Finally, not all cooking methods are presented here--lighter cooking methods obviously result in less vitamin losses.

VITAMIN LEVELS IN RAW VS. COOKED FOODS
(uncorrected for water loss)

(Note: Amounts are expressed in units per 100g portion of food.
In the table below, "boiled" = boiled, drained, without salt.)

Food	Water Content (%)	Vitamin Assay
		B6 (mg)	Folate (mcg)	B12 (mcg)	A (IU)	E (mg)
Broccoli, raw	91%	.16	71	0	1,500	1.7
Broccoli, boiled	91%	.14	50	0	1,400	1.7
Beef liver, raw	69%	.95	250	69	35,000	.67
Beef liver, braised	66%	.91	220	71	35,000	?
Beef liver, pan-fried	56%	1.4	220	112	36,000	.63
Carrots, raw	88%	.15	14	0	28,000	.46
Carrots, boiled	87%	.25	14	0	24,000	.42
Almonds, dried	4%	.11	59	0	0	24
Almonds, dry-roasted	3%	.07	64	0	0	5.5
Mung bean sprouts, raw	90%	.09	61	0	21	.01
Mung bean sprouts, boiled	94%	.05	29	0	14	.01
Tomatoes, red, raw	94%	.08	15	0	620	.38
Tomatoes, boiled	92%	.09	13	0	740	.38
Mackerel, Atlantic, raw	63%	.40	1.3	8.7	165	1.5
Mackerel, cooked, dry heat	53%	.46	1.5	19	180	?

Here is the average vitamin loss, expressed in percentages, and corrected for the effect of water loss:

VITAMIN LOSSES FROM COOKING
(corrected for water loss)

(Expressed as average percentage decrease from value observed in raw food.)

Food	Water Content	Vitamin Assay
		B6	Folate	B12	A	E
Average Losses (% lost compared to Raw value)
Uncorrected for water loss	8	-5	12	?	3	3
Corrected for water loss	--	3	20	?	11	11

Overall vitamin losses due to cooking are relatively modest. While there are a few inconsistencies in the above tables due likely to differing samples, globally we see that, on average, cooking does destroy vitamins, but the consequences are not catastrophic. Average vitamin losses after correction for water loss range from about 10 to 25% in most cases. Also, vitamin losses correlate with what our textbook by Kreutler et al. [1987] said, but not precisely, so obviously heat is only one of the many factors which affect vitamin content.

Table illustrates dietary variety is as significant to nutrition as cooking. Finally, given the extreme variability in vitamin content of different foods, there is a strong case to be made that variety is at least as important as cooking practices: someone eating 95% raw fruit is far more likely to be vitamin-deficient than someone eating a 100% cooked diet, but including a variety of foods like liver, green vegetables, etc.

• First of all, anti-vitamins present in some foods can be destroyed by heat. (For example, avidin, which in raw egg whites binds to biotin.)

• Gelatinization of starch. As mentioned previously, cooking significantly improves the digestibility/bioavailability of starchy foods such as tubers (potatoes, yams, etc.), squashes, grains, and legumes through the process of gelatinization.

• Effect of cooking on bioavailability of beta-carotene. Let's mention here the effect of heat on carotenoids [Erdman et al., 1993]. Some plant foods, such as orange or dark green vegetables like carrots or spinach, contain hundreds of carotenoids, some of which are antioxidants and thus are protective against cancer. The most well-known is beta-carotene, which converts in the body as vitamin A. The absorption and transport are quite complex and not yet well-understood.

  From Erdman et al. [1993], beta-carotene absorption can be as low as 1-2% from raw vegetables such as the carrot. Mild heating, such as steaming, appears to improve the extractability of beta-carotene from vegetables, and also its bioavailability. (Remark: "extracted" means put in contact with digestive juices. Nutrients in fibrous plants are not necessarily easily extracted; and something that is extractable may not necessarily have good bioavailability, i.e., be efficiently usable by the body.) However, additional heating can transform the naturally occurring trans double bonds into cis configurations, which reduces the biological value of beta-carotenes. For example, canning of sweet potatoes and carrots results in the conversion of 75% of all trans B-carotene to 13- or 9-cis isomers. (Note: Canning involves long periods at high heat, much more than ordinary steaming.) This means that 75% of the naturally occurring molecules of beta-carotene are transformed into molecules having the same chemical formula, but a different shape, and which are not usable by the body.

  From Johnson [1991], heat treatment of raw carrot juice at temperatures comparable to those at pasteurization and boiling does not change the carotenes, while heating at temperatures used during sterilization results in rearrangement of the carotene molecules and a decrease in total available carotenes (by transforming the naturally occurring trans into cis form). Note: sterilization typically uses temperatures of 180°C (356°F) for a few seconds, and kills all germs, whereas pasteurization is done at temperatures below boiling, and only kills most germs.

• Finally, folate in raw broccoli is only slightly more bioavailable than folate in cooked broccoli [Clifford et al. 1990].

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