A total of 10 websites selling NSs for diabetes were analysed (Table 1). All the websites were located in USA. In general the NSs were offered to promote healthy blood sugar levels and reduce the risk of diabetes and health-related conditions. Half of them suggested contacting a physician about taking NS, two of them only in the case of pregnancy, breastfeeding or suspected medical problems. Four websites contained the FDA disclaimer "This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease". On one website the FDA disclaimer was not complete. The company Xtend-Life stated that the product on offer was not a substitute for diabetes medication substitution, but this statement was in the FAQ section and there was no reference to the FDA. In only 3 out of 10 websites the manufacturer was clearly indicated. (Data not shown in table).
Scientific references supporting the NS offered were present on 3 websites only. The characteristics of references retrieved on the three websites are shown in Table 2. Sixty percent of the websites selected showed testimonials from customers who had bought the NS.
We found a total of 71 different ingredients in the 10 NSs selected and some of them were present in more than one supplement (Figure 2).
Research on PubMed regarding the ingredients of nutritional supplements (NSs). MAs, meta-analyses; NSs, nutritional supplements; RCTs, randomized control trials; SRs, systematic reviews.
A total of 10 NS ingredients were searched for on PubMed as they were present in at least 3 NSs. All the ingredients except two (Gymnema sylvestre and Vaccinium myrtillus) were analysed in RCTs (Figure 2).
In Additional file 1 are shown the characteristics of the studies investigating the ingredients selected and the main results. The largest RCT enrolled 447 subjects and regarded biotin. In the majority of cases (58%), the MA or SR retrieved only included a few studies (from 2 to 4).The largest study was an SR on chromium that encompassed 20 separate studies.
Reduction in fasting blood glucose, the most commonly used outcome measure, was statistically significant (p <; 0.05) for Gymnema sylvestre and alpha lipoic acid (ALA) in one of four studies, magnesium in two of three studies, chromium in four of six studies, zinc in one of four studies, biotin and Momordica charantia in one of three studies and Trigonella foenum-graecum in one of two studies.
Another outcome taken into account was the reduction in haemoglobin A1c, which was statistically significant (p <; 0.05) for Gymnema sylvestre, magnesium in one of two studies, chromium picolinate in one of four studies, zinc in one of two studies, biotin and Trigonella foenum-graecum in one study and Momordica charantia in one of two studies.
Adverse effects were rarely considered: in two of four studies for ALA, one of two studies for Camellia sinensis, two of three studies for biotin and Momordica charantia and one of two studies for Trigonella foenum-graecum.
Table 3 shows the dosage of the selected ingredient indicated in the NS offered by the websites and the tested dosage from the studies retrieved. For seven out of ten ingredients the dosage was specified on less than 50% of the websites. The daily dosage indicated for the NS offered by the websites and the dosage of the relative ingredient tested were comparable in the case of ALA, magnesium, chromium and zinc. In the other cases the dosages were quite different.
BMC Public Health. 2013;13(777) © 2013 BioMed Central, Ltd.
© 1999-2006 BioMed Central Ltd