Do I Need Vitamin C with my Iron Supplement?
The answer is mostly no, and it all depends upon which type of iron supplement you take. When considering Vitamin C and Iron absorption, remember that there are two types of iron supplements. Ionic Iron supplements are inexpensive to manufacture and are found in various forms as salts, chelates and polysaccharides. Heme Iron on the other hand is relatively expensive to manufacture and is available in the form of Heme Iron Polypeptide (HIP) which is refined from Bovine or Porcine Hemoglobin.
Vitamin C and Iron Absorption – How does it Work?
All Ionic Iron supplements require an acidic GI environment in order to properly dissolve and be available for absorption via the ionic transport pathway. Because of this Vitamin C (ascorbic acid) is added to many ionic iron supplements in an attempt to improve the relatively poor absorption of these products. Since HIP is absorbed by a totally different mechanism , HIP is far easier to absorb, absorbs more completely, and does not require an acidic GI tract to absorb well. Vitamin C does not help or hinder the absorption of HIP and thus it is not added to HIP containing iron supplements. A little known fact is that the mere presence of HIP actually enhances the absorption of the dietary ionic (non-heme) iron  such as additives that are found in “enriched” products like bread, cereals and snack cakes.
Vitamin C and Iron – The Risks
Is there any downside to taking Vitamin C with your ionic iron supplement? The answer is an emphatic YES! “Co-supplementation of ferrous salts with Vitamin C exacerbates oxidative stress in the gastrointestinal tract, predisposing individuals to ulceration, inflammatory disorders, and exacerbation of existing chronic disorders and may cause cancer”.  So, while it is inexpensive to take an Ionic iron supplement, adding Vitamin C to enhance absorption (as most such formulations do) may not be in the best interest of the individual taking the iron supplement. A better approach would seem to be to gain absorption via taking Heme Iron in the form of HIP as a supplement, thereby avoiding the GI oxidative stress associated with taking Ionic irons combined with Vitamin C.
1 Andrews, NEJM 2005; 353:23 pp2508-2509
2 Lynch, et. al., Am. Journal of Clinical Nutrition 1985; 41 pp 13-20
3 Fisher, et. al., Nutrition Journal 2004; 3:2