"That's what happens when you eat more than grass."
In addition to cooking for Gwyneth, Kate has also counted Miranda Kerr among her clients, working for her while she was still married to Orlando Bloom.
The chef revealed that the couple loved her light curries made with chicken or vegetables and coconut, salmon with quinoa and chicken salad.
Praising the model for 'walking the talk' when it comes to her diet, Kate added that she's not ultra-virtuous and abides by an 80/20 rule when it comes to healthy eating versus treats.
While Kate made Miranda sound like a breeze to work with, she had a slightly tougher task on her hands with Courteney Cox who liked to throw dinner parties for up to 50 people.
Kate said that she rose to the occasion, no matter how many she had to cater for, but things did go askew on one occasion where she accidentally grabbed olive oil instead of tequila to put in Courteney's margarita.
Another mishap occurred when Kate accidentally chipped a chunk out of the counter top in the home of Steven Speilberg and his wife Kate Capshaw.
She admitted breaking down in tears, fearing she would be fired, but luckily Kate loved her food and told her not to worry about it.
Kate's new revelation about Gwyneth Paltrow's strict regime is not the first time the star;s ultra rigid diet has raised eyebrows, as her various cookbooks sing the praises of avoiding gluten, sugar and dairy and hen's eggs.
"Every single nutritionist, doctor and health-conscious person I have ever come across... seems to concur that [gluten] is tough on the system and many of us are at best intolerant of it and at worst allergic to it," she insisted in her second cookbook, published in 2013.
And she caused further controversy with her comments about her children Apple and Moses, then eight and six, being left 'hungry' from not eating carbs.
"Sometimes when my family is not eating pasta, bread or processed grains like white rice, we're left with that specific hunger that comes with avoiding carbs," she said.
She also likes to kick off each New Year with a strict detox, which she shares with Goop readers - cutting out gluten, dairy, corn, soy, caffeine, alcohol, added sugar, red meat, shellfish and white rice.
Gwyneth has previously confirmed that she hasn't eaten meat in 20 years, but her former spouse Chris eased up on his strict vegetarian regime following their split in 2014.
"I was vegetarian for quite a long time and then for various reasons I changed," he told Radio 2's Steve Wright.
"My daughter's been vegetarian since she was born, so I keep getting tempted to go back. I don't eat very much meat."
