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Spinach is sometimes considered a good source of iron, especially when paired with ascorbic acid (vitamin C) as described in this answer.

If I am consuming spinach by blending it up in a smoothie, how much orange juice (or whole oranges, lemons, or limes) need to be added or consumed to provide enough vitamin C to help with iron absorption? Does the amount of vitamin C required go up when consuming larger amounts of spinach?

Also, does the freshness of orange juice make a difference to the amount needed?

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Consume equal parts spinach and orange/lemon juice for best absorption of iron. It’s okay to drink more orange or lemon juice than that, but it might be more costly.

The presence of vitamin C is most important when eating foods that also contain inhibitors of iron absorption. Orange juice and spinach are a great combination.

The promotion of iron absorption in the presence of AA is more pronounced in meals containing inhibitors of iron absorption.

A ratio of 7 mg ascorbic acid (vitamin C) to 1 mg iron is sufficient for foods with medium levels of inhibitors, though a higher ratio of 14:1 may promote absorption when high levels of inhibitors are present.

Meals containing low to medium levels of inhibitors require the addition of AA at a molar ratio of 2:1 (e.g., 20 mg AA: 3 mg iron). To promote absorption in the presence of high levels of inhibitors, AA needs to be added at a molar ratio in excess of 4:1, which may be impractical.

A single serving of spinach (30 grams, or approx 30 baby leaves) contains 0.81 mg iron. To support absorption, the serving should be paired with 5.7-11.4 mg vitamin C. Orange juice is very rich in vitamin C, and 11.4 mg vitamin C is provided by just 25 ml of orange juice. Just one tenth of a cup of orange juice is sufficient to pair with one serving of spinach.

Here's a quick summary. Roughly speaking, equal parts spinach and citrus (orange or lemon juice) enable good digestion of the non-heme iron in spinach.

                               Orange juice (ml)  Lemon juice (ml)
One serving spinach (30 g)                    25                30
Five servings spinach (150 g)                125               150

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