Unless you eat a lot of grass-fed butter, raw milk, cheese and egg-yolks daily (or you happen to like liver, or approve of it in the diet) – you may be deficient in vitamin A. Even a mild deficiency can inhibit the immune system, etc.
Vitamin A is crucial for the proper functioning of the thyroid (e.g., "The effects of vitamin A deficiency and vitamin A supplementation on thyroid function in goitrous children." J Clin Endocrinol Metab. 2004 Nov). This means that male testosterone, and male and female fertility will be compromised when vitamin A is low in the body:
"...It has long been known that vitamin A is required in rats and mice for testicular functions...In summary, vitamin A is essential for normal female reproductive function..."
(Kalampokas T, Shetty A and Maheswari A (2014) Vitamin A Deficiency and Female Fertility Problems: A Case Report and Mini Review of the Literature. J Women's Health Care 3:210. doi: 10.4172)
Vitamin A is also crucial for the proper functioning of the immune system, eye-sight, and much more:
"This vital nutrient is needed for the growth and repair of body tissues; it helps protect mucous membranes of the mouth, nose, throat and lungs; it prompts the secretion of gastric juices necessary for proper digestion of protein; it helps to build strong bones and teeth and rich blood; it is essential for good eyesight...and contributes to the health of the immune system…the thyroid gland requires more vitamin A than the other glands, and cannot function without it…Vitamin A inhibits the effects of phytic acid and increases absorption of iron from whole wheat. (Arch Latinoam Nutr Sep 2000;50(3):243-8). Vitamin A supplementation increases absorption of iron and folic acid in women in Bangladesh (Am J Clin Nutr Jul 2001;74(1):108-15)."
(Sally Fallon, Mary G. Enig)
"Vitamin A is involved in regulating the growth...of virtually all cells in the human body. Vitamin A has important roles in embryonic development, organ formation during fetal development, normal immune functions, and eye development and vision...Vitamin A deficiency is associated with an increased susceptibility to infections, as well as to thyroid and skin disorders...Animal food sources rich in preformed vitamin A include dairy products...Vitamin A was initially coined 'the anti-infective vitamin' because of its importance in the normal functioning of the immune system...vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin, the oxygen carrier in red blood cells...Vitamin A deficiency often coexists with iron deficiency and may exacerbate iron deficiency anemia by altering iron metabolism...Even children who are only mildly deficient in vitamin A have a higher incidence of respiratory complications and diarrhea...In North and West Africa, vitamin A deficiency and iodine deficiency induced-goiter can coexist in up to 50% of children..."
"More recently, researchers have found vitamin A to be important to many other processes. These include preventing childhood mortality, preventing childhood asthma, promoting pubertal development, protecting against oxidative stress, protecting against environmental toxins, preventing kidney stones, regulating the amount of fat tissue in the body, regulating blood sugar, and protecting against fatty liver disease."
Indeed, many may be weak or often sick due to the lack of foods such as grass-fed butter as a daily part of the modern diet:
"At the end of the First World War, a physician named Bloch discovered that a diet containing whole milk, butter, eggs and cod liver oil cured night blindness...In one important experiment, Bloch compared the results when he fed one group of children whole milk and the other margarine as the only fat. Half of the margarine-fed children developed corneal problems while the children receiving butterfat and cod liver oil remained healthy…The actual discovery of vitamin A is credited to a researcher named E. V. McCollum…The year was 1907…He discovered that rats fed pure protein, pure skim milk, sugar, minerals and lard or olive oil for fat failed to grow. When he added butterfat or an extract of egg yolk to their diets, their health was restored…He discovered a fat-soluble factor in certain foods that was essential for growth and survival. This was named 'fat-soluble factor A'…Research by Osbourne and Mendel, published just five months after McCollum's study, found that cod liver oil produced the same results as butter in rat studies, thus confirming the early work of Mori in Japan…The scientific term for vitamin A is retinol, because of its presence in the retina of the eye...In recent decades, much vitamin-A research has focused on its role in preventing cancer...Only animal fats contain vitamin A and vitamin A is present in large amounts only when the animals have a source of carotenes or vitamin A in the diet, such as green pasture, insects and fish meal…"
(Sally Fallon, Mary G. Enig)
In light of the above history, we can better understand why the Bible specifically reveals that the Lord Jesus ate butter as a child, in order to grow! Olive oil is certainly a foundational part of a healthy, Biblical diet, but we are specifically told that He ate butter:
Isaiah 7:15 Butter and honey shall he eat, that he may know to refuse the evil, and choose the good.
Dairy was indeed a major part of the Biblical diet, insuring necessary vitamin A and other nutrients:
Levitical 20:24 But I have said unto you, Ye shall inherit their land, and I will give it unto you to possess it, a land that floweth with milk and honey...
The suggested RDA for vitamin A (for men and adolescents) is 900 mcg. A large egg yolk may contain 80 mcg. A portion of cheese is around 70-90 mcg. A tablespoon of butter is 95 mcg. A cup of milk is in the same neighborhood as the above levels, depending upon the source. Many nutrition researchers believe that the RDA for vitamin A is way too low. Therefore, it is easy to see why many people, depending solely on natural foods, may be deficient in this vital nutrient. It would take a lot of dairy each day even to fulfill the RDA.
The idea that most people can easily obtain vitamin A from vegetables is a myth. For starters, it is reported that about 45% of the population carries at least one gene variation that significantly hinders the conversion of beta-carotene into vitamin A:
"Unfortunately, the vast majority of popular books on nutrition insist that humans can obtain vitamin A from fruits and vegetables…Even worse, FDA regulations allow food processors to label carotenes as vitamin A...Under optimal conditions, humans can indeed convert carotenes to vitamin A…Early studies indicated an equivalency of 4:1 of beta-carotene to retinol. In other words, four units of beta-carotene were needed to produce one unit of vitamin A. This ratio was later revised to 6:1 and recent research suggests an even higher ratio. This means that you have to eat an awful lot of vegetables and fruits to obtain even the daily minimal requirements of vitamin A, assuming optimal conversion…But the transformation of carotene to retinol is rarely optimal. Diabetics and those with poor thyroid function, a group that could well include at least half the adult US population, cannot make the conversion. Children make the conversion very poorly and infants not at all…zinc deficiency and even cold weather can hinder the conversion of carotenes to vitamin A, as does the lowfat diet…Carotenes are converted by the action of bile salts, and very little bile reaches the intestine when a meal is low in fat…Butterfat stimulates the secretion of bile needed to convert carotenes from vegetables into vitamin A, and at the same time supplies very easily absorbed true vitamin A…It is very unwise, therefore, to depend on plant sources for vitamin A."
(Sally Fallon, Mary G. Enig)
"The vitamin A activity of β-carotene is variable and surprisingly low in women...The vitamin A activity of β-carotene, even when measured under controlled conditions, can be surprisingly low and variable...The carotene in fruit...grains...and oils...seems to be more effective as a source of vitamin A than that in dark-green leafy vegetables...The human intestine has a limited capacity to absorb intact β-carotene..."
(The American Journal of Clinical Nutrition, Volume 75, Issue 5, 1 May 2002)
Assuming optimal conversion, eating excessively large amounts of certain vegetables raises the risk of thyroid inhibition from various antinutrients and goitrogenic properties.
Many have turned to liver to insure high amounts of vitamin A in the diet. A portion of turkey liver, or a cup of chicken giblets (i.e., the necks, hearts, livers, and gizzards of poultry), can have over 5000 mcg of vitamin A. Likewise, a small serving of beef liver can have over 4000 mcg. There is therefore a controversy among holistic nutrition researchers over the danger of these high vitamin A levels from livers. This is especially the case with cod liver oil. Many products on the market that go by this name often have synthetic vitamin A added in extremely high amounts.
The synthetic form of vitamin A (in supplements or added to foods) is linked to numerous problems, and may even hinder the absorption of true vitamin A.
Is liver a healthy part of the Biblical diet? Modern Jews reportedly eat liver if it is prepared carefully. Notice a report from the 18th century:
"M. Paul Martin Alberti, the first eighteenth-century writer to broach this topic, states that the Jews refrain from eating liver unless it is completely roasted and all blood has been removed."
(Judaism in Christian Eyes)
The Bible is not easy to understand on this topic. For example, kidneys are forbidden, along with the caul above the liver:
Leviticus 3:4 And the two kidneys, and the fat that is on them, which is by the flanks, and the caul above the liver, with the kidneys, it shall he take away.
There is no mention of specific liver prohibition, unless it is somehow assumed from Leviticus 3:4, etc.
If we assume that liver is meant to be eaten, the fact remains that each animal only has one liver. It would not appear to have been a daily part of the family meal - and certainly not in any great amounts (at least as far as cattle, goats, and other larger livestock animals are concerned).
Perhaps the answer is therefore in the amount consumed. There is some evidence that high amounts of liver in the diet may inhibit the thyroid and bring other serious problems:
"Thyroid Enlargement Following Liver Feeding in Rats...That the feeding of liver to animals produces an enlargement of the thyroid gland was reported by Hunt, who found that the thyroid of mice fed on a diet of oatmeal and liver was larger and more vascular than of those on a diet of eggs, crackers and milk. Marine produced a hyperplasia of the thyroid in brook trout by feeding them with liver and heart. Burget found that rats kept under hygienic conditions and fed a high protein diet, consisting of fresh liver and lean beef mixed with a little oatmeal or bread crumbs, developed a hyperplasia of the thyroid...Remington reported that in a low iodine goiter-producing diet, the replacement of a part of the wheat gluten by dried pig liver aggravated the degree of goiter..."
(H.C. Hou, Proc. Soc. Exptl. Biol. Med., 1940).
"All the liver fed animals showed distinctly heavier thyroids..."
(Burget, G. E. Attempts to produce experimental thyroid hyperplasia. Am. J. Physiol. 44:492-503. 1917)
In these cases, perhaps the suppression of the thyroid was due to excess vitamin A in from the liver diet:
"In the 1940s, Simkins demonstrated that patients with hyper-thyroidism, were successfully treated with high dose of vitamin A..."
(Haugen B.R. (2004) The Effect of Vitamin A, Retinoids and Retinoid Receptors on the Hypothalamic-Pituitary-Thyroid Axis)
"Barker et al. (l95l) found that a 500-times normal dose of vitamin A produced 50 percent inhibition of thyroid activity in rats...Thyroid activity was not inhibited in the rats when a 100-times normal dose was administered."
(Dana Justin Wolf, The Effect of a Dietary Goitrogen and Vitamin A on Thyroid Function in Cattle)
Based on the information in this article (until further light is gained), it appears wise to make sure lots of organic egg yolks, grass-fed butter, raw milk and cheeses are a daily part of the diet.
Perhaps poultry liver, etc., from clean, organic, free-range animals is safe and ordained by God to eat. Perhaps it should be limited to once a week or so, to insure adequate amounts of vitamin A, but not to raise levels excessively high. A small amount of true cod liver oil, with no synthetic additives, etc., as the amount of Vitamin A is monitored throughout the week.
As to the use of desicated liver capsules (from chemical free cattle), again, the levels of vitamin A in the daily diet should be carefully measured.
It is a fact that the kidneys (especially) collect more toxins or heavy metals than the rest of the animal. Liver, to a lesser degree, also collects more toxins. If liver is eaten, it should be washed and roasted, from safe, clean sources.
The advocates of liver argue that it is the synthetic vitamin A that is causing the most problems. They also argue that only certain brands of cod liver oil should be purchased.
Those who are careful to avoid thyroid inhibitors and get enough iodine, will profit greatly from adequate vitamin A levels. So many areas of the body are affected, including the immune system.