Diet and the Heart: Here’s Advice That’s Ripe for Rethinking

Science doesn’t like dogma, especially when it comes to diet and heart disease.

Research continually refines what we know about links between what our patients eat and their cardiovascular health.

Indeed, the new Scientific Report of the 2015 Dietary Guidelines Advisory Committee commissioned by the federal government includes some surprising departures from previous advice. Old beliefs have been overturned and new research avenues opened. Some controversies have heated up. Things are moving fast.

In case you missed something, here’s a roundup of recent developments in our understanding of diet and heart disease.

Fat

Evidence remains robust that elevated serum cholesterol, specifically LDL cholesterol, is a strong predictor of coronary artery disease in patients of all types and ages. Reducing LDL cholesterol remains a primary goal of preventive treatment for coronary artery disease. Trans and saturated fats are known to raise levels of LDL cholesterol in the blood. The questions are: How much? and Is it significant?

Butter, beef and bacon

Recent well-regarded studies suggest that the answers to these questions are much more nuanced than previously thought. Yet for over 50 years, patients have been counseled to avoid butter, whole milk and cream as well as beef, bacon and other meats based on their high saturated fat content. Steven Nissen, MD, Chair of Cardiovascular Medicine at Cleveland Clinic, is often asked by the media to explain this apparent reversal of medical opinion.

“High cholesterol is a metabolic condition that can only be moderately influenced by diet,” says Dr. Nissen. “Most circulating cholesterol is produced by the liver. Dietary cholesterol accounts for only about 15 to 20 percent of serum cholesterol. Changing the diet typically has only a modest effect on serum cholesterol levels.”

Indeed, the above-cited report of the government’s advisory committee concludes that “available evidence shows no appreciable relationship between consumption of dietary cholesterol and serum (blood) cholesterol.”

Fat bans spur unhealthy substitutions

Dr. Nissen believes previous recommendations against saturated fats contributed to the current epidemics of obesity and diabetes by promoting substitution of foods that are high in trans fats, simple sugars and carbohydrates.

“If you use a bit of butter to flavor your cooking, it’s not a sin,” he says. Moderation is the key to consuming fat. “There are a lot of reasons to hedge one’s bets, but you don’t have to absolutely avoid saturated fats. You just want to keep them under control.”

Trans fats, also known as hydrogenated vegetable oil, are consistently linked to an increased risk of heart disease and should be avoided, Dr. Nissen notes.

Go Mediterranean

So what went wrong? “High-quality research requires meticulous methodology of the sort that’s evolved only recently with development of the randomized controlled trial,” Dr. Nissen says. “Research before the modern era relied mostly on observational studies, with all their inherent biases.”

One study that Dr. Nissen strongly endorses is the PREDIMED investigation, published in the New England Journal of Medicine in 2013. Looking at a high-risk population of 7,500 people, it found that a Mediterranean diet including extra virgin olive oil or nuts reduced the incidence of major cardiovascular events.

“This is a high-quality study that blows the low-fat diet myth out of the water,” Dr. Nissen observes. “It’s good news for people advocating a sensible, balanced and tasty diet. Eat a Mediterranean diet, live long and enjoy life!”

Salt

In the mid-20th century, Cleveland Clinic researcher Irvine Page, MD, bucked the tide of contemporary opinion about hypertension and proposed what he called the mosaic theory, which postulated that hypertension resulted from a variety of factors, including heredity, environment, hormones and diet.

Since Dr. Page’s time, diet — particularly dietary sodium — has come to dominate the popular view of the causes of hypertension. Virtually all dietary guidelines now recommend lowering of salt intake as part of a lifestyle for both prevention and control of hypertension.

Some new research, however, suggests salt’s role in blood pressure level may have been overemphasized, at least for the general population. For instance, a recent study of more than 8,000 adults published in the American Journal of Hypertension found only a modest relationship between salt intake and systolic blood pressure — and the relationship failed to remain significant after accounting for factors like age and body mass index. Indeed, the study identified body mass index as the most important modifiable risk factor associated with systolic blood pressure.

“The science here is pretty murky,” says Dr. Nissen. “Some studies show that salt intake is linked to heart disease, particularly hypertension. Other studies are not so clear. We tell people that if they have high blood pressure, they should probably try to reduce salt in their diet. A low-salt diet high in fruits and vegetables does seem to lower blood pressure and might keep them off medication. If an individual doesn’t have hypertension and is otherwise healthy, reducing salt is not so much of a priority. You don’t want to overdose on salt, but it may not be as big a problem as the public has been led to believe.”

Red meat and eggs

For decades, eggs and red meat have topped the list of foods implicated in cardiovascular disease because of their high levels of dietary cholesterol. But the new science-led exoneration of fat described above would seem to let red meat and eggs off the hook. And it largely has, at least when it comes to fat. But a whole new line of investigation focused on intestinal bacteria is keeping eggs and red meat in the spotlight.

Going for the gut

Stanley Hazen, MD, PhD, Cleveland Clinic’s Section Head of Preventive Cardiology and Rehabilitation, has published one study after another (detailed here) linking the metabolic product of bacterial digestion of substances found in red meat and egg yolks with development of pathologies ranging from atherosclerotic plaque to heart failure to chronic kidney disease. According to Dr. Hazen’s studies, increased blood levels of a compound formed by gut microbes following consumption of foods like red meat and egg yolks are associated with increased risk of adverse cardiovascular outcomes, including death, even when controlling for other risk factors and standard blood test results.

Briefly, here’s how it works: Red meat and eggs (plus some dietary supplements and energy drinks) contain choline and carnitine, which gut bacteria metabolize into TMA (trimethylamine). TMA travels to the liver, where it is converted to TMAO (trimethylamine-N-oxide) and released into the bloodstream. There, TMAO becomes a factor that promotes vascular inflammation and formation of unstable plaques in arterial walls. The influence of TMAO on cardiovascular disease is significant enough to have prompted Dr. Hazen to develop an assay to assess cardiac risk by measuring plasma TMAO.

Still eat meat?

So what are the dietary implications of these findings? When Dr. Hazen conducted a study published in Nature Medicine comparing 51 habitual meat eaters with 26 vegetarians or vegans, he found that the vegetarians and vegans had much lower concentrations of plasma TMAO than did the meat eaters.

But recommendations must await further studies. “While multiple studies with thousands of subjects show high TMAO levels to be predictive of future cardiovascular events, no trials have yet directly tested whether lowering TMAO reduces cardiac risk,” Dr. Hazen explains.

For now, he recommends moderation. “Current evidence suggests that people who eat a lot of red meat should consider cutting back,’’ he says, noting that the same goes for eggs.

Back to the future?

In the 1973 movie Sleeper, Woody Allen plays a health food store owner who awakens from suspended animation into a future world where scientists have reversed the current wisdom and now consider deep-fried foods, steak, cream pies and hot fudge to be the real health foods. The joke always gets a big laugh. But we have moved somewhat in that direction, at least where fat is concerned.

One thing we know for certain is that dietary recommendations will continue to be fine-tuned as scientists continue to study food’s complicated effects on health.

(To share these insights with your patients, share this related patient-friendly infographic on diet and heart health.)