I have looked through 100s of vintage magazines ranging from the 1920s to the 1960s. I have yet to see (at least that I can remember) one of the ads that have been circulating about how to gain weight in an actual vintage magazine. I have seen ads about losing weight in almost every magazine, though not as prevalent as it is in current magazines.
Another thing I look at is that these outlets never mention what type of magazine these ads came from. Like I stated above, I have yet to see an ad like this in any Better Homes and Gardens, McCalls, Ladies Home Journal, or Life magazine. If the ads were geared toward just gaining weight and being healthy, I would assume that I would see them in the every day housewife magazines.
With that being said, though, we do have an unhealthy view toward weight these days. With media at every corner, the "perfect" body is constantly thrown in our face. Ads are airbrushed to death to give us an unrealistic view of what healthy is. But, we are getting bigger. Women in general were much smaller 60 years ago. Those of us who wear vintage clothing know that it isn't uncommon to find a dress with a 24 inch or smaller waist. From a quick search online, we have gone from an average size (including middle aged women) of 37-27-39 in the 1950s to a modern 38-34-40.
According to Livestrong.com, in the 1950s 33% of adults were overweight and 9.7% were clinically obese with a bmi over 30 . Now, that number has jumped to 30.5% of adults are clinically obese. That is quite a jump. Why? One issue is that we have a more sedentary lifestyle. Most women during that time stayed at home. Keeping the house clean and food on the table means they moved - a lot. Dishwashers and automatic washing machines were not as common in the 40s and early 50s as they are today. Now it is just the opposite. Rarely do you find a woman who has the ability to be a stay at home mom or housewife. This is either because they have to work outside the home, or they want to. Either way working away from home generally means sitting in front of the computer - all day. But, this isn't new information. And neither is the fact that our food has gotten increasingly more package and full of preservatives and modified substances.
What can we do about? I don't think looking back in time with rose colored glasses on is going to help. Times are different. As much as we wish we had a time machine, we have to do what we can to live a healthy lifestyle with today's circumstances.