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food source · 5
The liver is where animals store vitamin A: cooked pork liver has about 5400 µg RAE per 100 g, against an adult recommended intake of 700–900 µg a day, so 50 g covers about three days. Keep portions small; the UK NHS advises against eating liver and liver products in pregnancy.
Mechanism · A lifetime's stock
Animals store the vitamin A they eat in the liver. A piece of pork liver holds the preformed retinol, that is, ready-made vitamin A, that the pig built up; unlike beta-carotene, it is not converted only as the body needs it. That makes it extremely dense, and for people who are not short of vitamin A, eating a lot of it becomes a burden.
Safety · Keep the portion small
Cooked pork liver has about 5400 µg RAE per 100 g (raw liver about 6500; braising lowers it somewhat), while the US adult Recommended Dietary Allowance (RDA) is 700–900 µg RAE a day, so 50 g of cooked pork liver covers about three days. The UK NHS advises against eating liver and liver products in pregnancy, because the vitamin A in them can harm the baby. Do not stack pork liver with cod-liver oil or vitamin A supplements either, especially for children.
The orange of carrots comes from beta-carotene, which the body turns into vitamin A as it needs it. Beta-carotene does not dissolve in water; it has to enter tiny droplets formed with dietary fat, called micelles, before the small intestine can take it up, so a carrot eaten plain and one eaten with a little oil deliver different amounts.
Mechanism · Cut on demand, not ready-made
Carrots contain no ready-made vitamin A (retinol); they supply the raw material. An enzyme in the cells of the small intestine cuts beta-carotene down the middle and converts it to retinol, and when the body already has enough vitamin A, this step slows down and the extra pigment settles in the skin. That is why orange palms from eating a lot of carrots are harmless, not poisoning. Ready-made vitamin A from animal foods has no such brake.
In practice · Add oil, cook lightly, skip megadoses
Beta-carotene has to dissolve into micelles, the tiny oil droplets that bile forms from dietary fat and pigment, before it can be absorbed, so cutting carrots up, cooking them until just soft and adding a little oil is the best way to eat them. High-dose beta-carotene supplements are a different matter: in two large randomized trials, ATBC and CARET, smokers who took them developed more lung cancer, not less, so people who smoke or used to smoke should not take high-dose beta-carotene supplements. The amounts you get from vegetables are not part of that concern.
The orange in orange-fleshed sweet potato is beta-carotene, which the body converts into retinol as it needs it. It dissolves in fat, so eating it with a little fat raises absorption considerably.
Mechanism · The orange is raw material
The orange in orange-fleshed sweet potato is beta-carotene, which the wall of the small intestine cuts and converts into retinol as needed; when the body already has enough vitamin A, this step slows, so the surplus does not keep turning into vitamin A. How much each person converts also varies. Purple-fleshed sweet potato is colored mainly by anthocyanins, and white and yellow flesh has little pigment, so none of them are sources here.
In practice · It wants oil, not dry heat
A drizzle of olive oil on steamed sweet potato, or eating it alongside a dish with some fat, helps the beta-carotene get absorbed. The cooking method also affects blood glucose: in a study of ten sweet potato varieties, boiled sweet potato had a glycemic index (GI) of about 41–50 and baked or roasted about 79–94 (Bahado-Singh 2011). For vitamin A, what counts is fat, not dry heat.
The orange of pumpkin is beta-carotene, which the body cuts into vitamin A as it needs it. As with carrots and sweet potatoes, it is absorbed better when eaten with a little fat.
Mechanism · Orange is raw material, oil the carrier
The orange of pumpkin is beta-carotene, which the body cuts into vitamin A as needed. It does not dissolve in water and has to enter micelles formed with fat before it can be absorbed; when the body already has enough vitamin A, conversion slows, so the surplus does not keep turning into retinol.
Numbers · Vitamin A and blood glucose, counted apart
Cooked pumpkin is about 90% water. Its glycemic index (GI) is on the high side, but a 150 g serving holds only about 12–15 g of digestible carbohydrate, so its glycemic load is not high; its vitamin A and its effect on blood glucose are two separate ledgers. Pumpkin also contains some potassium, which people who need to limit potassium should count.
The orange of mango flesh comes from beta-carotene, which the wall of the small intestine cuts and converts into vitamin A as the body needs it. It dissolves only in fat, so eaten on its own a good share passes straight through the gut; a little fat alongside makes it worth much more.
Mechanism · Eaten alone, some passes through
Beta-carotene does not dissolve in water; bile and the fat in food have to wrap it into tiny oil droplets before it can reach the cells of the gut wall. Inside the cell, a dedicated enzyme cuts it down the middle and converts it into vitamin A, and when the body already has enough vitamin A, this step slows down. That is why getting vitamin A from orange fruit does not build up to toxic levels the way swallowing high-dose vitamin A can.
In practice · Keep two ledgers apart
Eating mango with a little yogurt or a few nuts helps the beta-carotene get absorbed. Mango is also a sweet fruit, so its vitamin A and its effect on blood glucose are separate ledgers; dried mango concentrates the sugar and is no substitute for fresh fruit.
cofactor · 2
→Zinc
The liver needs zinc to make retinol-binding protein (RBP), the protein that carries stored vitamin A around the body. When zinc is short, vitamin A cannot be shipped out even if the liver's stores are adequate, and blood vitamin A runs low, looking like vitamin A deficiency; zinc has to be corrected first.
Vitamin A is the raw material for rhodopsin, the light-sensing protein in the retina's rod cells, so the first thing to fail when it runs short is vision in dim light, known as night blindness. It governs seeing in the dark, not how nearsighted you are; marketing often blurs the two.
regulates · 1
Vitamins C and E and the carotenoids together form the lung's antioxidant network, but the benefit from the diet as a whole is modest. A single nutrient at a high dose backfired: in the ATBC trial, beta-carotene supplements raised lung cancer in smokers by 18%.