April 24, 2008

4 Min Read
Nutrition Q&A

Does soy affect thyroid gland function?

Foods that inhibit thyroid function are called goiterogens. There are, in fact, some studies in animal and in vitro models suggesting soy and its isoflavones inhibit thyroid hormone production.1 Several well-designed human studies have provided a clearer understanding of soy's effect in humans.

In one study, three soy powders containing different levels of isoflavones—one powder containing virtually none, the second a modest level (1 mg/kg) and a third a high level (2 mg/kg)—were given to 18 postmenopausal women for three months each in a crossover-design study. Researchers measured thyroid hormones and noted only small changes between the groups.2

In a larger and more recent study, researchers worked with 73 postmenopausal women for six months. Group one's protein powder contained casein from nonfat dry milk, group two's had isolated soy protein containing 56 mg isoflavones and group three's had isolated soy protein with 90 mg isoflavones. The results showed the soy protein groups had minimal effects on thyroid function.3

Still, some people are likely "thyroid sensitive" to soy protein or its isoflavones. Therefore, people with a history of thyroiditis or those who are on a marginally iodine-deficient diet should be cautious. But for the majority of individuals, I think soy is unlikely to have any long-term negative effects on thyroid function.

I've heard berries have a powerful antioxidant called ellagic acid. Has this phytochemical been well researched?

Berries—blackberries and raspberries in particular—are extremely rich in a phenolic compound called ellagic acid. Ellagic acid appears to work as a potent anticarcinogen by binding cancer-causing chemicals and improving the body's ability to get rid of them. In essence, ellagic acid improves the body's ability to detoxify these substances.4,5

In one 30-week study, rats fed a diet consisting of 5 percent whole, freeze-dried raspberries had 39 percent fewer tumors than rats not fed the berries, while those fed a 10 percent raspberry diet had 49 percent fewer tumors.6 Ellagic acid has been shown to inhibit chemically induced cancer in the lung, liver, skin and esophagus of rodents.7 Ellagic acid may inhibit events associated with both the initiation and promotion stages of carcinogenesis, a promising ability considering the limited number of synthetic agents that can achieve this. Human studies are under way.

Does taking too much vitamin A increase osteoporosis risk?

Researchers found when animals consumed massive amounts of vitamin A, their bone remodeling was affected. This led some researchers to suspect high vitamin A intake could contribute to fracture risk in humans. In the Nurse's Health Study, more than 72,000 women, aged 34 to 77 years, were studied to see if vitamin A intake, as measured by dietary questionnaires, correlated in any way with hip fracture risk. The researchers found that women in the highest quintile of total vitamin A intake from diet and supplements [>=3,000 mcg per day of retinol equivalents (one RE equals 3.33 international units)] had a significantly elevated relative risk of hip fracture (RR=1.48) compared with women in the lowest quintile of intake (<1,250 mcg per day of RE). Beta-carotene did not contribute significantly to fracture risk, only preformed vitamin A.8

This is important information and again suggests there is a range of safety for everything, even nutrients. Some important conditions warrant extra vitamin A, even at high dosages for short duration. However, we should pay attention to this evolving research. Taken as a supplement long term, Vitamin A doses should probably be less than 5,000 IUs, unless specifically advised otherwise.

Dan Lukaczer, N.D., is director of clinical research at the Functional Medicine Research Center, a division of Metagenics Inc., in Gig Harbor, Wash.


1. Chang HC, Doerge DR. Dietary genistein inactivates rat thyroid peroxidase in vivo without an apparent hypothyroid effect. Toxicol Appl Pharmacol 2000;168(3):244-52.

2. Duncan AM, et al. Modest hormonal effects of soy isoflavones in postmenopausal women. J Clin Endocrinol Metab 1999;84(10):3479-84.

3. Persky VW, et al. Effect of soy protein on endogenous hormones in postmenopausal women. Am J Clin Nutr 2002;75(1):145-53.

4. Ahn D, et al. The effects of dietary ellagic acid on rat hepatic and esophageal mucosal cytochromes P450 and phase II enzymes. Carcinogenesis 1996;17(4):821-8.

5. Szaefer H, et al. Effect of naturally occurring plant phenolics on the induction of drug metabolizing enzymes by o-toluidine. Toxicology 2003;186(1-2):67-77.

6. Kresty LA, et al. Chemoprevention of esophageal tumorigenesis by dietary administration of lyophilized black raspberries. Cancer Res 2001;61(16):6112-9.

7. Stoner GD, Mukhtar H. Polyphenols as cancer chemopreventive agents. J Cell Biochem Suppl 1995;22:169-80.

8. Feskanich, D, et al. Vitamin A intake and hip fractures among postmenopausal women. JAMA 2002;287(1):47-54.

Natural Foods Merchandiser volume XXIV/number 7/p. 41

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