Diversifying Diet

A little-known diet improves cardiovascular health through several distinct mechanisms. 

An illustration of foods included in the portfolio diet.

Illustration by Flora Bai

Diversifying one’s assets is useful not only in finance but also in diet, according to an October study from the Harvard T.H. Chan School of Public Health (HSPH). Though not many people have heard of the “portfolio diet”—consisting of plant-based foods proven to lower unhealthy cholesterol, such as nuts, oats, berries, and avocados—it is one of the easiest ways to improve long-term cardiovascular health. “The idea was that each of these foods lowers cholesterol quite minimally, but if you make a whole diet based on these different foods, you will see large reductions in [unhealthy] cholesterol,” said Andrea Glenn, an HSPH postdoctoral research fellow in nutrition and the lead author of the study. The more of these foods one eats, the higher the protection—but one need not include them all to reap the diet’s benefits, she said. “Like a business portfolio, you can choose the ones you want.”

Previous research had shown that the portfolio diet can reduce levels of low-density lipoprotein (LDL) cholesterol, the “bad” cholesterol that can lead to buildup in arteries and increase risk of heart disease and stroke. A 2005 study found that the portfolio diet reduced LDL cholesterol levels by around 30 percent over the course of four weeks—the same effect achieved by statins, cholesterol-lowering drugs, over the same period. But these previous studies were all shorter-term clinical trials ranging from four weeks to six months. Last year’s epidemiological study, which tracked participants’ health outcomes over the course of 30 years, was one of the first to examine the diet’s long-term effects.

Glenn and colleagues, including professor of epidemiology and nutrition Walter Willett, drew upon data collected on the diets of more than 200,000 adults who had enrolled in long-term health studies begun in the mid-1980s or early 1990s and concluded in 2016 or 2017. Every four years, participants—none of whom had heart disease at the outset—filled out questionnaires about their diets and had their health outcomes recorded. Researchers scored each participant’s eating patterns based on their compliance with the portfolio diet. They found that, by the end of the 30 years, those who adhered most closely to the diet had a 14 percent lower risk of heart disease and stroke compared to participants with lower scores.

“The portfolio diet…was linearly and consistently associated with a 14 percent lower risk of cardiovascular disease, coronary heart disease, and stroke,” Glenn said. “There was also a linear dose relationship with all three outcomes—so, that means that the more portfolio diet foods you add to your diet, the greater risk reduction you will have.”

The portfolio diet isn’t the only eating pattern to make headlines for its potential to prevent cardiovascular incidents such as heart disease and stroke. The Mediterranean diet has been known for decades to protect heart health; the Dietary Approaches to Stop Hypertension (DASH) diet was developed in the early 1990s to lower blood pressure. The portfolio diet isn’t mutually exclusive of these other eating patterns, and in fact shares similarities with them: all tend to emphasize unprocessed, plant-based foods such as whole grains, fruits, and vegetables.

“It’s good to have multiple options for different dietary patterns, because people need to be able to follow them for a long time,” Glenn said. “We also saw that only partial adoption of [the portfolio diet] still gave cardiovascular benefits. So you don’t have to follow every single part of the diet to a tee—maybe you’ll follow a couple of them, or all of them halfway, and you’ll still see some benefits.”

The portfolio diet differs from other heart-healthy eating patterns by emphasizing multiple plant-based food components. These include proteins like legumes and soybeans; viscous fiber found in oats, barley, berries, okra, eggplant, and apples; phytosterols from nuts and seeds; and monounsaturated fatty acids such as those found in avocados and olive oil.

“In this study, what’s unique is that we looked at a plant-based dietary pattern that combined multiple plant-based components,” said co-author Frank Hu, Stare professor of nutrition and epidemiology and professor of medicine. “The benefits are not just from fiber or from plant-based fats or proteins,” he continued: the combined components enhance the health benefits beyond what individual ones can offer alone. The plant-based portfolio diet has the added advantage of being healthy not just for humans, but for the planet, he said: “As we’re facing the climate crisis, I think it’s important for us to be mindful not just of human health but also environmental impact.” (See “Healthy Plate, Healthy Planet,” March-April 2020, page 34.)

The exact processes by which the portfolio diet lowers cholesterol vary. Important among them is the simple act of replacement: “If you’re replacing red meat with tofu, you’re going to have less saturated fat in your diet,” Glenn said. The plant-based components also possess properties that help lower cholesterol. Foods with viscous fiber (which dissolves in water to form a gel-like substance in the digestive tract) bind bile acids in the intestines, forming a complex that is eventually excreted. The body must then produce more bile acids to replace those lost—a process that uses up cholesterol. Phytosterols (compounds structurally similar to cholesterol, which are found in nuts and seeds) compete and interfere with cholesterol absorption in the intestines, which leads to the excretion of unabsorbed cholesterol. And legumes contain a protein that has been shown to inhibit hepatic apolipoprotein B (apoB) synthesis; when apoB levels decrease, cholesterol synthesis is also reduced.

The scientists plan to do further research to understand the biological mechanisms underlying the benefits of the diet, Hu said. They are conducting studies to determine metabolites (molecules produced during metabolic processes) associated with the portfolio diet that researchers can measure in people’s blood, making it easier to track adherence to the diet in future studies. They also hope to examine the diet’s potential impacts on other chronic illnesses, such as type two diabetes and some cancers, and on mortality.

Beyond its ability to prevent illness, the eating pattern may also “result in a healthier aging pattern,” said co-author JoAnn Manson, Bell professor of women’s health at Harvard Medical School. “That’s another area of great interest to us: maintaining physical function and slowing cognitive decline, memory loss, and cognitive aging.”

In the meantime, it is clear that the diet improves cardiovascular health—and that it isn’t an all-or-nothing approach. “Be open to trying new foods, maybe pick a few items from the portfolio diet and see if you can swap them with things you already eat,” Glenn said. “That can be a really great way to just get started on adding some of these plant-based foods that can lower cholesterol into your diet.”

Read more articles by: Nina Pasquini

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