34% of Americans are obese and nearly two thirds are overweight . This is a jump of more than 200% since 1970 when low fat diets started gaining momentum. Since the same period, the number of diabetic cases has grown 400%. Fat-free food labels are only worsening the obesity crisis if we believe Gary Taubes and Dr. Peter Attia of Nutrition Science Initiative. These are not mere numbers for academicians, but they have started causing growing concern in society as well.

Awareness about keeping healthy and taking proper nutrition is spreading fast due to the above reasons. Vitamins and supplements have become a part of the lifestyle nowadays and this is why the wellness industry is doing well. GNC Holdings (NYSE:GNC), Herbalife (NYSE:HLF) and Nu Skin (NYSE:NUS) have gained relevance in the wellness & dietary supplements market. Let's see if they are worth investing in as well.

GNC: A well-established name
GNC's business is run through three different segments – retail, franchise, and manufacturing/third-party. So it is well diversified.

GNC is the least controversial of the three companies in this space, and has been growing top and bottom lines through strong comparable-store sales growth. In the third quarter, for example, it reported an 8.2% increase in same-store sales due to  a strong increase in consumer traffic. In addition, GNC.com grew 31.7% in the previous quarter.

On the back of strong comps, third-quarter consolidated revenue increased 8.7% versus the same period last year to $675.6 million, and diluted earnings increased 24.6% versus the year-ago period to $0.76 per share. The robust results were accompanied with a revision of the full year outlook, which aims at earnings growth of 22% to 24%.

Besides this, GNC offers a dividend that yields 1%. There is room for improvement here as the company has ample free cash flow with a payout ratio of 21 %.The company has also been good at returning money to shareholders through share a repurchase program. Since its IPO in April 2011, GNC has returned far more cash to shareholders than it raised in the IPO, and it's no surprise that shares have appreciated by a whopping 260% since then.

In order to boost growth, GNC acquired the U.K.-based web-only retailer Discount Supplements. Expanding in the U.K. and other European and Scandinavian markets should result in growth going forward.

GNC has been leading the health and wellness market for over seven decades. It has 8,100 locations in 54 countries worldwide and tremendous brand awareness . All this makes GNC a trusted brand where consumers can go for fulfilling the needs of their healthy lifestyle.

Going forward, GNC is confident on maintaining store count growth of approximately 115 net new stores each year for the foreseeable future. It sees potential for approximately 5,000 total stand-alone GNC stores in the United States .

Nu Skin: On a fast growth track
Nu Skin has been making good strides as a successful direct marketing company, dealing with nutrition and personal care products. It has bet big on China and it has already achieved the target of $1 billion in sales in Greater China in the month of October, ahead of schedule. Moreover, markets like Taiwan improved by 79% and Hong Kong by 69%.

All in all, Nu Skin has shown great growth recently. Third-quarter revenue jumped a whopping 76% over the prior year to $928 million. Additionally, earnings grew 107%, reaching $1.80 per share. Buoyed by a strong performance, it revised the full-year guidance upwards.

Herbalife: Cheap and exciting
Herbalife also posted a robust third quarter, posting net sales of $1.2 billion, 19% above last year's third quarter. The global rise of obesity has been the main driver behind this growth. The company reported a 44% jump in earnings, which came in at $1.41 per share. Herbalife's marketing practice, like many direct-selling programs through individual distributors, has been questioned. There have been rumors that FTC may look into the legality of the whole scheme, but it hasn't happened yet.

But Herbalife has received good endorsements recently. It won a Belgian appeals court ruling that rejected claims that Herbalife is a pyramid scheme. The court concluded that the income that its distributors earn from others recruited to buy or sell its products isn't a violation of European consumer protection laws.

So, it looks like herbalife's model is finding acceptance in Europe, and if the same happens in the U.S., the company could be a good investment. It is pretty cheap with a P/E ratio of just 15 and also pays out a dividend with a yield of 1.70%. For the next five years, analysts are projecting annual earnings growth of 15% for Herbalife, which is quite impressive.

The takeaway
All three companies have capitalized on peoples' awareness about leading a healthy lifestyle. While GNC is a well-established name, Herbalife and Nu Skin have also been making their mark. The growth expectations for all three companies look promising, with Nu Skin's bottom line expected to grow at 29% a year over the next five years and compared to GNC's expected 20%.

But the cheapest of the three is Herbalife, and if you don't have many apprehensions about its business model then you can certainly consider it for your portfolio.

Fool contributor Vinay Singh has no position in any stocks mentioned. The Motley Fool has the following options: long January 2015 $50 calls on Herbalife Ltd.. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.