Title: Biochemical Measures of Nutritional Status 2
Key words: iron, haemoglobin, serum ferritin, zinc, serum zinc, metallothioneine, 24 hour urinary sodium, diarrhoea, potassium, plasma retinol, carotenoids, Vitamin A, plasma 25-hydroxyvitamin D, B-vitamins, erythrocyte glutathione reductase, riboflavin, red cell and plasma folate, serum B12, plasma Vitamin E, cholesterol, plasma Vitamin C,
Date: Jan 2001
Category: 14. Measurement
Author: Kate Neil (NS3)
Biochemical Measures of Nutritional Status
Haemoglobin and serum ferritin are the usual assessments1,2,3. Serum ferritin is one of the best available indices of long-term intakes in populations4. It reflects both overload and deficiency. As such, it needs interpreting with caution4. Raised serum ferritin can result from: inhibited haemoglobin synthesis, the acute phase reaction, and liver damage associated with iron overload4. Low haemoglobin levels may reflect low dietary iron, but low intakes of vitamin B12, folate, helminthic infestations4. A variety of diseases will reduce circulating haemoglobin4. Raised erythrocyte protoporphyrin levels, circulating transferring receptor, and low percent saturation of transferrin would be diagnostic of insufficient diet intake in conjunction with very low serum ferritin4.
Plasma/serum zinc used in childrens' surveys3 . Serum zinc is not a good indicator of status and of intake for population groups, though probably does distinguish between the extremes of intake4. Measuring metallothioneine is a promising indicator5.
Sodium and potassium
Urine collections are routinely assayed for their sodium and potassium content2. Urinary excretion of sodium is generally a good indicator of dietary intake, because faecal excretion of sodium is minimal4. The within-person variability in sodium excretion is of the order of 30%, necessitating a substantial number of observations to gain precision in the overall means for each individual4. Ensuring a complete 24-hour collection affects interpretation4. Research suggests that at least eight 24-hour urine collections are needed for good interpretation4. Similarly, eight 24-hour urine collections for potassium show good correlation between calculated intake and excretion4. Severe and prolonged diarrhoea can cause major losses4.
Retinol and carotenoids
Plasma retinol and several carotenoids are assessed1,2,3. Vitamin A deficiency is rare in the UK, dietary intake being only a minor determinant of plasma levels4. Approx 90% of vitamin A is stored in the liver. Plasma retinol plateaus and is very low only when the liver is virtually exhausted4.
Plasma is currently the only biochemical marker used for carotenoids4. Plasma beta-carotene is considered to reflect fairly long-term habitual intakes4. Dietary intake of carotene was positively associated with circulating alpha-carotene and beta-carotene in an adult UK survey2. Substantial between-individual variation has been shown in a supplementation study4.
Plasma 25-hydroxyvitamin D determination is used in surveys1,3. 25-hydroxyvitamin D is used for status because it is half way through its conversion to the active form, 1,25-hydroxyvitamin D, and is the best indicator of medium/long-term exposure6 from both dietary and endogenous sources4. 1,25-hydroxyvitamin D is not used as it is under tight homeostatic control6. Exposure to sunlight varies and should be considered when interpreting 25-hydroxyvitamin D status4.
B2 is commonly evaluated1,2,3. by stimulating erythrocyte glutathione reductase4. This is a measure of tissue saturation and long-term riboflavin status4. It is stable, extremely sensitive at low levels of riboflavin intake, and the test appears independent of age or gender4. Similar findings for B1 and B6 functional assays4.
Red cell and plasma folate alongside serum B12 is used in some surveys1,3. This provides a complete picture of folate status4.
Plasma vitamin E is commonly assessed4. Vitamin E is closely correlated with total blood lipids, particularly the cholesterol fraction4. Vitamin E:cholesterol ratio has been suggested to be more biologically relevant4.
Plasma vitamin C often measured. Plasma levels reflect daily intake. Leukocyte levels are a better index of tissue stores6.
5 Human Nutrition and Dietetics, 9th Edn, Edited by Garrow and James, Churchill Livingstone, 1993