A picture she posted of her chubby 16-year-old self on Instagram last Tuesday sparked online criticism after she wrote about her journey of whittling her "fat" figure down to its current svelte shape.
At 1.69m tall, Tay now weighs 54kg.
Some netizens felt she did not look fat back then, and criticised her for having an unhealthy obsession with "being thin".
Wrote netizen hazelbong: "Honestly, I think people who are really chubby will face self esteem issues because of your statement."
Another netizen wrote: "Why's your perception of beauty so twisted? Can't imagine what you would teach your daughter..."
Tay, who has a three-year-old son, posted a photo of her recent slender self alongside the controversial photo.
She wrote in the caption: "Oh my goodness! Met a college friend a few days ago who sent me this old picture of mine. I nearly died seeing myself so chubby at 16 years old!"
"Look at those arms and thighs... Now you believe me? Those who commented that I was never fat before?"
"From that young age I swore I would find the best slimming method... so that I will never be fat again!"
When asked about the backlash she has received, Tay told The New Paper that these people had misunderstood her. She said that her obsession was not with vanity, but with optimal health.
She explained that "looks can be deceiving", and only body composition tests can accurately determine how "fat" a person is.
The tests would also reveal a person's visceral fat levels, she said.
"Even though (one may not look fat) and may even be in the healthy weight range, the way the fat is distributed will determine if you are prone to diseases (such as fatty liver), and that can be detrimental to health," she added.
At 16, Tay said her weight was 67kg, which meant that her Body Mass Index (BMI) was 23.5.Her BMI is now 18.9.
Under the BMI classification, one is considered underweight if the number is less than 18.5, normal weight if it falls between 18.5 and 24.9, and overweight if it is between 25 and 29.9.
A person is considered obese if his BMI is above 30.
Although she was in the normal weight range in the past, she claimed she had a "terrible" body fat percentage. She cannot remember the figure, but insists it was well over the norm for women.
According to the American Council on Exercise, women qualify in the "fitness range" only when their body fat is between 21 and 24 per cent.
Higher body fat percentages put them in the "average" or "obese" categories.
It was only when Tay turned 20 that she succeeded in achieving a body fat level of 22 per cent, which she now maintains.
She said she has always been interested in health studies since a young age and wanted to lose the extra fat for her long-term health.
The health coach and nutrition consultant at fitness centre Body Inc said: "Many people (who saw my post) said that I was not fat then, and I am too skinny now.
"I have been giving talks for the Health Promotion Board for the last two years so I know well enough that I have the ideal weight and body fat percentage now."
Tay admitted that her struggle with fat loss as a teen was fraught with many failures.
She said that she tried every method that was on the market, such as replacement diets, slimming pills and slimming shakes.
In her most desperate moments, she even considered going for liposuction.
"In the end, I realised that for long-lasting results, simply getting proper nutrition and exercise is the best.
"A good diet means choosing nutrient-dense, low-calorie foods and cutting down on sugars, processed foods, fried food and fat.
"Stair-climbing is my all-time favourite exercise now, because it burns more calories than running."
Big belly could spell danger
Overweight women who saw Jacelyn Tay's Instagram post had mixed reactions to it.
Housewife Chan Joong Li, 39, who is 1.64m tall and weighs 75kg, said: "Her post was like a wake-up call for me.
"I've been overweight since young, but because I've always been confident of my curves, I've never bothered with my fat.
"I will be going to take the body composition test to assess my risk for diseases and I will have to lose the weight if need be.
"I have two young kids so I want to be healthy so that I can take good care of them."
Businesswoman Glenda Lee, who is 1.7m tall and weighs 90kg took a different view, acknowledging that Tay had a right to hers.
Said the 25-year-old: "I do health screenings every year and have always been in the pink of health.
"I am a fat woman who is healthy. This is possible, just in case everyone who reads Jacelyn's post suddenly starts trying to lose weight because they get scared."
"My advice to people is to get their health checks done. If there is nothing wrong (with your health), then just be happy with the size that you are."
While health coaches and fitness trainers The New Paper spoke to differed widely on weight loss methods and what it meant to be fat, they did agree on the idea that visceral fat can be harmful to health.
An excess of visceral fat is known as central obesity, or "belly fat", in which the abdomen protrudes excessively.
Newly-developed tests such as the Body Volume Index are specifically designed to measure abdominal volume and abdominal fat.
Excess visceral fat is linked to Type 2 diabetes, insulin resistance, inflammatory diseases, and other obesity-related diseases.
Said Ms Samantha Giles, health coach and founder of health centre Sprouting Wholeness:
"I can tell you that where you accumulate fat (visceral fat in particular) increases the risk of developing certain diseases.
"When I work with clients I don't just focus on losing fat per se, because losing fat is complex and involves things like getting inflammation under control, getting hormones balanced and managing stress levels."
Mr Jovin Koh, 28, fitness trainer and owner of sports and fitness company Brand New Start, who also saw Tay's Instagram post, said that she may have what is termed "skinny fat".
He said: "This means that one may look slim but may actually have fat.
"I think that most woman are very critical about themselves, always thinking that they are fat."
"I think this is shaped by the demands of their job and their own insecurities."
"(In my opinion), for body fat to affect your health, an individual needs to be very overweight, consume junk food daily, have irregular meal timings and a lack of exercise and lead a sedentary lifestyle."
This article was first published on Dec 22, 2014. Get The New Paper for more stories.