The Harvard Nurses' Health Study II followed 18,555 women and identified specific dietary patterns associated with a 66% lower risk of ovulatory infertility. The key findings: replace trans fats with monounsaturated fats, choose plant protein over animal protein, favor full-fat dairy over low-fat, take an iron supplement, and eat slow carbs instead of refined ones. A Mediterranean-style diet pattern has since been linked to improved IVF outcomes in multiple studies.
The Harvard Nurses' Health Study
This is the largest prospective study ever conducted on diet and fertility. Researchers at the Harvard School of Public Health followed 18,555 married nurses from 1991 to 1999, tracking their diets and pregnancy outcomes. The results, published in Obstetrics & Gynecology (2007) and later compiled in the book The Fertility Diet, identified a combination of dietary factors that, when followed together, reduced ovulatory infertility risk by 66%.
The key word is “ovulatory infertility.” This dietary pattern specifically addresses anovulation and irregular ovulation — the most common cause of female infertility. It does not address tubal factor, male factor, or other structural issues. But since ovulatory dysfunction accounts for approximately 25–30% of all infertility, the dietary impact is clinically meaningful.
The Five Key Findings
1. Replace Trans Fats with Unsaturated Fats
Every 2% increase in calories from trans fats (instead of carbohydrates) was associated with a 73% increase in ovulatory infertility risk. Trans fats increase insulin resistance, promote inflammation, and interfere with ovulation. While trans fats have been largely eliminated from the US food supply since the FDA ban, they still appear in some imported products, fast food, and margarine.
Action: Cook with olive oil or avocado oil. Eat avocados, nuts, and fatty fish (salmon, sardines). Avoid anything with “partially hydrogenated oil” on the label.
2. Choose Plant Protein Over Animal Protein
Replacing 25 grams of animal protein with 25 grams of plant protein was associated with a 50% lower risk of ovulatory infertility. The mechanism likely involves insulin sensitivity — plant proteins tend to have lower insulin-stimulating effects than animal proteins, particularly red and processed meat.
Action: Add beans, lentils, chickpeas, tofu, and nuts as protein sources. You don't need to go vegetarian — just shift the ratio toward more plant-based meals each week.
3. Favor Full-Fat Dairy Over Low-Fat
This was one of the more surprising findings: each serving of low-fat dairy per day increased ovulatory infertility risk by 11%, while each serving of full-fat dairy decreased it by 27%. The hypothesis is that the process of removing fat from dairy also removes fat-soluble hormones (estrogen, progesterone) that may support ovulation, while concentrating non-fat components that may inhibit it.
The practical shift
Switch from skim or low-fat milk to whole milk. Choose full-fat yogurt over fat-free. Have a small serving of ice cream instead of frozen yogurt. This is a temporary dietary shift for the TTC period — once pregnant, you can return to whatever dairy preference you like. The caloric difference is modest: whole milk has about 60 more calories per cup than skim.
4. Eat Slow Carbs Instead of Fast Carbs
High glycemic-index foods (white bread, white rice, sugary drinks, potatoes) cause rapid blood sugar and insulin spikes. Chronic insulin elevation disrupts ovulation, particularly in women with PCOS or insulin resistance. The Nurses' Health Study found that a high glycemic load was associated with significantly higher ovulatory infertility risk, especially in women without diagnosed PCOS.
Action: Replace white bread with whole grain, white rice with brown rice or quinoa, sugary cereals with oatmeal. Pair carbohydrates with protein or fat to slow absorption.
5. Take an Iron Supplement from Plants or Supplements
Non-heme iron (from plants and supplements) was associated with lower ovulatory infertility risk. Women who consumed 40+ mg/day of supplemental iron had a 40% lower risk compared to women consuming less than 20 mg/day. Heme iron (from meat) showed no protective effect.
The Mediterranean Diet Connection
Since the Harvard study, multiple smaller studies have linked Mediterranean-style diets to improved fertility and IVF outcomes:
| Study | Finding |
|---|---|
| Vujkovic et al. (2010), Fertility & Sterility | Couples following a Mediterranean diet had 40% higher odds of pregnancy through IVF |
| Karayiannis et al. (2017), Human Reproduction | Women scoring highest on Mediterranean diet adherence had 2.7x higher IVF success rates |
| Gaskins et al. (2019), Human Reproduction | Higher Mediterranean diet adherence associated with more eggs retrieved, better embryo quality, and higher live birth rates |
| Sun et al. (2020), Nutrients | Mediterranean diet improved sperm concentration and total motile count in male partners |
The Mediterranean diet overlaps substantially with the Harvard findings: high in plant protein, olive oil, whole grains, fish, and vegetables; low in red meat, processed food, and refined carbs. It also includes moderate amounts of full-fat dairy, particularly yogurt and cheese.
The TTC plate
- Half the plate: Vegetables and fruits (aim for variety and color)
- Quarter: Whole grains (brown rice, quinoa, oats, whole wheat)
- Quarter: Protein — emphasize plant sources (lentils, beans, tofu) and fish; limit red meat to 1–2 servings/week
- Fats: Olive oil, avocado, nuts as primary fat sources
- Dairy: 1–2 servings/day full-fat (whole milk, full-fat yogurt)
- Supplements: Prenatal vitamin with iron + folate; vitamin D; omega-3 DHA
What to minimize
- Sugar-sweetened beverages: Associated with lower fertility in both partners. One soda per day may reduce per-cycle fecundability by 20–25%.
- Trans fats: Check labels for partially hydrogenated oils. Essentially eliminated in the US but still present in some products.
- Excessive caffeine: Keep under 200 mg/day (roughly one 12 oz coffee). Higher intake may delay conception and increase miscarriage risk.
- Alcohol: No proven safe amount for TTC. Even moderate drinking (3–6 drinks/week) has been associated with reduced fertility in some studies.
- Processed meat: Hot dogs, bacon, deli meat — associated with lower sperm quality in men and potentially impaired ovulation in women.
Supplements Matter Too
Diet is the foundation, but targeted supplements can fill critical gaps. See our evidence-based prenatal guide.
Read the Prenatal Vitamin Guide