These body-weight 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 U.K. There is 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.

Another explanation: People who are naturally very slim may feel free to eat more junk food, smoke, skip exercise or engage in other unhealthy behaviors compared to those who are overweight—all of which could raise their risks for health issues, says Geir Lorem, a professor of health at the Arctic University of Norway who has studied the ties between low BMI and mortality.

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. (If you’re curious about your own BMI, the National Institutes of Health maintains an online BMI calculator.)

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. (A tablespoon of olive oil contains 119 calories, according to the USDA.)

It’s also helpful to drink your fats. Romano recommends blending up a calorie-dense smoothie made with peanut butter, avocado, nuts, yogurt or milk, seeds and other fatty foods, along with a little fruit or healthy greens. Just make sure you’re drinking your smoothie after a meal—not before—so you won’t lose your appetite, Ravussin adds.

For older adults or those who typically don’t eat a lot of protein, getting more of it may also be helpful. Protein supports muscle maintenance and growth, both of which can be challenging for adults over age 65, Ravussin says. While overdoing it on protein could carry some risks for young and middle-aged adults, there’s evidence that those 65 and older benefit from eating up to 1.3 grams of protein per two pounds of body weight each day. (That’s 110 grams of daily protein for a 170-pound person, which is well above the 46 to 56 grams currently recommended by the National Academy of Medicine for adults aged 19 and older.)

To hit these protein targets, protein powder supplements containing natural animal- or plant-based sources of protein may be helpful, says Dr. Mark Moyad, director of preventive and alternative medicine at the University of Michigan. These are casein, whey, hemp or soy-based protein powders. But Moyad warns against other types of weight-gain supplements, especially pills that claim to build muscle. “These pills can be contaminated with steroids or steroid-like compounds,” he says. They’re also not subject to much regulatory oversight, and their long-term health risks aren’t known, he says.

Finally, exercise is a great way to pack on pounds and increase your BMI. Muscle weighs more than fat, and resistance training in particular is a good way to build strength, says Ravussin. Pumping iron also increases bone density and strength, which can help combat frailty and its attendant risks. Exercise also tends to stimulate appetite, he says, so you may find it easier to consume more calorie-dense foods after a workout.

If you’ve tried all this and you haven’t added weight the way you had hoped, it’s time to talk with a doctor or dietitian. It’s possible you have an undiagnosed health issue—something like an overactive thyroid, lactose intolerance or celiac disease—preventing you from gaining weight, Moyad says.

It’s also possible you’re just not eating as much as you think you are. “There are people who are always on the move and have high stress and miss countless meals, and when we have them talk to a dietitian, they realize they’re not getting even close to the number of calories needed for basic weight gain,” Moyad says.

You may never be a beefcake. But you should be able to add healthy weight with the right combination of diet tweaks and muscle training.