Liver is about the most concentrated dietary source of pre-formed vitamin A (retinol) that you could hope to find, along with a number of other vitamins. Like in us humans, animals store retinol in their liver and the more they consume the more gets stored in their liver.
Historically, plant-eating livestock like cows and sheep would have sourced their vitamin A in the form of beta-carotene from the plants they ate, which they convert into retinol as required. Other more omnivorous animals like chickens and pigs may have gotten some pre-formed retinol in their diets, depending upon what they could eat.
The advances in animal nutrition in during the 20th Century have resulted in much more targeted livestock nutrition to optimise animal health and productivity. This includes the widespread use of nutritional supplements in modern livestock farming. When vitamin A is supplemented in the diet it is usually added in the form of retinol.
In 1992 M J Scotter and colleagues published an analysis of the liver retinol content of different farm animals in the UK in a survey of animal livers for vitamin a content.
In Table 1 (below) they compared their results to previous reports going back to 1960. You can see that, compared to 1960, the liver retinol content of all the animals increased if you look at the mean values. Looking at the range of individual results there were also more animals with really high levels of liver retinol. All groups showed a wide range of retinol content.
(There was no measure for chicken liver in 1960 to compare with)
A later paper from 1998 reported that vitamin A levels hadn’t changed much during the 1990s. A survey of vitamin A concentrations in the liver of food-producing animals.
“The concentrations of liver vitamin A were similar to those found in other surveys (Scotter et al. 1992). There was no evidence that vitamin A concentrations in liver had increased over the past decade, suggesting that the vitamin A supply in animal diets from all sources, including supplements, had remained at a similar level throughout.”
Another more recent study in 2006 found that the amount of retinol in the livers of farm animals in Austria was a lot lower than from other European countries with more industrial farming systems. Vitamin A content (retinol and retinyl esters) in livers of different animals
“In the small- and medium-sized farms (dominating in the agricultural production in Austria), animal breeding is being practised less extensively, without higher vitamin A supplementation, than in other European countries, such as Germany or the UK. Many of the farms tend to be quite self-sufficient and only a few of them buy modest amounts of commercial feeding supplies for the animals. Moreover, the slaughter of the animals was done on the farm or in the closed slaughter-house. Therefore the application of the vitamin A preparation to minimise the stress before slaughter could be excluded.”
As an example of the results:
“The mean amount of vitamin A observed in Austrian calf livers was one-fifth (1/5) of the Danish (10.3 mg RE/ 100 g), one-ninth (1/9) of the British (18.8 mg RE/100 g) and one-fifteenth (1/15) of the German (28.8 mg RE/ 100 g) average liver vitamin A contents.”
“The Austrian cattle showed a mean liver vitamin A content about 4.5–5.5-times lower than those of German (17.9 mg RE/100 g) and Danish cattle (22.3 mg RE/100 g)…”
“It was observed that the average amount of vitamin A measured in Austrian pig livers was… just a third of that found in German pig livers (36.3 mg RE/100 g) (BgVV, 1995). The”
“The average content of vitamin A found in chicken livers was about 2, 4 and 6-times lower than the mean values reported for British (9.7 mg RE/100 g), for Slovak Republic (17–20 mg RE/100 g) and German chickens (33.5 mg RE/100 g), respectively.”
They concluded that:
“The variations of liver vitamin A concentrations among the species were a result of differences in race and age, but especially of different contents of vitamin A in the supplied feed.”
The liver you eat today may contain rather more vitamin A than the liver your grandparents ate when they were young. Whether this is of nutritional importance to you eating it is less certain. Liver is a valuable source of many nutrients and remains an important part of my diet.
One situation it may be of relevance is the question of liver and retinol consumption during early pregnancy, which is a topic beyond this blog post.
Perhaps the moral of this story is that what you food eats can be important and that modern farming can alter our food in ways that you might never think about.
Scotter MJ, Thorpe SA, Reynolds SL, Wilson LA, Lewis D J. Survey of animal livers for vitamin a content. Food Additives and Contaminants 9, 237–242 (1992).
Retail samples of livers from calf (23), ox (18), lamb (17), pig (15), chicken (16) and turkey (1) were analysed to determine levels of vitamin A (all-trans-retinol) and to aid assessment of the effects of using vitamin supplemented compound feedingstuffs for livestock. For comparison, 22 liver samples from lambs reared on diets not containing vitamin‐supplemented compound feedingstuffs and four samples of liver from ox which had received supplemented feed but not during the last four months prior to slaughter were also analysed. The chosen method of analysis utilized saponification, solvent extraction and normal‐phase high-performance liquid chromatography with fluorescence detection. For all species analysed, the levels of vitamin A ranged from 10 to 1100 mg/kg, with all but seven at or below 400 mg/kg. For lamb and ox livers, the mean levels were 310 mg/kg and 200 mg/kg respectively for retail samples. The mean levels were 220 mg/kg (lamb) and 120 mg/kg (ox) in liver samples from animals fed controlled diets. The results are of the same order as those reported over recent years.
Howells LC, Livesey CT. A survey of vitamin A concentrations in the liver of food-producing animals. Food Additives and Contaminants. 15, 1, 10-8 (1998)
“High oral doses of vitamin A have been shown to be teratogenic and accordingly the World Health Organization has recommended that the daily intake for pregnant women should not exceed 3.3 mg. Liver contains high concentrations of this vitamin and consequently it has become necessary to assess its level in the livers of food animals. A survey of fresh, frozen, imported and home-produced retail liver samples from calf (42), ox (121), lamb (228), pig (133) and chicken (125) was carried out between August 1992 and April 1993 using a modified version of the method of Ashoor and Knox (1987) to determine retinol and retinyl esters. Interlaboratory comparison showed no significant difference in results for this method and a method which employed hydrolysis of retinyl esters, and there were significant advantages in specificity, simplicity, cost and quality control. The mean +/- sd total vitamin A concentration calculated as retinol equivalents, for all species surveyed was 139 +/- 96 with a range of 3-1267 mg/kg liver. Mean values for individual species were: calf 188 +/- 125, ox 142 +/- 110, lamb 173 +/- 104, pig 174 +/- 118 and chicken 97 +/- 44 mg/kg liver. No change in the vitamin A concentrations in liver were observed compared with previous surveys in the UK, which suggests that the intake of vitamin A from dietary and supplementary sources by different species has not changed. The data from this survey confirm that liver cannot be recommended as a food for pregnant women.”
Majchrzak D. Fabian E. Elmadfa I. Vitamin A content (retinol and retinyl esters) in livers of different animals. Food Chemistry. 98, 4, 704-710 (2006)
In the present study, 90 animal livers of five different species (pig, cattle, calf, chicken, turkey) were examined for their vitamin A contents. The investigation of extracted vitamin A included all-trans retinol, retinyl palmitate, stearate, oleate and linoleate, expressed as retinol equivalents (RE). The separation of the various chemical forms was done using HPLC. The liver vitamin A contents ranged between 6.5 and 18.9 mg RE/100 g in pigs, from 1.1 to 6.7 mg RE/100 g in cattle and from 1.6 to 16.6 mg RE/100 g and 2.7 to 21.5 mg RE/100 g in chickens and turkeys, respectively. The livers of calves contained the smallest amount of vitamin A, with variation from 1.3 to 3.2 mg RE/100 g. Retinyl palmitate was the predominant form of vitamin A in the livers of investigated animals and contributed about 40% (avids) up to 75% (calf) of the total liver vitamin A contents. The results indicated that the lower levels of animal liver vitamin A, observed in our study, could be a result of small-structured agriculture in Austria. The variations of liver vitamin A concentrations among the species were a result of differences in race, age and the different feeding regimen.