Iron Deficiency Anemia
Ruling out iron deficiency anemia is one of my first considerations when a pre-menopausal woman presents with fatigue or low energy. Other suggestive symptoms include (but are not limited to): pale skin, frequent colds/infections, ringing in ears, shortness of breath, dizziness and fainting.
Besides iron's contribution to oxygen transport throughout the body (which is a pretty big deal), it also supports healthy immune functioning, energy metabolism, and acts as a co-factor for many enzymes in the body.

Let’s pretend that your blood work has revealed low iron. Supplementation can be great to replenish levels, but there are often overlooked factors that should also be addressed. This is especially true when supplementation doesn't seem to be raising your levels.
1. Lack of stomach acidity- Heme iron (from animal sources) is bound to proteins and requires acid and digestive enzymes to be released in order to be properly absorbed. Non-heme iron (from plant sources) also needs acidity to become stabilized in a form that can be absorbed. Stomach acidity can decline if you’re in sympathetic overdrive (a.k.a constant stress), eating under less than ideal conditions (such as multitasking, eating too quickly, on the go etc.) on a PPI, and/or have low thyroid function.
2. Integrity of gastrointestinal tract is impaired- Iron is absorbed in the small intestine, so any source of inflammation is going to limit this transport. This includes conditions like: dysbiosis, IBS, Celiac disease, IBD, infection, food sensitivities, autoimmune diseases etc.
3. Consuming foods that block absorption - Casein proteins, found in milk, eggs, and dairy products decrease iron absorption. Tannins and phytic acid found in plant-based foods like black or green tea, grains, nuts, tofu, cocoa, etc. have also been found to increase iron absorption, however - this is counteracted by their vitamin C content, which increases iron absorption. Bottom line, don't consume your iron supplement with a caffeine containing drink and consider eliminating or minimizing dairy consumption.
4. You’re simply not consuming enough - This is more common if you follow a vegan or vegetarian diet. I'll get into this more in Part II.
5. Heavy menstruation- Normally, you lose about 30-40mL of blood per cycle and it’s considered “heavy” when loss reaches ~60-80mL per cycle. No one actually measures their menses each month…hopefully…but you can do a rough calculation to find out how much you’re losing:
1 soaked normal sized tampon or pad= 5mL of blood
1 full diva cup=15mL of blood
∴using more than 12 pads or tampons per cycle could indicate a heavy cycle
6. Medications - There are quite a few, but some common ones that inhibit iron absorption which include: PPIs, NSAIDs, and antacids.
Stay tuned for Part 2 where I’ll discuss food sources of iron, the best supplements, and my thoughts on conventional reference ranges!
Please note this is to serve as general information, not medical advice. There can be more serious reasons for anemia which need to be investigated. Please see your medical care provider for individualized advice regarding your concerns.
References:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4086762/
http://sci-hub.io/10.1080/09637480500085820
http://www.aafp.org/afp/2007/0615/p1813.html
http://www.cemcor.ubc.ca/resources/very-heavy-menstrual-flow
http://lpi.oregonstate.edu/mic/minerals/iron#summary