I am interested in stocks (short-term investment) that would rise based on people making New Years resolutions for physical fitness or diets. Have anyone made any such investments?

Stock prices may not rise just cause of good sales or increased earnings. Countless of examples this year alone.

INTC reported good earnings last quarter and the shares plunged.

Jsut this week, ZM and CRM reported great earnings but prices plunge as well.

Then you have many struggling companies reporting poor earnings but shares rally up.

You’ll be extremely confused if you try to predict the movement on share prices based on news. Fundamentals and valuations work over a long period of time. Straying but coming back towards the rough direction of the intrinsic value.

In the short run, price action and market emotions drive price directions

Hard to say for the current on-going COVID-19 situation. Even if gym membership signs up rates increase over the new year, leading into their quarter growth/revenue report, it will be capped at certain levels due to potential further lockdowns (Canada recently went into lockdown again), and safe-distancing measures.

I think most home-based exercise equipment companies like Peloton has its growth already priced in due to the lockdown situation.

As for diet-based companies like Herbalife, not following it too close, hence I can't give you an opinion. But kudos to you for the creative thesis about going into the new year with these fitness resolution whatnots.

I would say just stick with companies with long-term growth prospects (tech), I don't think fitness/diet companies are volatile enough to capture short term spikes due to high revenue/earnings since they are not really as growth-oriented compared to tech stocks these days.