Low BMI in and of Itself Portends Poor Survival After Acute MI

Patients who are underweight at the time of hospitalization for acute MI face greater mortality risk than their normal-weight peers, and the difference cannot be explained by cachexia and frailty alone, according to a new observational study.

Notably, the apparent impact on survival persisted as long as 17 years after the initial heart attack.

Most studies on the topic of body mass index (BMI) in this context have “buried” underweight individuals among all the other categories, lead author Emily M. Bucholz, MD (Boston Children’s Hospital and Boston Medical Center, MA), told TCTMD. “In particular, [underweight patients] are often just somewhat dismissed within the papers as being a group that understandably has higher mortality, but no one really looks at the factors that contribute to that.”

The common assumption is that patients are cachexic or otherwise frail, she said. But based on the current findings, Bucholz noted, this does not seem to be the only explanation.

Bucholz along with senior author Harlan M. Krumholz, MD (Yale-New Haven Hospital, New Haven, CT), and coauthor Hannah A. Krumholz (Yale University, New Haven, CT), published the results online April 19, 2016, in PLOS Medicine.

Curves Separate Early

For the study, Bucholz et al analyzed data on nearly 58,000 patients that had been gathered as part of the Cooperative Cardiovascular Project, a cohort-based study of Medicare beneficiaries hospitalized for acute MI between January 1994 and February 1996. At baseline, 9.8% were underweight (BMI < 18.5 kg/m²) and 90.2% were normal-weight (BMI ≥ 18.5 to < 25 kg/m².

The crude mortality rate was higher for underweight versus normal weight patients at 30 days (25.2% vs 16.4%), 1 year (51.3% vs 33.8%), 5 years (79.2% vs 59.4%) and 17 years (98.3% vs 94.0%; P < 0.001 for all).

After adjustment for comorbidities known to cause cachexia—congestive heart failure, chronic obstructive pulmonary disease, cerebrovascular accident or stroke, cirrhosis/liver disease, chronic kidney disease, HIV infection or immunocompromised state, cancer, Alzheimer’s disease or dementia, and other terminal illness—as well as markers of nutritional status and frailty, underweight patients continued to have a higher risk of death.

A separate analysis restricted to patients without comorbidity showed similar 30-day mortality between the BMI categories but still found a 21% higher risk of death for the underweight versus normal-weight group at 17 years.

Asked whether patients are likely to stay in the same BMI category over such a long time span after an acute MI, Bucholz said that not many studies have investigated this question. But the limited data that do exist have “universally shown that patients—regardless of all the interventions that you throw at them, whether it’s weight counseling or exercise counseling—really do not change BMI categories,” she reported. “So there is very little we do, at least presently, to help patients get to healthier weights.”

The durability of the mortality difference in the current study stands out when looking at the Kaplan-Meier survival curves, she pointed out. “The curves separate . . . pretty dramatically in the first couple of days,” Bucholz noted, adding, “The greatest risk is occurring in the first 30 days to 1 year after AMI, but then you see the persistence of that effect over the long-term.”

Which of course begs the question of what can be done to mitigate that effect, she notes. Before an intervention can be designed, she said, more must be known about exactly why underweight patients have poorer survival.

What Can Be Done?

Commenting on the study for TCTMD, Carl “Chip” Lavie, MD, of Ochsner Medical Center (New Orleans, LA), pointed out that the study did not assess fitness or muscular mass/strength. “Fitness markedly impacts prognosis for almost any condition, especially in CVD,” he wrote in an email.

As such, he suggested that both nutrition and fitness are important means to helping underweight patients. “Increasing caloric intake, protein intake, and aerobic exercise to increase fitness . . . would be very helpful,” Lavie said, as would adding resistance training and the necessary dietary intake to increase muscle.

Bucholz acknowledged that fitness might be influential but pointed out that the dataset, though “rich,” was gathered at the time of hospitalization. Patients who had recently experienced an acute MI weren’t able to undergo testing for body fat composition or exercise tolerance so close to their event, she said. “I think it would be a fascinating study to do, ultimately, but probably not in the acute period after MI.”

The dose-response relationship often seen between BMI and mortality—with the highest risk seen in the underweight group and the lowest in the obese category—implies that potential targets for low-BMI patients are nutritional status and weight gain. “So if you have an underweight patient who’s admitted to the hospital,” Bucholz said, it’s worth asking, “What are we doing for them nutritionally? What are we doing to boost their overall caloric supplementation while they’re in the hospitalization and then after discharge? Nutrition is not something that anyone looks at when it comes to post-AMI care.”

In the heart-failure setting, she added, “several pharmaceutical agents do seem to increase weight gain and also improve . . . survival.” These have yet to be tested in the acute MI population, Bucholz noted.

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