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1. Fear of needles? Ooked out by the sight of blood?
It helps to not look at the needles, the bags, or anything else related to the process, at least the first few times through. I discovered that really helped me when I was new to the process! It does get easier -- my first donation was at age 17, and I'm now 36. Aside from pregnancy and bouts of the flu or whatnot, I've been a regular donor since, and the needles and all stopped bothering me in fairly short order. I just kept eye contact with people whenever possible, and whenever not possible, I kept my eyes closed.
2. Worries about Mother Nature and donating?
a generally healthy person will not be aversely affected by the coincidence of events. You might be a little woozier afterwards, or need a nap and an extra snack than what you've planned on, so keep an eye on how you feel. In either event, you need more fluids, more rest, and good attention to nutrition, anyway, so with both happening at once, just be extra attentive to being healthy. They won't let you donate if your iron is low, or if other conditions are in place that WOULD hurt you, so please be at ease there.
3. Some hints about good iron levels. Vitamin C does help blood iron levels. Eating a little more than the recommended 3-5 daily fruits/veg is good in many ways, for many people's health (talk to your doctor first, esp. with blood sugar and other issues), and will help improve iron levels. Make sure that you get a good diversity of fruits and vegetables, too! Also, if you're not vegetarian/vegan/pescatarian, super-lean red meat is recommended. Many grocers sell 90% lean beef, and some will also have 93% lean. Whole grains and beans are also recommended to help balance things out. Of course, it won't hurt to eat chicken, fish, eggs, turkey, etc., either.
4. If you're told that your veins roll a lot, you probably need to hydrate yourself better.