Heart Health, Explained
BMI: Overweight women, sudden cardiac death, and the usefulness of body mass index as a measure of health
Two recent studies examine obesity and its effect on health, drawing potentially divergent conclusions using the same standard — body mass index — as a means of determining risk for life-threatening cardiovascular disease.
In order to help explain what these two studies might mean for people seeking an understanding of their relative risk in the face of obesity, let’s examine the studies one at a time.
- BMI and Sudden Cardiac Death
- BMI as a Measure of Health
According to the first study, published in November in JACC: Clinical Electrophysiology, women who were overweight at the age of 18 are more likely to die of sudden cardiac arrest at any age, regardless of their current weight.
The results of the study were publicized in The New York Times on Dec. 7, 2015.
BMI and Sudden Cardiac Death Q&A
- What is Body Mass Index (BMI)? What does it tell us?
Body Mass Index (BMI) is a way of categorizing people as underweight, normal weight, overweight, or obese. It is a formula that incorporates a person’s height and weight.
- A BMI of 18.5 to 24.9 is considered normal weight
- A BMI of 25 to 29.9 is considered overweight
- A BMI of 30 to 34.9 is considered obesity, Class I
- A BMI of 35 to 39.9 is considered obesity, Class II
- A BMI of 40 or above is considered obesity, Class III
The categories are somewhat arbitrary and have changed over time. For example, overweight was considered in the past to be a BMI of 27 or higher.
There are limitations to using BMI to determine someone’s obesity status. While BMI may be useful for looking at a whole population, when it comes to a particular individual it may not always be accurate. One common example of an inaccurate BMI would be an athlete with high muscle mass. She may have a BMI of 27, which is considered overweight, but her body fat percentage may be normal because muscle weighs more than fat. Another limitation is that the normal BMI may vary based on race. For example, the BMI recommendation for Asians is lower than for Caucasians: a normal BMI would be lower than 23 for an Asian person.
More important than what BMI does tell us is what it doesn’t tell us. While BMI may indicate obesity, it does not necessarily translate to increased health risk, since the distribution of fat is important in determining health risk. Abdominal obesity (known as visceral fat) is associated with health problems such as diabetes, high blood pressure, high cholesterol, and heart disease, while subcutaneous fat (around the hips and legs for example) is not.
- What is a heart attack?
This is when one of the arteries on the outside of the heart that supplies blood to the heart muscle (coronary artery) becomes blocked. That part of the heart muscle no longer has a supply of oxygen and can be injured or even die, leaving permanent damage to the heart.
A coronary artery gets blocked slowly over time by a process called atherosclerosis, in which cholesterol plaque builds up, narrowing the canal through which the blood flows. During a heart attack, there is typically a crack or rupture in the cholesterol plaque, and a blood clot forms on the rupture, suddenly cutting off the blood flow to the heart muscle.
- What is sudden cardiac death (SCD)? What causes it?
Cardiac arrest is when the heart develops a lethal abnormal heart rhythm which results in the heart no longer pumping blood. Sudden cardiac death is defined as death from cardiac arrest that occurs within one hour of onset of symptoms.
Two of the most common causes of SCD, accounting for 80 percent of cases, are a new myocardial infarction (heart attack), or an old heart attack that has left heart damage or scarring. A heart attack is not the same thing as SCD (most people survive a heart attack), but it can lead to SCD.
Other causes of SCD are an enlarged heart (cardiomyopathy) or disease in one or more of the heart’s four valves that control the direction of blood flowing in and out of the heart. Some people are at increased risk for SCD due to abnormalities in the heart’s electrical circuitry from birth (congenital long QT syndrome), or due to certain medications.
The incidence of SCD in the United States is somewhere between one in 1,000 to one in 2,000 deaths per year. Men die of SCD more often than women by a 2:1 ratio. Incidence varies by race as well, being more common in African Americans than in Hispanics and Caucasians.
It is worth noting that there are other causes of sudden death that could be misdiagnosed as SCD. One example is a pulmonary embolism (PE), a blood clot to the lung. In this first study, a definitive cause of death was often not available (see below).
- Where did the researchers find the subjects? Why no men? What is the significance of using this cohort of subjects?
The study participants were those enrolled in the Nurses’ Health Study, an observational study of actively willing participants that began in 1976. Initially, 121,700 female nurses in the United States volunteered to participate. The ages ranged from 30 to 55. After exclusions for various criteria, the final analysis for this article included 72,484 women.
The authors did not state why this particular study cohort (which did not include men) was chosen. Most likely it was chosen because it is a rich database of information on a large number of people followed over a long period of time. It is interesting that they chose a study of women only, as heart disease in general has long been considered a disease of men and overlooked in women for decades even though more women than men die of cardiovascular disease every year.
- What were the researchers looking for? How were the measurements and endpoints determined?
The authors were looking for an association between BMI and sudden cardiac death (SCD), particularly SCD that was primarily due to a heart rhythm abnormality. They also looked at associations between BMI and fatal or nonfatal heart attacks.
BMI was based on self-reported weight and height. The women self-reported (at ages 30-55) their weight at age 18 by recall.
SCD determination was based on reports from next of kin or postal authorities, searches of the National Death Index, reviews of medical records, autopsy reports, and interviews with family members. If the death was unwitnessed or occurred during sleep when symptom-free during the prior 24 hours, it was considered probable SCD as well and was also included in the study results.
Fatal heart attack was confirmed by autopsy report, hospital records, or death certificate. Probable fatal heart attack was also included if indicated by a death certificate or a family member’s report.
Nonfatal heart attack was determined by participants’ questionnaires and confirmed by medical record review when available.
- What did the researches learn? Why is this significant?
The strongest association the researchers found was that women with a higher BMI during early adulthood had a greater risk of SCD.
They also concluded that:
- Weight gain of >20 kilograms (about 44 pounds) during early-to-mid adulthood was associated with twice the risk of SCD.
- Obesity may be a stronger risk factor for SCD for middle-aged as opposed to older women.
- Excess weight or weight gain may have an early and cumulative impact on SCD risk that is not completely reversed by weight loss later in life.
- Higher BMI was associated with lower risk of SCD in women with known heart disease (see obesity paradox later in this article).
- Excess body weight likely influences both heart attack and SCD risk through atherosclerotic pathways and adverse effects on blood pressure, cholesterol, and insulin resistance.
- Women with BMI >35 had a higher risk of SCD even after adjusting for risk factors, so extreme obesity may increase SCD through other (non-atherosclerotic) pathways such as heart muscle enlargement and change in the heart’s electrical conduction.
- Excess weight in early adulthood/being overweight for a longer period may lead to early changes in heart structure and function which may increase SCD later in life.
- What conclusions did the researchers reach in terms of how to apply their results to patient care? What should doctors and patients do with this new information?
The authors recommend strategies for maintenance of healthy weight throughout adulthood as a method of SCD prevention in the general population, particularly among women.
Recommendations for patient management should not change based on this study alone. While an association between BMI and SCD was made, it cannot be said that the higher BMI caused the increase in SCD.
- What were the limitations of this study? Do we have any similar data for men? What would be a good next step for researchers to learn more on this subject?
There were several limitations to this study. The researchers were able to draw conclusions about associations but not causality. The fact that weights were obtained by self-report (including recall from many years prior) is a limitation — even being a few pounds off could result in being in a different BMI classification. There were no direct measures of clinical risk factors such as blood pressure, cholesterol levels, and diabetes. All were self-reported. The diagnosis of SCD was very often not definitive, even including reports from “postal authorities.” The population studied was mostly white, educated women involved in health care and not representative of the U.S. population as a whole.
Also, an observational study of this sort involves only those participants who chose to sign up and limits the ability of researchers to extrapolate results to wider populations as in a randomly selected sample.
In previous research (which included men), the data have shown an association between obesity and SCD in some studies but not in others. Age has also played a factor in these studies.
A good next step might be to do a retrospective study of women and men who die of confirmed rhythm-related (not due to heart attack) SCD, to determine what risk factors these patients may have had. A prospective study may be difficult to do, as at-risk subjects would have to be followed for decades.
Heart Health Explained is a collaboration of Women’s Health Research at Yale and the OhioHealth Healthcare System, a nationally recognized not-for-profit organization with providers across 46 counties, offering a holistic approach to prevention, treatment and rehabilitation of heart disease. OhioHealth is staffed by physicians, psychologists, nutritionists and nurses who answer the questions of the moment on heart and vascular health.
The information provided here may help you make more informed choices. However, it is not a substitute for an individualized medical opinion or diagnosis, and everyone should always consult with their personal physicians to make decisions about their condition or treatment.