N r w

Thiamin-vitamin B,

B. Coenzyme Function

Thiamin

Glyceralde-hyde-3-P

Trans-ketolase

Sedoheptulose-7-P Xylulose-5-P

C. Thiamin in the Citrate Cycle Pyruvate

Thiami n-P-P !ยง) Pyruvate dehydrogenase

Acetyl-CoA

Oxaloacetate

I a itr

Malate

Isocitrate

Fumarate

Succinate

Citrate cycle 1

a-Ketoglutarate

Thiamin-P-P

Succinyl-CoA

a-Ketoglutarate dehydrogenase a-Ketoglutarate a-Ketoglutarate dehydrogenase

  1. Effects of Thiamin
  2. a-Splitting 0

Pyruvate decarboxylase

2. a-Condensation 0

Acetolactate e

II I

(C5 aldose) (C, ketose) Transketolase (C, aldose)

Glycerin aldehyde-5-P Sedoheptulose-7-P

(C7 ketose)

Fructose-6-P + HOPO,2

Phospho-

ketolase HwO O

Acetyl-P

Transition stage

Thiamin:

Occurrence and Requirements

Thiamin is found in all animal foods (A). Good sources are several species of fish (e. g., farmed catfish, sole, Florida pompano) as well as liver and muscle, especially porcine (e.g., ham), and egg yolk. Good plant sources are whole grains, potatoes, legumes, soy milk, and acorn squash. Like most B vitamins, most of the thiamin in grains is found in the seed coat that is removed during the refining process. This also applies to rice, which loses most of its thiamin during polishing.

Thiamin requirements vary with energy expenditure. Vitamin balance research shows that ~0.5 mg of thiamin per 1000 kcal (4.2 MJ) used is needed daily to maintain erythrocyte trans-ketolase activity as well as sufficient tissue thiamin levels. Current recommendations, according to which women need up to 1.1 mg and men up to 1.2 mg thiamin/d, are based on this research. An additional 0.3 mg/d is recommended during pregnancy and lactation. These amounts are based on average energy intakes; in case of increased energy use due to hard physical labor or very intensive workouts, requirements increase accordingly. Chronic alcohol abuse reduces thiamin absorption and metabolism. Fortification of alcoholic beverages with thiamin has been suggested to prevent this effect. It should be noted that thiamin is water soluble, as well as sensitive to heat and oxidation. An approximate 30% loss during food preparation must, therefore, be included in any calculation. In a normal, mixed diet, most of the thiamin comes from animal products. If a shift to more plant-based foods occurred, which would be desirable, thiamin intakes might become problematic unless there were a parallel shift to more whole-grain products. In the U.S. this is less crucial with regard to thi-amin, since adequate thiamin supply of the general population has been insured by mandatory enrichment of refined flour. No UL has been established.

The classic vitamin Bj deficiency syndrome (C) is beriberi. The wet form causes generalized edema; in the dry form, nerve lesions dominate; in breastfed children it may be associated with carbohydrate intolerance (infantile beriberi). Beriberi was common in the U.S. before government-mandated enrichment began in the 1940s. In 1932 alone, there were nearly 50 000 cases of beriberi. Based on thiamin's biochemical functions, there are two main categories of symptoms: cardiovascular disturbances, with insufficient blood supply, edema, cardiac insufficiency; and neurological disturbances like impaired sensibility, cramping, paralysis, and anxiety. Whereas now, the occurrence of classic beriberi is usually restricted to developing countries, the latter symptoms are found in the U.S. mostly due to alcohol abuse: nearly 40 % of alcoholics who require treatment suffer from polyneuropathy and 3-10 % develop Wernicke-Korsakoff syndrome, characterized by mental confusion, amnesia, impaired short-term memory, and sometimes psychosis.

- A. Occurrence and Daily Requirement -

The daily requirement of 1 mg vitamin B1 is contained in:

1 kg trout, bass

500g salmon 300g sole, eel

1 kg veal

500 g beef (muscle meat)

300g beef liver, pork liver

100 g pork

Nutrition For Alcoholics

2 kg fruit 300g peas

2 kg fruit 300g peas

200g whole-grain flour

750g instant oatmeal

1 kg polished rice, refined wheat flour

200 g rice, rolled oats

I- B. Recommended Intakes (DRI/AI*, 2000)

Life Stage and Gender Group

Age

Thiamin (mg/d)

Thiamin (mg/kg)

Infants

0 - 6 mo

0.2*

~ 0.3*

7 - 12 mo

0.3*

~ 0.3*

Children

1- 3 y

0.5

3 - 8 y

0.6

Boys and girls

19 - 13 y

0.9

Boys

14 - 18 y

1.2

Girls

14 - 18 y

1.0

Men

> 19 y

1.2

Women

> 19 y

1.1

Pregnancy and lactation

14 - 50 y

1.4

- C. Deficiency Symptoms

Riboflavin Deficiency Symptoms

Riboflavin: Chemistry, Metabolism, and Functions

Riboflavin or vitamin B2 was isolated from yeast in 1932 and its structure described soon thereafter (A): it is a tricyclic nitrogenous ring system with a C5 side chain, in which the last hydroxy group can be esterified with phosphoric acid. Riboflavin is the short name suggested by IUPAC; the old names "ovoflavin" and "lactoflavin" should no longer be used.

Foods contain free riboflavin as well as protein-bound flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), all of which are called vitamin B2. Free riboflavin is absorbed in the proximal small intestine after dephosphor-ylation. Inside the mucosa cells, the enzyme riboflavin kinase phosphor-ylates it back to FMN. Absorption in the intestine is active and subject to saturation kinetics. At higher concentrations, however, passive diffusion may occur. In the blood, free riboflavin, FMN, and FAD are bound to albumin or to ribo-flavin-binding proteins (RFBPs). Storage capacity for riboflavin depends largely on the available amount of apoprotein. Maximal reserves last 2-6 weeks, much less in case of protein deficiency. Vitamin B2 is excreted through active secretion into the renal tubules either as riboflavin or as metabolites like 7-a-hydroxy riboflavin. Its concentration in urine can be used as an indicator of B2 status: <40 |ag riboflavin/g creatinine indicates riboflavin deficiency. Riboflavin's biochemical functions are based on the oxidoreductase effects of FMN and FAD as coenzymes or prosthetic groups of enzymes (B). Because of the yellow color of the coenzyme they are called flavoproteins or flavin enzymes. To date, more than 60 such enzymes have been found, many of which play key roles in various metabolic processes. FMN and FAD are able to attach hydrogen to riboflavin's N atoms in positions 1 and 5 and can thus function as hydrogen donors. Hydrogen acceptors can be ubiquinone in the respiratory chain, NAD+, or even oxygen (formation of H2O2). Most flavin enzymes contain FAD, such as acyl-CoA dehydrogenase, which catalyzes the first step in the p-oxidation of fatty acids. FMN can be found, for instance, in NADH-dehydrogenase (respiratory chain) and in amino acid oxidases. Some flavin enzymes contain additional metal cofactors like Fe, Cu, Mn, or Mo.

The flavin component may be cova-lently attached to the apoprotein, as in the mitochondrial enzymes of the respiratory chain, in succinate dehydro-genase (SuccDH), or in the monoami-ne oxidases (MAO). Lastly, prosthetic groups may be loosely and reversibly attached to the protein, as for instance in xanthine oxidase or glutathione reductase.

FMN and FAD (C) form predominantly in the liver, kidneys, and heart under hormonal control (e. g., T3). Their synthesis requires ATP.

- A. Chemistry

CHwOH

H

C i

OH

H

C

OH

H

C

OH

H

C

H

H-C

N

8

v

r 22^o

jj,

j\4 jnh

H-C

N

N"

C

O

Riboflavin

- vitamin B2

B. Cellular Metabolism -

Riboflavin

Flavokinase O

Enzyme complexes

Covalently bonded to some enzymes (SuccDH, MAO)

|- C. Biosynthesis of FMN and FAD-

I Riboflavin

I II

Flavin mononucleotide (FMN)

HO OH

Riboflavin:

Occurrence and Requirements

In accordance with its metabolic role, vitamin is ubiquitous in foods (A). It is most abundant in yeast, but yeast does not typically play a great role in human nutrition. Milk contains ~0.2 mg vitamin B2/IOO g. Since riboflavin is protein bound, milk concentrates like cheese and ricotta have higher B2 contents. One serving of liver supplies several times the daily requirement. However, liver is not of great nutritional significance either. Fruits and vegetables contain very little riboflavin. With regard to grains, what goes for B2 applies to all B vitamins: germ and bran contain a relatively high amount of vitamin B2, ~two-thirds of which are lost during refining. Since vitamin B2 is heat-resistant, other forms of processing usually do not engender additional losses as long as any riboflavin leached into the water that is used during preparation ends up in the product. Vitamin B2 is extremely light-sensitive, though. If milk in a clear glass bottle is exposed to light for several hours, B2 losses may reach 80 %. The average loss to storage and processing amounts to 20%. Recommended intakes for vitamin B2 (B) are based on balance tests, in which elevated urinary excretion is used as a parameter indicating tissue saturation. Male adults with average energy nutrient intakes show a distinct increase in urinary excretion at >1.1 mg/d, indicating saturation. The resulting recommendations of 1.1 mg and 1.3 mg vitamin B2/d for females and males, respectively, include an additional safety margin. During pregnancy and lactation the requirement is increased by 0.3 and 0.5 mg/d, respectively. Trauma, malabsorption, and/or alcohol abuse, as well as the use of certain phar maceuticals (antidepressants, some oral contraceptives) may result in increased need. Actual intakes tend to be below requirements, especially in the elderly. Adolescents, particularly girls, obtain only three-fourths of the recommended amounts. In accordance with riboflavin's occurrence in foods, animal products contribute less than two-thirds of the total supply at present; about a third comes from milk and dairy products alone. A UL for ribo-flavin has not been established.

Several possible test parameters are available to determine B2 deficiency (C). A reliable method is the measurement of glutathione reductase activity (in erythrocytes) after stimulation with FAD (a-EGR-method). Clinical deficiency symptoms are extremely rare in industrialized countries. At first, they are unspecific (D) and affect mainly the mucous membranes of the head (e. g., stomatitis, inflamed buccal mucosa). B2 deficiency also affects Fe metabolism, causing hypo-chromic anemia in advanced stages. Sometimes there is also dermatitis, occasionally generalized. FMN- and FAD-dependent enzymes also play roles in the metabolism of other vitamins. This fact, and the fact that the vitamins often occur in foods together, explains why isolated B2 deficiency is uncommon. Instead it usually occurs in combination with other vitamin deficiencies.

- A. Occurrence and Daily Requirement-

The daily requirement of 1.5 mg vitamin B2 is contained in:

Examples Fibrous Root System

Life Stage and Gender Group

Age

Riboflavin (mg/d)

Riboflavin (mg/kg)

Infants

0- 6mo

0.3*

~ 0.4*

7-12 mo

0.4*

~ 0.4*

Children

1- 3y

0.5

4- 8y

0.6

Boys

9-13y

0.9

14-18 y

1.3

Girls

9-13 y

0.9

14 -18 y

1.0

Men

>19 y

1.3

Women

>19 y

1.1

Pregnancy

14 - 50 y

1.4

Lactation

14 - 50 y

1.6

Delicious Diabetic Recipes

Delicious Diabetic Recipes

This brilliant guide will teach you how to cook all those delicious recipes for people who have diabetes.

Get My Free Ebook


Post a comment