Food tables and labels provide information on the quantities of nutrients in foods. It is often assumed that eating the food will provide the individual with the listed nutrients. However the value provided on the labels may not be representative of the final absorption quantities in the individual. Of course the nutrient must be in the food to begin with for it to be absorbed, and as food tables are an estimate of the possible nutrition of a food, this is not always the case. Many factors such as processing, growth conditions of plants and animals, storage and environmental conditions can all influence the nutritional content derived from foods. However, even if the label is accurate, there is no guarantee that the nutrients within foods will be absorbed. A number of factors transpire against the efficient absorption of various nutrients for a myriad of reasons. The final quantities of nutrients absorbed and metabolised by the individual may therefore not be representative of the quantities in the original food material.
Assessments of the absorption rates of nutrients from foods have been reported from whole foods in the nutritional literature. For example, one study1 assessed the absorption rates of pantothenic acid (vitamin B5) and pyridoxine (vitamin B6) by comparing the absorption values from a typical American diet with that of a purified liquid diet containing supplemental vitamins. Vitamin levels from all foods were assayed to confirm the levels within food accurately. Subjects consumed a semipurified liquid diet that contained a total of 1.1 mg of pyridoxine and 8.2 mg of pantothenic acid for 35 days. Supplements of pyridoxal hydrochloride and calcium pantothenate made up some of the intake during this period. The subjects then consumed a typical American diet that contained 2.3 mg of pyridoxine and 11.5 mg of pantothenic acid for 35 days. Subjects then resumed the original semipurified liquid diet for 21 days, but this time the pyridoxal content of the diet was 2.7 mg per day.
Researchers then calculated the absorption of vitamin B6 and vitamin B5 from food based on the percentage of the vitamins absorbed from the whole food diet compared to the semipurified diet. The results from this calculation showed that the vitamins from the semipurified diet were more bioavailable. In comparison to the semipurified diet the vitamin B6 from the typical American diet was only 61 to 81 % as bioavailable (mean value 79 %). Similarly the pantothenic acid from the typical American diet was only between 40 to 61 % (mean 50 %) as bioavailable as the vitamins from the semipurified diet. Therefore the water soluble vitamins pyridoxine and pantothenic acid are not fully absorbed from the typical American diet. The authors calculated that the available pyridoxine in the typical American diet was 1.7 mg per day and the typical pantothenic acid content was 5.8 mg per day. These levels are very low and explain some of the health problems associated with consuming such as diet.
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