Vegetarianism, along with veganism (which excludes all animal products including eggs and dairy), is more popular than ever in Hong Kong and worldwide. The growing number of vegetarian and vegan options - eateries, products and even cookbooks - shows there is demand fuelling the supply.

Restaurant and cafe chains have hopped on the bandwagon - even 7-Eleven stores in the US, more than 100 of them, last year began carrying vegan versions of dishes such as pad Thai and linguine tikka masala. Celebrities such as rock star Ozzy Osbourne and actor-comedian Russell Brand have joined the vegan fray, too.

Ng, who turned vegan seven years ago, founded the Hong Kong Vegan Association in 2009, and in August that year started the Meat Free Monday Meetup (meetup.com/Meat-Free-Hong-Kong), a group that dines together at 7.30pm every Monday in a different vegetarian spot each week. The group's membership has grown from fewer than 10 to nearly 800, and about 30 join in each week.

At the University of Hong Kong, the Less-Meat Monday Campaign started by final-year student Helen Kwok Hiu-lam is gaining ground. Late last year, she managed to persuade campus caterers to offer a larger range of vegetarian dishes on Mondays, and to place a 'Green Grin' sticker on these items.

Kwok, 23, says the response has been encouraging and has spread beyond HKU. The English Schools Foundation has invited Kwok and her team to share their knowledge with student representatives. 'Hopefully, the meat-free-once-a-week idea will go ESF-wide soon - if not all over Hong Kong primary and secondary schools.'

There are many reasons to go vegan, but the main initial factor, it seems, is not usually health.

Eight years ago, Marcus Turner, 49, rid himself of all animal products, including leather and wool, for animal welfare reasons. Even his seven pet dogs are vegan. 'It's unnecessary and disgusting,' he says of factory farming.

Environmental issues are another popular motivation. A United Nations report in 2010 noted that as the global population surges towards a predicted 9.1 billion people by 2050, diets rich in meat and dairy are unsustainable, and a global shift towards a vegan diet is vital to save the world from hunger, and to avoid fuelling poverty and the worst effects of climate change.

Often, the health benefits of a plant-based diet encourage people to stick at it. 'Most people find they become healthier. You save a lot of money because you don't go to the doctor as much,' says Victor D'Aquino, 63, a board member of Club O, an organisation that promotes 'living a life according to the laws of nature' and serves free vegetarian dinners at its Mong Kok headquarters weekly.

Evidence of the benefits of plant-based diet is compelling. One of the most comprehensive studies is The China Study, which looked at dietary and lifestyle factors linked with disease mortality of 6,500 adults across 170 villages in China. Led by Cornell University professor emeritus T. Colin Campbell, it was a 20-year partnership with Oxford University and the Chinese Academy of Preventive Medicine that began in 1983.

Campbell, who has been researching diet, nutrition and disease since the late 1960s, says: 'I have come to see that the benefits produced by eating a plant-based diet are far more diverse and impressive than any drug or surgery used in medical practice. Heart disease, cancer, diabetes, stroke and hypertension, arthritis, cataract, Alzheimer's, impotence and all sorts of other chronic diseases can be largely prevented.'

He found that nutrients from animal-based foods increased cancer tumour development, while nutrients from plant-based foods decreased it. There is also 'impressive evidence' that good nutrition can reverse advanced heart disease, relatively advanced cancers of certain types, diabetes and other degenerative diseases.

Based on his research, Campbell authored The China Study, the book that converted former US president Bill Clinton to a plant-based diet - and led to an 11kg weight loss.

Vegetarianism is nothing new to Chinese culture, as Eastern religions such as Buddhism and Taoism promote such a diet. But Dr Neal Barnard, founder and president of the Physicians Committee for Responsible Medicine (PCRM) based in Washington, says traditional plant-based diets are being swamped with unhealthy foods and fast-food items, and this is driving obesity and diabetes rates in China 'through the roof'.

In a study he co-authored that was published in the American Journal of Health Promotion in 2010, 113 employees at a US insurance company went through a voluntary 22-week intervention programme. All participants had a body mass index of 25 and above, and/or a previous type-2 diabetes diagnosis.

The 68 people who were put on a low-fat, vegan diet lost an average of 5.1kg and cut 4.7cm from their waistlines. The rest, who kept to their habitual diets, gained 0.1kg and 0.8cm around the waist on average.

To motivate people to switch to a plant-based diet, PCRM launched the 21-Day Vegan Kickstart in 2009, a free online programme based on Barnard's research that offers daily recipes and expert and celebrity tips. It has helped more than 150,000 Americans lose weight and improve their health. A Chinese version (21DayKickstartChina.org) of the challenge was launched last month.

'Our research shows the best way to lose weight and reverse disease is to jump into a completely low-fat, plant-based diet for three weeks,' says Barnard.

But is such a diet a balanced one?

Zhejiang University researcher Duo Li, in a study published last year in the Journal of Agricultural and Food Chemistry, reviewed dozens of articles published in the past 30 years on the biochemistry of vegetarianism. He found that vegan diets tend to lack key nutrients such as iron, zinc, vitamin B12 and omega-3 fatty acids. Vegans therefore tend to have raised blood levels of homocysteine and lower HDL ('good' cholesterol) levels, both risk factors for heart disease.

Candy Sin Kam-ling, a registered dietitian with the World Cancer Research Fund Hong Kong, also mentions those nutrients, along with protein and calcium that a plant-based diet may lack. But she says: 'A vegan can still meet all nutritional requirements - provided the meal is well planned.'

That seems to be the experience of Agnes Tam Nga-yin, 25, whose parents were worried that she wouldn't get enough nutrition when she turned vegetarian seven years ago.

'However, time has proved that a plant-based diet is a lot healthier. I do not get an upset stomach, skin allergies, or catch a cold as often as before,' says Tam, a HKU research assistant. 'When I read or write essays, I feel so focused.'

Even Ng's golden retriever, a frail abandoned dog she adopted a year ago, is thriving on a vegan diet. 'It is now very fat,' she says.

Transitioning to a plant-based diet may be daunting for meat lovers, but Turner says it's not as hard as people imagine. 'I gave up smoking 20 years ago and it was 1,000 times harder. To go vegan, I did it in one attempt.'

Besides, with such a diet becoming more mainstream, there is a lot more information and products available to make the switch easy. Still, many vegans in Hong Kong agree that the city could do with more vegan options.

Cheese, for example, is often a stumbling block for potential vegans, says Turner. 'For some reason, cheese is one of the more difficult products to give up. To find a good vegan cheese is like finding the Holy Grail.'

He did eventually find this 'Holy Grail' in Scotland. Last year, Turner started importing the non-dairy Sheese - which, trust me, tastes remarkably like the real thing - into Hong Kong. 'The drastic step was to become the distributor so that I could get my own supply.'

But if you find cheese - or other animal products - difficult to give up, don't beat yourself up over it.

'I have tried being 'pure' vegan for some periods, but I'm very flexible with my diet,' says Shima Shimizu, 32, a raw food chef and founder of Sesame Kitchen cooking school. 'I don't think the 'purity' of your diet is important. You can experience the benefits of eating more fresh fruits and vegetables without being 100 per cent vegan. Listen to your body and try to make the best choices as much as possible.'