MADISON, Wis. -- The Madison area chapters of the American Red Cross are putting potential blood donors on alert that they are in need more donations.

The late-summer plea isn't unusual for the American Red Cross. Typically, they see blood supply levels fall at about this time of year.

Officials said that it's not summer vacations or donors' busy schedules causing the problem, but rather the quality of donors' blood that is usually to blame for an unstable supply, WISC-TV reported.

During summer many donors experience changes in diet and activity that lower the amount of hemoglobin -- or iron -- in their blood. That typically results in 40,000 to 50,000 donors being turned away nationally, officials said.

Sarah Stevermer, a communications specialist for the American Red Cross, said that what the Red Cross constitutes a "stable" reserve is five days' worth of blood. With that in reserve, Stevermer said, "If an emergency does arise, we have enough blood to send out right away. In the summer, it can get down to as low as a one-day supply and that's when we get concerned and we let the media know about the blood shortages."

Potential donors are advised to follow these steps before donating to make sure their blood has an adequate level of hemoglobin: Eat iron-rich foods such as spinach, broccoli, or beef; avoid iron blockers like caffeine or chocolate; get a good night's rest; eat a full meal before donating.

To make an appointment with the American Red Cross to donate blood, call 1-800-GIVE-LIFE or go to www.givelife.org.