In the ongoing low-fat versus low-carb diet debate, the low-fat side scored some points this week with the publication of two new studies.
One study, published in the journal Circulation, reported that replacing red meat with plant-based sources of protein decreased some of the risk factors associated with heart disease.
Both studies come with important caveats, so they aren’t going to end the often-heated dispute over whether red meat has a role to play in a healthy diet.
But their findings are interesting, nevertheless.
Effects on risk factors
For the Circulation study, researchers at Harvard University and Purdue University analyzed data collected from 36 previously published studies that had compared diets with red meat with diets that replaced the meat with a variety of other foods. Those other foods included chicken, fish, high-quality plant protein sources (such as legumes, soy products and nuts) and carbohydrates (including breads and cereals).
The studies were all randomized controlled trials (RCTs), which meant that the participants were randomly assigned to a particular diet for a particular period of time. In these studies, that time period ranged from two to 36 weeks.
The studies had a total of 1,803 participants, aged 22 to 70. In most of the studies, the participants had normal (healthy) cholesterol levels, but some included people who’d been diagnosed with high cholesterol.
The authors of the Circulation study looked to see how the diets high in red meat compared with the other diets in terms of four risk factors for cardiovascular disease: blood pressure and blood levels of total cholesterol, triglycerides and lipoproteins (including high-density lipoprotein, or LDL, and low-density lipoprotein, or HDL).
When the researchers compared the high-meat diets to a combination of all the other diets, they found no difference between them in regard to three of the four risk factors. The exception was triglyceride levels, which were higher in people who were on a red-meat diet.
Higher-than-normal levels of triglycerides can be a sign of metabolic syndrome, which increases the risk for heart attack, stroke and type 2 diabetes.
The researchers then examined the diets individually. This deeper dive into the data revealed that the diets that replaced meat with high-quality plant proteins, such as lentils, beans, peas and nuts, led to lower levels of both total and LDL (the so-called bad cholesterol).
These findings suggest, say the study’s authors, that RCTs have produced inconsistent findings regarding red meat’s impact on cardiovascular disease because — or, at least, partly because — of the content of the diets to which the red-meat diet was being compared.
“Asking ‘Is red meat good or bad?’ is useless,” said Meir Stampfer, the study’s senior author and an epidemiologist at Harvard University, in a released statement. “It has to be ‘Compared to what?’ If you replace burgers with cookies or fries, you don’t get healthier. But if you replace red meat with healthy plant protein sources, like nuts and beans, you get a health benefit.”
Stampfer and his co-authors conclude their study by saying healthy vegetarian and Mediterranean-style diets “should be recommended for their health benefits and to promote environmental sustainability.”
Effects on early death
In the second study, a team of researchers at the University of Eastern Finland analyzed data collected over two decades from about 2,600 men who had been part of a long-running Finnish study on heart disease risk factors.
The men had been enrolled in the study during the years 1984 and 1989, when they were between the ages of 42 and 60. Over the years, they had provided detailed information about their health habits, including the foods they ate.
For the current study, the men were followed for an average of 22 years. During that period, 1,225 of them died as a result of heart disease, cancer or other disease. The study found that men who favored animal protein over plant-based protein in their diet were at greater risk of being among the men who died.
Specifically, men in the study who consumed more than 7.0 ounces of meat per day were 23 percent more likely to have died from disease during the study than those who ate less than 3.5 ounces per day. The greater risk of death was found, however, only in men who had been diagnosed with type 2 diabetes, heart disease or cancer at the start of the study.
The study’s authors say their study’s findings underscore the need to do more research on the effects of high-meat diets on health, particularly among people with a pre-existing medical condition.
“However, these findings should not be generalized to older people who are at a greater risk of malnutrition and whose intake of protein often remains below the recommended amount,” said Heli Virtanen, a doctoral student at the University of Eastern Finland and one of the study’s authors, in a released statement.
Not the final word
Both studies have major limitations. For example, the individual RCTs used in the Circulation meta-analysis involved a small number of participants, a factor that impeded researchers from detecting statistically significant effects from dietary subcategories. Also, RCTs that ask people to change their diets often have low compliance, which could have affected the studies’ — and the meta-analysis’ — results.
In addition, the meta-analysis looked only at the effect of the different diets on risk factors for cardiovascular disease, not at their effect on the disease itself.
As for the Finnish study, it was observational, which means it can’t prove that a high-meat diet raised the risk of early death. Other factors, ones not accounted for in the study, could also explain the correlation.
So, the low-fat versus low-carb debate continues. These two latest studies, while intriguing, don’t resolve the issue.
In the meantime, Michael Pollan’s concise dietary advice seems to be holding up pretty well: “Eat food. Not too much. Mostly plants.”
FMI: You can read the Circulation study in full on that journal’s website. You’ll find an abstract of the American Journal of Clinical Nutrition study on that journal’s website, but the full paper is behind a paywall.