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."