Eat Your Leafy Greens: Dietary Nitrate Improves Metabolic Health, Study Finds

By Bridget Coyle-Asbil 

14 July 2022

We have all been reminded, countless times, of the importance of eating leafy greens.  Now, a new study led by Dr. Graham Holloway and post-doctoral fellow Dr. Henver Brunetta from the Department of Human Health and Nutritional Sciences suggests these superfoods could be even more beneficial than previously thought.

Leafy greens, along with beetroot, are an important source of dietary nitrate. Once thought to be harmful due to a suspected link to cancer, nitrate has since been shown to be indispensable to normal physiological functioning. This includes past research led by other investigators, which found that removing nitrates from the diet led to a significant decline in metabolic health.   

Those results led this duo to ask: could simply adding nitrates to the diet improve health in an animal already suffering metabolic issues?

“I’m fascinated by how we can simply use a natural compound from foods to improve our metabolic health,” says Brunetta. “You don’t need to rely solely on doctors or drug therapies, you can start by simply picking the right foods at the grocery store.”

The researchers launched their study by feeding mice a high fat diet to induce a metabolic condition called “insulin resistance”.  Insulin is a very important hormone, produced by the pancreas, that lowers blood sugar levels after a meal. When an animal is insulin resistant, the hormone isn’t working properly, and the pancreas has to work twice as hard to get the same metabolic response. This places stress on the pancreas and can lead to diabetes.

The high fat diet led to insulin resistance, as expected. But mice that were fed a high fat diet plus a nitrate supplement had improved metabolic profiles - suggesting a protective benefit from the nutrient against the harmful effects of the high fat diet.  

Seeking to understand what was happening at the cellular level, Brunetta and Holloway zeroed in on the skeletal muscle, a highly metabolic organ largely regulated by insulin. At first, they didn’t observe any major changes that could explain the insulin resistance, or how nitrate protected against it.

It wasn’t until they looked deeper into what was happening in the mitochondria, the well-known “powerhouse” organelle inside almost every cell. It was there that they saw the dysfunction caused by a high fat diet.

“Imagine you spill coffee inside your car,” Brunetta explains.  “From the outside, the car – or in this case, the skeletal muscle – still looks clean, but as soon as you open the door and look inside, it’s evident there’s a problem.”

What the researchers found was that mice who were given a high fat diet had lower levels of a specific protein called “Sirt1”, along with higher levels of damaged mitochondria compared to mice who also received the nitrate supplement.

Sirt1 acts as a sort of “cleaning crew” within a cell, fixing damage that occurs to other proteins when there are high levels of fats present, and mitochondria appear to be very susceptible to this damage. Importantly, consumption of nitrate prevented these responses and helped preserve mitochondrial function.   

To confirm beyond a doubt that Sirt1 was the key to nitrate’s positive benefits, the researchers used a genetic knockout mouse model, in which the production of Sirt1 protein was completely blocked. They discovered that mice lacking Sirt1 fed a high fat diet and nitrate supplement did not experience the beneficial effects of nitrate.

In other words, the ability of nitrates to improve metabolism was indeed wholly dependent on the presence of Sirt1.

The study highlights the fascinating way that a small and specific mechanism at the cellular level can lead to a noticeable and advantageous effect at the whole-body level.

“I’m more convinced than ever that mitochondrial health is vital to maintaining good systemic health,” says Brunetta. “Not just in terms of good metabolism but heart, kidney and even neurological function.”

This study was funded in part by the Natural Sciences and Engineering Research Council and Brazilian National Council for Scientific and Technological Development.

Read the full study in the journal Redox Biology.

Read about other CBS Research Highlights.