5 fascinating nutrition science studies: Milk, blood pressure, and more

Welcome to the eighth edition of ZOE Nutrition Science Roundups. This series shines a light on recent studies from the world of nutrition science.

We explain what the researchers did, what they found, and why it matters for your health. 

1. Iron supplements and gut bacteria

Around 14–18% of people in the United States take iron supplements, most commonly to treat iron deficiency anemia — a lack of iron.

Adequate iron levels are essential for human health. And many bacteria also need iron to survive.

Some experts are concerned that iron supplements might support “bad” gut bacteria. For instance, studies have shown that reducing access to iron limits the growth of “bad” bacteria.

So, a group of researchers set out to understand whether iron supplements alter your gut microbiome.

What did they do?

The team tested the impact of two types of iron supplements in 172 women in Cambodia.

Each participant received an iron supplement or a placebo each day. The trial ran for 12 weeks, and the participants provided poop samples at the start and end of the study.

What did they find?

Overall, the iron supplements didn’t affect the diversity of the participants’ gut microbiomes.

And a diverse gut microbiome is associated with good health, so that’s a positive finding.

However, the scientists did find a slight increase in levels of Enterobacteriaceae. This is a large group of bacteria, and some types can cause infections.

What should you do? 

Most people can get enough iron from their diet. Foods like red meat, seafood, spinach, peas, and broccoli contain high levels.

If you do need to take iron supplements, they seem unlikely to harm the diversity of your gut microbiome.

2. Different diets, different gut microbiomes

Over the last few decades, vegetarianism and veganism have grown in popularity. Flexitarianism — eating less meat and more plants — has also become common. 

A recent study examines how these different dietary patterns influence your gut microbiome, as well as your energy and fiber intake. 

What did they do?

The scientists analyzed the gut microbiomes of 258 participants. Each had followed a vegan, vegetarian, flexitarian, or Western diet for at least the past year.

The team also had information about what the participants were eating.

What did they find?

The researchers found that a lower intake of animal products was associated with a lower intake of calories and a greater intake of dietary fiber. 

Also, the participants’ gut microbiomes differed in diversity and composition.

For instance, those with a Western diet had more of a particular bacterium linked to insulin resistance, an increased risk of bowel cancer, and a higher risk of irritable bowel syndrome (IBS). 

Meanwhile, the vegan group had more bacteria associated with reduced cardiovascular risk factors.

Examples of these risk factors include high body mass index, blood pressure, and levels of blood fat.

What should you do?

At ZOE, we don’t believe that eating healthy necessarily means cutting out animal products. But we know that eating a wide variety of plants — and, therefore, fiber — can benefit your health.

So, this study is further evidence of links between a high-fiber diet, a happy gut microbiome, and a reduction in risk factors for disease.

ZOE’s Scientific Co-Founder Prof. Tim Spector recommends eating 30 plants each week to support your gut and overall health.

3. Plant compound and kidney cancer

Polyphenols are plant compounds linked to good health.

They’re a broad range of chemicals, and many have antioxidant and anti-inflammatory properties.

A recent study investigated links between polyphenols called anthocyanidins and kidney cancer risk.

Anthocyanidins occur in many plants. They’re responsible for the red and purple coloring in berries and other fruits.

What did they do?

The researchers followed more than 100,000 people for around 12 years. They had information about the participants’ health and diets.

What did they find?

The analysis revealed that higher consumption of anthocyanidins was associated with a lower risk of developing kidney cancer.

What should you do?

At ZOE, we know that colorful plants are excellent for your health.

So, try to “eat the rainbow.” Choose different-colored fruits and vegetables whenever possible.

And if you’re aiming to eat 30 plants a week, focusing on color can help. For instance, red, yellow, and green peppers count as three of your 30 because they contain different polyphenols.

The scientists behind the study call for more research to confirm their findings.

But in general, eating colorful fruits and veggies is a good way to support your overall health. 

4. Milk and blood pressure

High blood pressure, or hypertension, affects almost half of U.S. adults. And it’s a leading cause of cardiovascular diseases, like stroke and heart disease.

Some previous research has suggested that drinking milk is associated with having lower blood pressure. But other research in adults with higher blood pressure didn’t confirm this association

So, scientists in South Korea recently reopened the debate with a new study investigating links between drinking milk and having hypertension. 

What did they do?

The researchers accessed data from more than 60,000 participants who had filled out food questionnaires. 

The participants had their blood pressure checked at the start of the study and at the end, an average of 5 years later.

What did they find?

The analysis showed that people who drank the most milk were less likely to have hypertension than those who drank the least milk.

Also, the milk-drinkers’ blood pressure was less likely to worsen over the 5-year span.

What should you do?

Milk is rich in vitamins, minerals, and protein. It’s a welcome addition to a healthy diet.

So, if you like milk, enjoy it. And it might help you control your blood pressure, too.

If you’d like to learn more, we have a fascinating podcast on dairy.

5. Vegetarianism and heart disease

Globally, cardiovascular disease is a leading cause of death.

Many factors can contribute to your risk, including your diet. And there’s some evidence that a vegetarian diet might reduce your risk.

So, a new review and meta-analysis asks: Can switching to a vegetarian diet benefit people who already have cardiovascular disease or have a high risk of developing it?

What did they do?

The scientists identified 20 studies that had investigated this question. Together, the studies had looked at 1,878 participants.

The team pooled the data and crunched the numbers. 

What did they find?

The researchers found that people at risk of developing cardiovascular disease who switched to a vegetarian diet for an average of 6 months did get some benefits.

There were improvements in some risk factors, including improved levels of “bad” cholesterol and blood sugar, as well as reduced body weight.

What should you do?

As we’ve mentioned already, upping your intake of plant foods is an excellent way to support your health.

Animal products can still be part of a healthy diet. If you do decide to go vegetarian, it’s important to make sure you still focus on food quality. 

So, replace meat with healthy options, like tofu or nuts, rather than highly processed alternatives. 

The take-home

What should we make of the results of these interesting recent studies? Here are your key takeaways:

  1. If you take iron supplements, they don’t seem to impact your gut microbiome. But most people can get enough iron from their diet. So, unless you need to take supplements for health reasons, focus on food sources.

  2. Eating less meat is linked to getting more fiber and having a healthy gut microbiome. So, upping your fruit and veggie intake is a smart move.

  3. Polyphenols from colorful plants may reduce your risk of kidney cancer. And if they don’t, “eating the rainbow” is still likely to support your overall health.

  4. Drinking milk might be linked to reduced blood pressure. And if it isn’t, milk can still be part of a healthy diet. 

  5. If you’re at high risk of cardiovascular disease, going vegetarian might help. If you’re not ready to cut meat out entirely, reducing your meat intake and increasing your plant intake is still likely to promote good health.


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High dietary intake of whole milk and full-fat dairy products does not exert hypotensive effects in adults with elevated blood pressure. Nutrition Research. (2019). https://www.sciencedirect.com/science/article/pii/S0271531718309254 

Iron. (2023). https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/ 

Role of dietary fiber and energy intake on gut microbiome in vegans, vegetarians, and flexitarians in comparison to omnivores — insights from the Nutritional Evaluation (NuEva) Study. Nutrients. (2023). https://www.mdpi.com/2072-6643/15/8/1914

The effect of oral iron supplementation on gut microbial composition: A secondary analysis of a double-blind, randomized controlled trial among Cambodian women of reproductive age. Microbiology Spectrum. (2023). ​​https://journals.asm.org/doi/full/10.1128/spectrum.05273-22 

The top 10 causes of death. (2020). https://www.who.int/news-room/fact-sheets/detail/the-top-10-causes-of-death 

Vegetarian dietary patterns and cardiometabolic risk in people with or at high risk of cardiovascular disease: A systematic review and meta-analysis. JAMA Network Open. (2023). https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2807597