They state two differences between their product and plain Vitamin C
- It is not the free ascorbic acid but the calcium salt
- Vitamin C metabolites are present
The first aspect is supposed to lead to a better tolerance of the Vitamin C, as the calcium ascorbate is less acidic than the free ascorbic acid. The metabolites are supposed to increase the absorption of Vitamin C, according to the claims of the company behind Ester-C
On their homepage the company lists the two following studies as evidence for their claims:
The study about Vitamin C absorption found no difference in plasma levels, but a difference in the Vitamin C level in leukocytes:
No significant difference in plasma
vitamin C levels was observed when
comparing the different preparations.
However, at 24 hours, calcium
ascorbate with metabolites resulted in
significantly higher concentrations of
vitamin C in leukocytes (P<0.0001)
compared with vitamin C alone.
The study investigating if Ester-C is better tolerated found a positive effect:
Investigators concluded that Ester-C
compared with AA caused significantly
fewer epigastric adverse effects in
participants sensitive to acidic foods
and that Ester-C is much better
There is a third, older study not mentioned on the website that reportedly did not find any difference in Vitamin C absorption between Ester C and other products:
Comparison of the absorption and excretion of three commercially available sources of vitamin C.
Unfortunately this one does not have an abstract and the article is not freely available, I have to rely on the second-hand reporting for this one.
Those are pretty small studies (15 persons in the first one, 50 in the second one), I don't think they can settle those questions on their own. The absorption claim in general is not validated by the studies, except for the leukocytes. The better tolerance of the calcium ascorbate seems to be supported by one of the studies.