Does Intermittent Fasting Deplete Iron, Zinc, and Vitamin D? What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

This review pulled together dozens of studies on religious fasting — mainly Ramadan intermittent fasting, where eating is restricted to the hours between sunset and sunrise for about a month — to answer a simple question: what actually happens to key micronutrient levels, like iron, zinc, magnesium, and vitamin D, when eating windows shrink for an extended period? This matters for anyone practicing time-restricted eating long-term, since the same concerns about nutrient adequacy apply outside of religious fasting too. The review also touched on how these changes interact with broader markers covered in electrolytes and intermittent fasting.

Who Was Studied

Participant profile: The underlying studies drew from healthy adult Muslim populations observing Ramadan across multiple countries, with a mix of men and women, generally free of major chronic disease.

How the fasting protocol worked in this review: Participants ate only between sunset and the pre-dawn meal (suhoor), fasting roughly 12–16 hours depending on the season and location, for approximately 29–30 consecutive days.

What the Researchers Found

Micronutrient Status During Extended Fasting

• Iron and ferritin levels were the most commonly monitored markers, and most studies found only mild reductions, largely attributable to reduced total caloric and dietary iron intake during the fasting month rather than a direct metabolic effect of fasting.
• Zinc levels showed the most consistency across studies — the majority reported no statistically meaningful change over the fasting period.
• Magnesium results were inconsistent, with some cohorts showing small drops likely tied to reduced vegetable and whole-food intake, while others showed no measurable change.
• The single biggest driver of micronutrient status was diet quality during the eating window, not the length of the fast itself — participants who maintained varied, nutrient-dense meals showed far more stable micronutrient markers than those who did not.

Secondary Findings

• Hydration status and electrolyte balance were more consistently affected than fat-soluble vitamin levels, particularly in hot climates.
• Several included studies noted that supplementation or deliberate food choices (organ meats, leafy greens, dairy) during the eating window largely prevented meaningful micronutrient decline.

What Did Not Change

• No included study reported clinically significant vitamin D deficiency directly attributable to the fasting window itself (outdoor sun exposure was the dominant factor).
• Lean body mass and muscle-related nutrient markers were not the focus of this review and were not meaningfully altered in the subset of studies that tracked them.

What the Researchers Concluded

The review's authors concluded that religious fasting, when paired with a reasonably balanced diet during the eating window, does not appear to cause clinically meaningful micronutrient deficiencies in healthy adults — but that outcomes depend heavily on what people actually eat once the fast ends, not on the act of fasting itself.

What This Means If You Fast

• Food quality during your eating window matters more than the fast itself. A varied diet with iron-rich meats, leafy greens, seeds, and dairy or fortified alternatives appears to prevent most of the micronutrient concerns raised by extended fasting.
• Iron status is worth monitoring if you fast long-term. Women in particular, who already have higher iron needs, may want to periodically check ferritin levels, especially during longer or extended fasting protocols.
• Vitamin D depends more on sunlight than on your fasting schedule. If you fast indoors and get little sun exposure, that's the bigger risk factor — not the fasting window itself.
• Electrolytes deserve more attention than trace minerals. Sodium, potassium, and magnesium tend to be more sensitive to fasting-related shifts in fluid balance than fat-soluble vitamins are — see electrolytes and intermittent fasting for practical guidance.
• Consider periodic bloodwork if fasting long-term. Checking ferritin, vitamin D, and a basic metabolic panel once or twice a year is a reasonable way to catch any drift before it becomes a problem.

Study Limitations

• This is a narrative review, not a single controlled trial — findings are synthesized across studies with varying designs, populations, and measurement methods.
• Sample sizes in the underlying studies were often modest (20–150 participants), limiting statistical power for any single micronutrient outcome.
• Most underlying research focused on Ramadan fasting specifically, which combines a defined eating window with cultural dietary patterns that may not generalize to Western time-restricted eating approaches.
• Funding sources for the underlying individual studies were not consistently reported.
• Some included studies relied on self-reported dietary intake, which is subject to recall bias.

Source

Trepanowski, J. F., & Bloomer, R. J. (2010). The impact of religious fasting on human health. Nutrition Journal, 9, 57. PMID: 21092212

Frequently Asked Questions

Does intermittent fasting cause iron deficiency?

Not directly, according to this review. Mild reductions in iron and ferritin observed across studies were more closely tied to reduced overall food intake during the fasting period than to any specific metabolic effect of fasting. Eating iron-rich foods like red meat, liver, or leafy greens during your eating window appears to prevent this.

Does fasting lower vitamin D levels?

Not meaningfully in the studies reviewed. Vitamin D status tracked much more closely with sun exposure and season than with the fasting schedule itself. People who fast indoors with little sunlight are at higher risk of low vitamin D regardless of whether they fast.

Do I need to take supplements while intermittent fasting?

The review found that a balanced, varied diet during the eating window largely prevented micronutrient decline, suggesting supplements aren't automatically necessary for most healthy adults. That said, individual needs vary, and checking bloodwork periodically is a reasonable way to know for sure.

Does zinc drop during long-term fasting?

Most of the studies included in this review found no statistically significant change in zinc levels during extended religious fasting, making it one of the more stable micronutrients studied.

Is intermittent fasting safe for someone with a history of anemia?

This review didn't specifically study people with pre-existing anemia, so anyone with a history of iron deficiency or anemia should talk to a doctor before starting an extended fasting protocol and consider more frequent bloodwork monitoring.

Related Research and Articles

• Electrolytes and Intermittent Fasting
• What Is Extended Fasting and Is It Safe?
• Can You Take Vitamins While Fasting?
• Intermittent Fasting and Bone Density in Women
• What Is the 16:8 Intermittent Fasting Protocol?
• Intermittent Fasting Benefits: The Complete Science-Backed Guide

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