For most Americans with weight issues, the problem is carrying around too much, not too little. While obesity rates have surged in North America since the 1970s, the proportion of underweight people has remained low—less than 5% of the population, according to a study in The Lancet .
These bodyweight trends mean that most health experts are focused on helping people lose weight to avoid disease. But there are also some potentially serious health consequences associated with being clinically underweight, which is usually defined as having a body mass index (BMI) of 18.5 or below.
“There are many epidemiologic studies that indicate that underweight in adults and older people is associated with higher risk of death or mortality,” says Kay-Tee Khaw, a professor of clinical gerontology at the University of Cambridge in the UK. There are a range of possible explanations why. For some, a low BMI coupled with unexplained weight loss may be an indication of an underlying medical condition such as cancer.
Also, bone and muscle (not just fat) contribute to body weight and BMI. Being underweight may be an indicator of loss of bone and muscle mass—and therefore frailty—particularly in older adults, Khaw says. Frailty can increase a person’s risks for weakness, falls, broken bones and other health problems.
It’s important to point out that being skinny is not the same as being clinically underweight. A 5’10” man who weighs 140 pounds has a BMI of 20—still within the range of normal and healthy. The same is true of a woman who is 5-foot-3 and weighs 113 pounds.
But let’s say you’re intent on gaining weight—either because you just feel too slim, your BMI is below 18.5 or you’re an older adult who’s worried about becoming frail. How should you go about it?
Start by adding more healthy, nutrient-dense foods to your diet, says Eric Ravussin, a professor and chair in diabetes and metabolism at Louisiana State University’s Pennington Biomedical Research Center. High-fat foods like olive oil, avocados, nuts and nut butters, seeds, and fatty fish are all great options. Replacing low-fat dairy products with full-fat dairy, like whole milk, cheese and Greek yogurt, is another way to squeeze more healthy fat into your diet, he says.
If you’ve read about the recent studies tying consumption of these healthy and fatty foods to lower body weights, you may be confused by Ravussin’s advice. How could the same foods help people both gain and lose weight? In the context of a typical Western diet, swapping these fatty foods in for low-fat, heavily processed and sugary foods may help people feel more full and consume fewer total calories throughout the day, which can aid weight loss, says Alicia Romano, a clinical registered dietitian at the Frances Stern Nutrition Center at Tufts Medical Center. But these foods are still calorie-dense, so if you eat enough of them, they can help you add healthy weight. “Fatty foods have the most calories per gram—an average of nine, compared to about half that for protein or carbohydrates,” Romano explains.
To fit more of these foods into your diet, Ravussin recommends eating three large meals and adding a few 300-calorie snacks in the morning and afternoon. “With obesity, we recommend that people don’t snack,” he says, “but if you’re trying to gain weight, more frequent eating is a winning situation.” Your meals shouldn’t be composed solely of fats; you’ll want to eat a healthy range of foods, including plenty of vegetables, whole grains, legumes and protein, to avoid any nutritional deficiencies.
“Take a bag of almonds or walnuts and measure out a cup,” Romano advises. “One cup is about 800 calories.” You can eat some of these nuts as a snack and sprinkle more of them onto salads at mealtimes to inflate your calorie intake. Drizzling olive oil onto everything is another way to pump up your calorie total in a healthy way, she says.