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I have been under word-of-mouth impression that while vitamin A is most replete in meats, a vegan who eats nutrient rich vegetables or sweet potatoes and carrots intakes their full share. However, the chemical reaction necessarily to create vitamin A in these vegetables requires cooking, which poses an additional challenge for raw vegans.

I was not easily able to validate these claims either direction from a few google searches. Do veggies fulfill vitamin A requirements, and does cooking impact this?

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Sources of Vitamin A

There are two dietary sources of vitamin A:

  • Taking up retinoids (vitamers of vitamin A) directly, which occurs in animal based, non-vegan food.
  • Converting a form of provitamin A into vitamin A. Provitamin A is abundant in lots of vegetables.

A quick note on supplements

Both forms can also be taken as a food supplement. Retinol overdose is harmful, especially during pregnancy. Provitamin A cannot be overdosed so easily since conversion to vitamin A stops with a high vitamin A blood level.

(Given the long discussion on the other answer I felt that this needed to be addressed, even if out of scope for the question.)

Provitamin A sources

The most common source of provitamin A is β-carotene. It occurs in all orange vegetables, such as carrots and sweet potatoes, but also in many others.

Other sources include α-carotene and γ-carotene. Not all carotenes are a source of provitamin A.

Provitamin A conversion

In a healthy human, provitamin A is converted into vitamin A. Conversion depends on the specific source of provitamin A, and other factors.

The actual question: Cooking and raw preparation

The bioavailable amount of vitamin A depends greatly on the preparation. In studies the following preparation methods of carrots improved β-carotene blood levels:

  • Slicing, ideally homogenizing (e.g. blending)
  • Cooking
  • Extracting juice
  • Preparation in fat

Effects of cooking

Cooking a carrot can have several effects:

  • The carrot is usually sliced in smaller pieces.
  • Fat is added.
  • Carrot cells break and release β-carotene.
  • The carrot is heated, and β-trans-carotene is converted to β-cis-carotene.

The first three of these increase the β-carotene blood level. The last one is potentially a problem, since β-cis-carotene is not converted to vitamin A as effectively as β-trans-carotene.

It seems like there is no decisive result yet whether the process of heating is beneficial or not. But certainly either option can provide you with sufficient amounts of provitamin A.

My recommendation: Grate your carrot and add some linseed oil for a great raw salad. Blend it for a great smoothie.

Main source: National Academies of Science - Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc, and the plentiful sources therein.

  • "whether the process of heating is beneficial or not" -- Since you mention earlier there are multiple preparation methods that improved β-carotene blood levels, do you mean "whether heating is more beneficial than slicing, blending, or adding fat"? I hesitate to edit it myself because I'm not clear on your meaning :) – Erica Feb 7 '17 at 20:10
  • @Erica, no, I mean whether heating in itself is beneficial at all. As far as I know there is no study that blends carrots with some oil and then lets some people eat it raw and some people eat it cooked. It's only known that the combination of blending and cooking is beneficial. Please do edit if you see potential for clarification. – Turion Feb 8 '17 at 9:28
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TL;DR:

  • Do veggies fulfill Vitamin A requirements?

  • Does cooking impact vitamin absorption?

    • Research is not conclusive. Chewing your raw veggies well can be as effective for absorption as cooking.

Background

Vitamin A comes into two forms:

  • retinoids (mostly found in animal food: shrimp, salmon, eggs, milk, yogurt, etc.)
  • carotenoids (mostly found in plant food: sweet potato, carrots, spinach, kale, etc.)

For effective maintenance of health we require both types. In some people, their bodies actually convert carotenoids to retinoids so they're generally ok even while not taking any animal food. Others with genetic influences, digestive problems, etc., are at a disadvantage as this conversion doesn't happen for them.

The retinoid form is of particular concern for those who are pregnant.

Impact of Cooking

Carotenoid Forms [Vitamin-A found in plant food]

The jury is still out regarding the impact of cooking and processing on carotenoids in food. High-carotenoid foods like carrots typically have the vast majority of their carotenoids occurring in all-trans form. While this form can provide excellent health support, it is not as readily available to the bloodstream or to our cells as another form called the cis form.

The cooking of a plant food decreases the total amount of all-trans carotenoids found in the food, but it also increases conversion of many all-trans carotenoids into their more available cis form. As a net result, some studies show better support of carotenoid levels in the blood after consumption of a cooked plant food product (like tomato paste) than a non-cooked plant food product (like fresh tomatoes). In the case of tomatoes, the carotenoid of greatest interest has been lycopene, and several studies have shown better support of blood lycopene levels from cooked and processed tomato products versus fresh tomatoes.

However, other factors may have played an important role here, including the breaking apart of cells in the tomato during processing. The crushing of the tomato cells may have made the cell contents more readily available for digestion and metabolism and thereby improved blood levels of tomato constituents, including lycopene. If this sequence of events played a key role, it gives all of us a very good reason to eat slowly and do an outstanding job chewing our food (including tomatoes). Thorough chewing could accomplish the same result as industrial processing, i.e., breaking open of most tomato cells and providing us with easier access to their nutrients.

Top 25 Vitamin A-Containing WHFoods Retinol and Carotenoid Content*

+---------+-----------------------+-------------------------------+------------------+----------------------------+---------------------+---------------------------+----------------+
| Ranking |         Food          | Total for All Forms (mcg RAE) | Retinol (mcg RE) | Total Carotenoids (mcg RE) | Beta-carotene (mcg) | Lutein & Zeaxanthin (mcg) | Lycopene (mcg) |
+---------+-----------------------+-------------------------------+------------------+----------------------------+---------------------+---------------------------+----------------+
| 1       | Sweet potato          | 1922                          | 0                | 3844                       | 23018               | 0                         | 0              |
| 2       | Carrots               | 1019                          | 0                | 2038                       | 10108               | 312                       | 1              |
| 3       | Spinach               | 943                           | 0                | 1887                       | 11318               | 20354                     | 0              |
| 4       | Kale                  | 885                           | 0                | 1771                       | 10625               | 23720                     | 0              |
| 5       | Mustard greens        | 865                           | 0                | 1732                       | 10360               | 8347                      | 0              |
| 6       | Collard greens        | 722                           | 0                | 1444                       | 8575                | 11774                     | 0              |
| 7       | Turnip greens         | 549                           | 0                | 1098                       | 6588                | 12154                     | 0              |
| 8       | Swiss chard           | 536                           | 0                | 1072                       | 6391                | 19276                     | 0              |
| 9       | Winter squash         | 535                           | 0                | 1071                       | 5726                | 2901                      | 0              |
| 10      | Romaine lettuce       | 409                           | 0                | 819                        | 4912                | 2173                      | 0              |
| 11      | Bok choy              | 361                           | 0                | 722                        | 4333                | 65                        | 0              |
| 12      | Cantaloupe            | 271                           | 0                | 541                        | 3232                | 42                        | 0              |
| 13      | Bell peppers          | 144                           | 0                | 288                        | 1494                | 47                        | 0              |
| 14      | Parsley               | 128                           | 0                | 256                        | 1536                | 1691                      | 0              |
| 15      | Broccoli              | 121                           | 0                | 241                        | 1449                | 1685                      | 0              |
| 16      | Asparagus             | 91                            | 0                | 181                        | 1087                | 1388                      | 54             |
| 17      | Sea vegetables        | 81                            | - -              | - -                        | 973                 | - -                       | - -            |
| 18      | Chili peppers         | 80                            | 0                | 160                        | 810                 | 17                        | 1              |
| 19      | Tomatoes              | 75                            | 0                | 150                        | 808                 | 221                       | 4631           |
| 20      | Basil                 | 56                            | 0                | 112                        | 666                 | 1198                      | 0              |
| 21      | Papaya                | 131                           | 0                | 262                        | 756                 | 246                       | 5045           |
| 22      | Shrimp                | 102                           | 102              | 102                        | 0                   | 0                         | 0              |
| 23      | Eggs                  | 75                            | 74               | 75                         | 5.5                 | 176                       | 0              |
| 24      | Brussels sprouts      | 61                            | 0                | 121                        | 725                 | 2012                      | 0              |
| 25      | Grapefruit (pink/red) | 59                            | 0                | 119                        | 706                 | 8                         | 1453           |
+---------+-----------------------+-------------------------------+------------------+----------------------------+---------------------+---------------------------+----------------+
RAE = retinol activity equivalents, RE = retinol equivalents 
* All values are listed per serving size as identified on our website (whfoods.com).
- Provided that your body has the ability to effectively convert carotenoids into retinoids, 
  you'll actually end up with more retinoid forms of vitamin A by eating any of these 
  Top 25 Vitamin A foods, even though all of these are plant foods that do not directly 
  provide any Vitamin A in retinoid form.

Source: World's Healthiest Foods

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