NS235 Unit 2 Vitamin Profile Example

Plate to Pyruvate Dehydrogenase: A Unit 2 Vitamin Profile of Thiamin and Every Stage Where It Can Fail

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Purdue University Global

NS235: Micronutrient Metabolism

Unit 2 Vitamin Profile

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[Date]

Food values are typical figures rounded from standard tables; the reference intake report and the deficiency review cited are real.

Why Thiamin

Thiamin, vitamin B1, was the first B vitamin identified, and the disease caused by its lack, beriberi, has shaped nutrition history. It is a useful profile because its path from food to function has several points of vulnerability: it is scattered thinly in foods, easily lost in cooking and processing, absorbed by a carrier that can be saturated or impaired, stored in small amounts and needed constantly by the body's energy pathways (Whitfield et al., 2018).

Stage 1: Foods

Thiamin is found in many foods but in small amounts. Rich sources include pork, legumes, sunflower seeds, whole grains and enriched grain products. A 3-ounce pork chop supplies about 0.8 milligrams, a cup of cooked black beans about 0.4 milligrams and a slice of enriched white bread about 0.1 milligrams. The Recommended Dietary Allowance for adults is 1.2 milligrams for men and 1.1 milligrams for women (Institute of Medicine, 1998).

Refining is a major cause of loss. Milling removes the bran and germ of grains, where most thiamin is concentrated. Polished white rice keeps only a fraction of the thiamin in brown rice, which is why beriberi spread in populations that came to depend on polished rice. In the United States, enrichment of refined flour and grain products restores thiamin and makes them an important source.

Thiamin and Carbohydrate

The relationship between thiamin and carbohydrate also shapes need. Because thiamin is used in the metabolism of glucose, the reference intakes note that requirements rise with energy and carbohydrate intake, and early allowances were expressed per 1,000 kilocalories. Diets very high in refined carbohydrate and low in thiamin, such as those based on polished rice or sugar, therefore carry the greatest risk. Absorption of thiamin, like that of other water-soluble vitamins, occurs mainly in the upper small intestine (Goodman, 2010).

Stage 2: Cooking and Storage

Thiamin is water-soluble and sensitive to heat, particularly in neutral or alkaline conditions. Boiling leaches it into cooking water, and long cooking destroys part of it. Adding baking soda to vegetables to preserve their color, an old practice, speeds thiamin destruction. Some raw fish and shellfish contain thiaminases, enzymes that break down thiamin, and tea and coffee contain compounds that can reduce its availability, though these matter mainly when intake is already marginal (Whitfield et al., 2018).

Stage 3: Absorption

Thiamin is absorbed mainly in the upper small intestine. At low intakes, it is taken up by specific carrier proteins, an active process that can become saturated; at high intakes, some is absorbed by passive diffusion. Chronic heavy alcohol use impairs thiamin absorption and storage and is the most common cause of thiamin deficiency in high-income countries.

Stage 4: Activation

Inside cells, thiamin is converted to thiamin pyrophosphate, also called thiamin diphosphate, by adding two phosphate groups. This is the active coenzyme form.

Stage 5: Function

Thiamin pyrophosphate is the coenzyme for several key enzymes in energy metabolism. Pyruvate dehydrogenase, which converts pyruvate to acetyl-CoA, links glycolysis to the citric acid cycle. Alpha-ketoglutarate dehydrogenase, within the citric acid cycle, requires it. Branched-chain alpha-keto acid dehydrogenase, which breaks down the carbon skeletons of leucine, isoleucine and valine, requires it. Transketolase, in the pentose phosphate pathway, requires it as well (Institute of Medicine, 1998).

Without thiamin pyrophosphate, pyruvate cannot become acetyl-CoA, so cells that depend on glucose, above all in the brain and nerves, lose access to most of the energy glucose could provide, which is why thiamin deficiency shows itself first in the nervous system and the heart.

Stage 6: Storage and Turnover

The body stores only about 30 milligrams of thiamin, much of it in muscle, and turns it over quickly. With a thiamin-free diet, signs of deficiency can appear within weeks. This small reserve is why a steady daily supply matters.

Deficiency

Beriberi takes two main forms. Dry beriberi affects the nerves, causing weakness, numbness and pain in the limbs. Wet beriberi affects the heart, causing an enlarged heart, rapid heartbeat and fluid retention. In people with heavy alcohol use, thiamin deficiency can cause Wernicke encephalopathy, with confusion, eye movement problems and unsteady gait, which can progress to Korsakoff syndrome, with lasting memory loss. Thiamin deficiency remains a public health concern in some populations that rely on polished rice, and in high-income countries it occurs mainly with alcohol use disorder, after bariatric surgery, with prolonged vomiting and in some hospitalized patients (Whitfield et al., 2018).

How Thiamin Status Is Assessed

Because blood thiamin levels change quickly with recent intake, laboratories often assess thiamin status through the activity of transketolase in red blood cells, measured with and without added thiamin pyrophosphate. If adding the coenzyme raises the enzyme's activity substantially, the cells were short of it. Whole-blood thiamin diphosphate can also be measured directly. In practice, clinicians often treat suspected deficiency with thiamin before results return, especially in people with alcohol use disorder, because treatment is safe and delay can cause lasting harm (Whitfield et al., 2018).

Why Glucose Can Make Deficiency Worse

A striking feature of thiamin deficiency is that giving glucose to a deficient person can precipitate symptoms. Glucose increases the flow through pyruvate dehydrogenase and the other thiamin-dependent enzymes, using up what little thiamin pyrophosphate remains. This is why hospitals give thiamin before or with glucose to patients at risk, such as those admitted with alcohol intoxication or severe malnutrition. The example shows how closely thiamin is tied to carbohydrate metabolism.

Who Is at Risk

1. People with alcohol use disorder, because of low intake, poor absorption and impaired storage.

2. People after bariatric surgery, especially with vomiting.

3. Older adults with low intakes.

4. People with conditions causing prolonged vomiting, including severe morning sickness in pregnancy.

5. Populations relying on polished rice without enrichment.

Toxicity

Thiamin has no upper limit in the reference intakes, since the review found no reports of harm from high intakes by mouth (Institute of Medicine, 1998). The kidneys excrete excess readily.

Summary of Failure Points

1. Food: low thiamin in refined, unenriched grains.

2. Preparation: losses in cooking water, from heat and from alkaline additions.

3. Absorption: carrier saturation and impairment by alcohol.

4. Activation: requires conversion to thiamin pyrophosphate.

5. Storage: small reserve that depletes in weeks.

Thiamin's place at the junction of glycolysis and the citric acid cycle explains both its importance and the speed with which deficiency appears. Each failure point has a named cause, and each suggests where prevention can act: enrichment, cooking methods, attention to alcohol use and supplementation for high-risk groups.

References

Goodman, B. E. (2010). Insights into digestion and absorption of major nutrients in humans. Advances in Physiology Education, 34(2), 44-53. https://doi.org/10.1152/advan.00094.2009

Institute of Medicine. (1998). Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. National Academies Press. https://doi.org/10.17226/6015

Whitfield, K. C., Bourassa, M. W., Adamolekun, B., Bergeron, G., Bettendorff, L., Brown, K. H., Cox, L., Fattal-Valevski, A., Fischer, P. R., Frank, E. L., Hiffler, L., Hlaing, L. M., Jefferds, M. E., Kapner, H., Kounnavong, S., Mousavi, M. P. S., Roth, D. E., Tsaloglou, M.-N., Wieringa, F., & Combs, G. F., Jr. (2018). Thiamine deficiency disorders: Diagnosis, prevalence, and a roadmap for global control programs. Annals of the New York Academy of Sciences, 1430(1), 3-43. https://doi.org/10.1111/nyas.13919