Weighted vests are increasingly showing up in fitness classes and on jogging tracks. Social media influencers tout them as a way to keep bones healthy, increase performance and even accelerate weight loss. The vests add resistance to the torso through pockets with removable weights. But experts say there’s a lack of robust research on their benefits, so it’s unclear how much of the hype surrounding them is justified. They say some people, such as seniors or those who are pregnant, should be cautious about using them. And they urge people to seek guidance before using them for the first time.