We posted part one of this blog last week where we discussed the two types of iron found in food as well as the food compounds that help to increase iron absorption. You can read part one here.
This week we’re continuing our conversation on iron absorption by discussing those compounds that inhibit its absorption. Such compounds not only inhibit the iron found within that food item, but also inhibit the absorption of iron found in other foods eaten at the same time. Knowing which foods inhibit absorption will help you decide how best to organize your meals and determine which foods should be eaten separately from other foods. On the other hand, if you are looking to lower your iron intake, these foods will help achieve this when paired with iron-containing foods.
Food compounds that inhibit iron absorption include calcium, phytates, oxalates, tannins, and polyphenols.
Serving Size Determines Nutrient Amount
All foods discussed below are those that contain the nutrient in question per serving size. This means that a person can eat more (or less) than one serving to affect nutrient intake. A serving size is considered to be 1 cup raw vegetables or ½ cup cooked vegetables. It is also 1 piece of fruit the size of a lightbulb, or 2 pieces of fruit if they are small (like kiwis). Dairy products are one cup for milk and yogurt and 1/3 cup for cheeses. Grains and legumes are both a half-cup serving while nuts are one handful.
MyFoodData.com is a great tool for listing the breakdown of nutrients in a food. The Cronometer app displays this same information as well and might be more convenient to use when standing in a grocery store.
When To Eat Iron-Inhibiting Foods
Let’s outline the various compounds found in food that can alter how much iron your body gets. In an effort to increase our absorption of iron, we not only want to increase our intake of foods that enhance iron absorption (discussed in part one), but we also want to decrease our intake of iron inhibiting foods by eating them away from iron-containing meals. What this means is to separate these foods by at least 2 hours on either side of an iron meal. In this way, foods that inhibit iron would make for great snacks in between meals.
Calcium
Calcium plays a large role in preventing iron absorption. It is unique in that it is the only known compound that affects both types of iron. All other compounds that affect iron absorption only affect non-heme sources and have no effect on heme iron. Since calcium inhibits both heme and non-heme iron, it is best to avoid eating high-calcium foods with any iron-containing meal, especially a meat meal.
Studies have shown that consuming 300 mg or more of calcium with an iron-containing meal will prevent between 56% and 59% of all iron from being absorbed. However, eating less than 300 mg of calcium with that same meal greatly decreases how much iron is inhibited (meaning that smaller amounts of calcium in food will not have a huge effect on inhibiting iron from your meal). Consuming 30 mg or less of calcium has minimal effects on inhibiting absorption of iron.
Below is a list of foods with less than 30 mg of calcium per serving:
Vegetables:
- Mushrooms
- Corn
- Garlic
- Eggplant
- Potatoes
- Bell peppers
- Onions
- Green leaf lettuce
- Tomatoes
- Cucumber
- Squash
- Swiss chard
- Zucchini
- Cauliflower
- Beets
- Carrots
- Snow peas
Dairy and Eggs:
- Ghee
- Butter, pat
- Whipped butter
- Cream cheese
- Cottage cheese
- Whipped cream
- Egg, one
Meat:
- Bacon
- Salami
- Game meat (deer, bison)
- Steak
Other:
- Nuts (besides almonds)
- Beans (besides black beans and blackeyed peas)
- Whole grains (besides amaranth)
Let’s also check out the foods high in calcium (100 mg or more per serving). Be sure to eat these foods away from iron-containing meals, especially when consuming more than one serving (amounting to more than 300 mg of calcium).
- Firm tofu (1720 mg)
- Soy milk (300 mg)
- Soybeans (175 mg)
- Blackeyed peas (100 mg)
- Almond milk (480 mg)
- Plain yogurt (450 mg)
- Cows milk (300 mg)
- All cheeses (250-300 mg)
- Whey protein powder (200 mg)
- Sesame seeds (280 mg)
- Chia seeds (180 mg)
- Leafy greens (100-245 mg)
- Grapefruit juice and orange juice (350 mg)
- Canned sardines (325 mg)
- Salmon (180 mg)
- Shrimp (125 mg)
- Oysters (122 mg)
- Walleye (175 mg)
- Trout (123 mg)
- Pork chop (105 mg)
- Teff (123 mg)
- Amaranth (115 mg)
A Note About Eggs
Although eggs are low in calcium, which thus does not alter iron absorption in significant ways, the egg yolk is another story. Egg yolks contain phosphoprotein, a compound that impairs iron absorption. One egg yolk can inhibit absorption of iron by 28% when consumed with an iron-containing meal. For this reason, it is recommended to eat eggs away from meals that contain iron.
Phytates and Oxalates
Phytates and Oxalates are both naturally occurring compounds found within various vegetables including nuts, beans, and whole grains. Collectively, these compounds function to protect living plants against being eaten and to help in the germination of their seeds. In humans, however, phytates and oxalates are not necessary to the human diet and even inhibit some absorption of iron and calcium. Because of this, these compounds can be ill-suited for some individuals and when eaten in excess in one sitting. For this reason, phytates and oxalates are oftentimes considered to be “antinutrients” since they can rob us from absorbing nutrients. Those with low iron should consider decreasing their intake of phytates and oxalates or eating them separately from iron-containing foods.
Phytate Foods and How to Lower Them
Phytates are found in high concentrations in beans, whole grains, nuts, seeds, and soy products (soy milk, soybeans, and tofu). These compounds are very effective at inhibiting iron absorption, up to 50%-65% of non-heme iron!
The good news is that lowering phytate concentration in foods is very easy. Most antinutrients are water-soluble. This means you can use any method of preparation that introduces water to the food and the antinutrients will leach out of the food and into the water. Soaking these foods in water overnight is the best way to lower phytates. Phytate foods that are best for soaking are raw beans, nuts, and whole grains (such as quinoa and rice). Keep in mind that preparing any food in water will cause it to leach out all water-soluble compounds, not just antinutrients. This would include any vitamin C and the B vitamins, especially thiamin. Whenever you use water to remove phytates, always be sure to discard the used water. In the case of raw beans, always rinse the beans after discarding the soaking water. If you are using canned beans you do not need to soak them prior to using, but you do need to rinse them thoroughly. The phytates (and lectins, another kind of antinutrient) are present in the surrounding liquid found in the can, so it is very important to strain out all of this liquid and rinse until all the bubbles are gone from the strainer.
Other than soaking and rinsing phytate foods, you can also lower phytate content by sprouting. Seeds, whole grains, and raw beans are the best candidates for sprouting (also called germination). Sprouting causes phytates to degrade.
Fermentation is another way to lower the phytate content in foods. Whole grains and raw beans are ideal for fermenting. Simply soak whole grains overnight, or at least for a few hours. Add a splash of an acidic liquid to the soaking water, such as apple cider vinegar or lemon juice. In the case of raw beans, be sure to soak overnight at least. Replace the soaking water at least once during this time, being sure to rinse the beans prior to re-soaking. Once again, add an acidic liquid to the soaking water. Although soy products are high in phytates, tempeh is not. This is because tempeh is made by fermentation. Even with its low phytate content, however, tempeh is high in calcium (180 mg per cup) and so should be consumed in smaller amounts (quarter cup servings instead).
Perhaps the easiest way to prevent phytates from inhibiting the absorption of iron is to consume these foods away from an iron-containing meal (at least 2 hours away on either side of the meal). Keep in mind that eating phytate foods on their own (without preparing them in the above mentioned ways) will still inhibit the non-heme iron present in those foods. But it won’t affect the iron absorption of your meal eaten a couple hours later.
Lastly, we can consider serving sizes. This is especially true for nuts and seeds, where usual serving sizes are too small to contain any significant amount of phytates. Be sure to abide by correct serving sizes: nuts should be a small handful and seeds are no more than a tablespoon or so.
Phytates are not degraded by heat, so cooking these foods will not decrease their phytate content.
Oxalate Foods and How to Lower Them
Aside from phytates, oxalates are the other “anti-nutrient” that inhibits iron absorption (as well as calcium). Oxalates are found in a variety of foods rather than in entire food groups, making them more difficult to memorize when at the store. When wanting to increase iron absorption, it is best to limit oxalate foods to no more than 50 mg of oxalates per day. Below is a list of foods that contain high amounts of oxalates (over 50 mg of oxalates per serving):
- raw spinach
- Rhubarb
- rice bran
- Buckwheat
- Almonds
- soy products
- sweet potato
- white potato
- navy beans
- dark chocolate
- Beets
- Millet
- okra
Oxalates are also found in Swiss chard however they are not bioavailable (able to be used) by the body.
Oxalates are degraded by moist heat cooking. They are also water-soluble (like phytates). Preparing and cooking oxalate foods in water will leach out and break down its oxalate content. Steaming, boiling, and pre-soaking, are the most effective methods for greatly reducing oxalates. As you can see from the list above, each of those foods respond well to one or more of these methods. Navy beans, buckwheat, and nuts are the only ones that can be soaked, and the rest can be boiled and/or steamed. As with phytates, always be sure to discard the used water and remember that soaking or cooking in water will cause food to leach more than just phytates, but also all water-soluble compounds (like vitamin C and the B vitamins). Unlike phytates, oxalates are not removed through fermentation or sprouting.
Another way to lower oxalates is simply to eat those foods that contain low to moderate levels (less than 30 mg of oxalates per serving). Some examples of low oxalate foods are:
- Fruits: bananas, blackberries, blueberries, cherries, strawberries, apples, apricots, lemons, peaches
- Vegetables: mustard greens, kale, bokchoy, broccoli, cabbage, cauliflower, mushrooms, onions, peas, zucchini
- Grains and starches: white rice, corn flour, oat bran
Tannins and Polyphenols
Tannins and polyphenols are both potent iron inhibitors. Polyphenols are a class of phytonutrients. There are over 500 types of polyphenols, one of which is tannins. Other types of polyphenols you may have heard of are flavonoids, quercetin, and isoflavones. Polyphenols, as a group, help plants function and stay healthy. Tannins, in particular, make fruits and herbs unpalatable to deter being eaten. They are responsible for the bitter taste of unripe fruit, tea, coffee, wine, and chocolate. When it comes to iron, studies show that tannins and polyphenols in beverages specifically (not food) significantly inhibit non-heme iron absorption from a meal, by as much as 60% to 90%!
Tannins
Coffee and black tea contain the most tannins while white tea and oolong tea contain moderate amounts. The amount of tannins in a typical mug of coffee can inhibit non-heme iron absorption by 40% and black tea inhibits non-heme iron by 64%. Be sure to drink coffee and black tea away from iron-containing meals, at least 2 hours on either side of a meal.
Polyphenols
As with tannins, coffee and black tea top the list for polyphenol content. Green tea also tops the list here. Studies have shown that beverages containing between 100 to 400 mg of polyphenols per serving (standard mug size), inhibit absorption of non-heme iron by 60%-90%. Coffee, black tea, and green tea, each contain between 200-400 mg of polyphenols per serving, so it’s important to separate these beverages by at least 2 hours on either side of an iron-containing meal.
It’s important to note that tannin and polyphenol content are present in both caffeinated and decaffeinated coffees and teas. Their concentration is the same strength and unaffected by the amount of caffeine present.
For those who need to increase their iron, it’s important to be aware of which foods to combine together in the same meal and which foods to separate from each other. There are many compounds that inhibit the absorption of iron from other foods in the same meal. Calcium is the only compound that inhibits iron absorption of both heme and non-heme iron. Antinutrients, phytates and oxalates, block the absorption of iron and other minerals. Lastly, tannins and polyphenols, specifically in coffee and tea, can block iron absorption up to 90%! Foods containing these compounds should be consumed away from an iron-containing meal so that they don’t disrupt the iron found in other foods.