for normal
function, growth and maintenance.
• They are not a source of calories.
• They are Non – caloric organic nutrients
• Needed in very small amounts
• They are Facilitators – help body processes proceed;
digestion, absorption, metabolism, growth etc.
• Some appear in food as precursors or pro vitamins

35. VITAMIN COMPARISON
Fat Soluble Water Soluble
Vits A, D, E, K Vits B & C
Absorption Lymph then Directly into blood
blood / protein carrier
Storage Long term Short term
Excretion Less readily excreted Kidneys detect
– remain in fat storage and remove in urine
sites

36. • Fat-soluble are stored in the liver and fatty tissues.
• These are not readily excreted from the body.
• Water-soluble vitamins travel in the blood and are
stored in limited amounts.
• These are readily excreted from the body through
urine.

37. Fat-Soluble Vitamins
• Vitamin A
• Vitamin D
• Vitamin E
• Vitamin K

38. Water-Soluble Vitamins
• Vitamin B-Complex
– Thiamin (vitamin B1)
– Riboflavin (vitamin B2)
– Niacin (nicotinamide, nicotinic acid)
– Vitamin B6 (pyridoxine, pyridoxal,
pyridoxamine)
– Folacin (folic acid)
– Vitamin B12
• Vitamin C

39. Fat soluble vitamins
stored in tissues
Examples
A
D
E
K

40.  VITAMIN A
Vitamin A is a Fat Soluble Vitamin
 Found in the fats and oils of food.
 Absorbed into the lymph and carried in blood with protein transporters =
chylomicrons.
 Stored in liver and body fat and can become toxic if large amounts are consumed.
 Foods from animal source provide various retinoids that are easily converted to
retinol in the body.
 Foods from plant source provide carotenoids
 Over 500 carotenoids found in nature
Less than 10% with provitamin A activity
-carotene highest activity
Must be acted upon in the gut or by the liver to form retinol

41. Vitamin A (precursor – beta carotene)
3 forms: retinol (stored in liver), retinal, retinoic acid
•3 forms of vitamin A important for health
•Retinal
•Retinoic acid
•Retinol (key player; can be converted to other forms. It is
stored in the liver)
•β-carotene (a carotenoid or pigment) in yellow/orange foods is
a potent provitamin A

42. Function, Deficiency Signs & Sources
Vitamin A
Deficiency signs: retarded growth in the young, the development of a
peculiar condition around the eyes known as Xerophthalmia, night
blindness and reproductive disorders.
Sources: whole milk, carotene, animal body oils (cod fish and tuna),
legume forages and can be synthetically produced.

43. • Functions
• Contributes to the production of retinol pigments which are needed for vision
in dim light
• Necessary for maintaining the normal functioning of glandular and epithelial
tissue which lines intestinal, respiratory and urinary tracts as well as skin and
eyes
• Supports growth especially skeletal growth
• Anti infective, high infection-lowered immune-vitamin A deficiency
• May protect against some epithelial cancer such as bronchial cancer
• Sources
• Animal source: liver, egg, butter, cheese, milk, fish, and meat
• Fish liver oils are the richest source of retinol

44. VITAMIN D
• Body can make from sunlight
• Production occurs in liver and kidney
• 1 mcg of VIT D = 40 IU of VIT D

45. Vitamin D - The sunlight vitamin
• Vitamin D is a group of fat-soluble vitamins.
• There are several forms of vitamin D,
The 2 most common are
1. Ergocalciferol (vitamin D2)
2. Cholecalciferol (vitamin D3)
Which are known together as Calciferol.

46. FUNCTIONS
 Calcium homeostatis
 Bone formation and maintenance
 Maintain blood calcium level to facilitate the proper interaction
between nerves and muscles.
 They are considered pro-hormones or the precursors to
hormones, and are essential to endocrine health.
Vitamin D is critical to bone health and immune system function.

47. DEFICIENCY
 RICKETS IN CHILDREN
 OSTEOMALACIA IN ADULTS
EPIDEMIOLOGY
1. RICKETS
 Characteristically more severe in children between the ages 1-3 years, when they are
growing rapidly and the limb bones have to support an increasing weight
 Deficiency in young infants
 Recognized in very young infants born with low birth weight-attributed to mother being
unable to supply sufficient vitamin.
2.OSTEOMALACIA
 A disease of multiparous women who are unable to replace calcium from their wombs lost
to foetus

48. • RECOMMENDED DIETARY ALLOWANCES
• 400 microgram is recommended by ICMR

49. VITAMIN - E
In 1933 vit E was identified to be an essential vitamin and found to comprise
a range of substances known as TOCOPHEROLS and TOCOTRIENOLS
Alpha tocopherol is the active form and the supplements are the derivatives
of this.
1mg of Tocopherol equivalents = 1 IU of tocopherol
RECOMMENDED DIETARY ALLOWANCES
ICMR Requirements of Vitamin E is 0.8mg/g

50. FUNCTIONS
1. Act as an antioxidant
2. Mixes with lipids ad protects the lipid molecules from oxidation.
3. Considered the body’s first line of defense against any lipid oxidation.
4. Protects cell membranes against oxidising free radicals.
5. Prevents the oxidation of beta carotene and vit A in the intestine.
6. Required for the normal functioning of the immune system
7. Regulating the production of prostaglandins may control the aggregation of blood
platelets during the formation of blood clots.
8. Regulation of the production of various hormones.
9. A WHO study found that improved VIT E status was strongly co related with reduced
death rates from CVD

51. • It is widespread in food
• Easily destroyed by heat processing
SOURCES

52. DEFICIENCY
1. Neurological Disorders
 Vit E deficit is found to produce uncoordinated movement (ataxia) weakness and sensory
disturbances.
 In born error of metabolism known as Isolated Vit E deficiency occurs at birth
2. Haemolytic Anaemia
 Vit E is a fat soluble vitamin and is a antioxidant
3. Retinopathy – defective functioning of the retina of the eye
4. Platelet and Lymphocyte Malfunction - severe liver disease

53. VITAMIN K
• It is called a Koagulation Vitamin
• Aids in blood clotting
FUNCTION
 It is used by the liver in the synthesis of various substances
needed for blood clotting
 Sources : Leafy green vegetables (spinach, , kale, cauliflower, broccoli), avocado,
kiwi, squash, green beans, herbs like (parsley, oregano, pepper, etc)

54. DEFICIENCY
•May arise in infants called neonatal haemorrhage
•It is rare in adults
RECOMMENDED DIETARY ALLOWANCES
a. ICMR committee considered that no recommendation is required for this
Vitamin.
b. But if occasionally seen in premature newborn infants a dose of 0.5 – 1.0mg
of vitamin K is administered by the intramuscular route.
c. The committee on Medical aspect of food policy of United Kingdom
concluded that 1 mcg/kg/day is both safe and adequate for adults

55. Vitamin C (Ascorbic acid)
Function: has an effect on the metabolism of calcium in the body
Deficiency signs: none demonstrated in livestock. Human deficiency:
scurvy (swollen and painful joints and bleeding gums) and brittleness of
bones.
Sources: citrus fruits, tomatoes, leafy vegetables and potatoes.

56. • Water-soluble

57. B – 1 THIAMINE
Important in:
– Producing energy from carbohydrates
– Proper nerve function
– Stabilizing the appetite
– Promoting growth and good muscle tone
– ATP production

59. Vitamin B1 (Thiamin)
Function: required for the normal metabolism of carbohydrates.
Deficiency signs: loss of appetite, muscular weakness, severe nervous
disorders, general weakness and wasting (BeriBeri). No known toxicity is known
with high dose of vitamin B1.
Sources: raw, whole grains and especially their seed coats and embryos; fresh
green forage; and yeast, milk and rumen synthesis.

60. ICMR Recommendations
Warnings : B-1 is nontoxic even at high
dosages
GROUP THIAMINE
(mg/day)
Men – Sedentary 1.2 mg
Moderate 1.4 mg
Heavy work 1.6 mg
Women – Sedentary 0.9 mg
Moderate 1.1 mg
Heavy 1.2 mg
Pregnant Woman + 0.2 mg
Lactating Woman
0 – 6 months + 0.3 mg
6 – 12 months + 0.2 mg
GROUP THIAMINE
(mg/day)
Infants 0 – 6 months 55 microgram / kg
6 – 12 months 50 microgram / kg
Children 1 – 3 yrs 0.6 mg
4 – 6 yrs 0.9 mg
7 – 9 yrs 1.2 mg
Boys 10 – 12 yrs 1.1 mg
13 – 15 yrs 1.2 mg
16 – 18 yrs 1.3 mg
Girls 10 – 18 yrs 1.0 mg

61. INTRODUCTION - Riboflavin
 Riboflavin – Vitamin B2 also called the yellow enzyme
 Recognised as a vitamin in 1917.
 Relatively stable vitamin that is resistant to acid, heat and oxidation.
 It is unstable in the presence of alkali and light.
 Soluble in water hence some losses occur when small pieces of riboflavin
containing food are cooked in large amounts of water for long periods.
 Upto 70% of the riboflavin in milk can be destroyed by the action of sunlight on
milk during four hours of exposure to sunlight.

62. FUNCTIONS
1. It is important for body growth and red blood cell production and helps in releasing energy
from carbohydrates
2. Plays a role in the energy metabolism of all cells.
3. Also involved in activation of vitamin B6 & conversion of folic acid to its coenzymes.
4. The proteins to which they become attached are known as flavoproteins.
5. Riboflavin essential for growth; involved in production of corticosteroids, formation of RBC,
Gluconeogenesis, Thyroid enzyme regulatory activity.
6. It plays an important in dehydrogenation reactions.
7. It is involved in production of hormones in the adrenal gland, the formation of red blood
cells in the bone marrow, synthesis of glycogen and catabolism of fatty acids.

63. Deficiency/Toxicity
• Deficiencies are rare although some people may take in
marginal amounts
• Drug and alcohol users and restricted caloric intake
• Ariboflavinosis
• Toxicity– not observed

64. Food Sources
• Most plant and animal foods
• Milk and milk drinks and yogurt supply about 15% in the
American diet
• Fortified cereals, bread and bread products contribute about
10%
• One of four vitamins added to enriched products

65. Dietary Recommendations for Riboflavin
• RDA is 1.1 mg for women and 1.3 mg for men per day
• Pregnancy and lactation increase energy needs and need for
riboflavin

66. Niacin (Nicotinic Acid)
• Made from tryptophan; essential nutrient if protein intake is
inadequate.
• (60 mg tryptophan to 1 mg niacin, which is referred to as
1 niacin equivalent (NE). )
• RDA is 14-16 NE/day for adults
• NE include preformed and niacin derived from tryptophan

67. NIACIN -B3
• Sources: Foods rich in tryptophan, such as dairy products, can
compensate for not consuming enough niacin in the diet
because the body can convert tryptophan to niacin.
• Good sources of niacin are avocados, dates, tomatoes, leaf
vegetables, broccoli, carrots, sweet potatoes, asparagus,
mushrooms, liver, heart and kidney, chicken, beef, fish: tuna,
salmon, milk, eggs and mushrooms.

68. Niacin deficiency
• Pellagra
• Epidemic in southwest in early 1900’s as corn became staple;
niacin is bound by protein
• Deficiencies found in southeast if subsisting on diet of corn,
molasses and salt pork
• International health issue
• High doses can cause flushing, itching, gout, and liver damage
(rarely) and high blood sugar.

69. VITAMIN B5 (PANTHOTENIC ACID)
• Function: pantothenic acid is needed to release energy from
sugars, starches, and fats. It helps to support the adrenal glands.
Sources: Small quantities of pantothenic acid are found in nearly
every food, with high amounts in whole-grain cereals, legumes,
eggs, meat, and royal jelly
• Deficiency can lead to fatigue, listlessness, sensations of weakness,
burning foot syndrome.
• No RDA(rarely found); AI(adequate Intake) is 5 mg/day
• Excess:No toxicity is known with over dosage.

70. VITAMIN B6 (PYRIDOXINE)
• function of Vitamin B6 :Pyridoxine is essential for the metabolism of carbohydrates, amino
acids, and fats, as well as for normal nerve function and for the formation of red blood
cells.
Sources :Foods rich in vitamin B6 are spinach, bell peppers, turnip greens, garlic, tuna,
cauliflower, mustard greens, banana, celery, cabbage, mushrooms, asparagus, broccoli, kale,
collard greens, Brussels sprouts, cod and chard.
Recommended daily allowance: RDA is 1-3 mg/day
vitamin B6 deficiency
Deficiency of pyridoxine can lead to anemia, nerve damage, seizures, skin problems, and
sores in the mouth.
Excess :Over dosage of pyridoxine can damage the nerves and cause sensory neuropathy.

71. FOLIC ACID (B12)
• function
Folic acid with vitamin B12, is necessary for the formation of normal red blood cells and the synthesis of DNA.
Folate is also necessary for normal development of a fetus's nervous system. A low intake of folate may increase
the risk of fractures in older adults.
Sources
Leafy vegetables such as spinach, asparagus, turnip greens, legumes such as dried or fresh beans, peas and
lentils, fortified grain products, sunflower seeds, citrus foods are rich in folic acid.
Recommended daily allowance:RDA is 400ug/day (is it most prevalent vitamin deficiency?)
• 0-8 years=65-200mcg/d, 9-18 years=300-400mcg/d
symptoms of folic acid deficiency?
Deficiency results in anemia, mouth ulcers, diarrhea, depression, confusion, anemia, and brain defects in fetus.
• Excess(overdosage)Folate is generally not toxic.

72. Vitamin B12
• Function of Vitamin B12?
Vitamin B12 with folic acid is necessary for the formation and maturation of red blood cells and the
synthesis of DNA. Vitamin B12 is also necessary for normal nerve function.
Sources
Vitamin B12 occurs in foods that come from animals. Plants are poor sources of vitamin B12. Thus in
vegetarians, supplementation may be required.
Recommended daily allowance: RDA is 2.4 gum per day; B12 not present in plant foods
• 0-8 years=0.4-0.9mcg/d; 9-18 years=1.2-2.4mcg/d
vitamin B12 deficiency
Vitamin B12 is stored in the human body for 3-5 years and hence deficiency is rare. Deficiency results
in anemia, peripheral neuropathy, memory loss. In infants it can lead to tremors.
Excess /overdosage:
No known toxicity is known with excess intake of vitamin B12.

73. References
• http://www.nytimes.com/health/guides/nutrition/fat/overview.html
• http://www.ask.com
• http://www.prohealth.com
• http://www.cdc.gov