Businesses in Singapore generally do not have their own cafeterias (the locals call them canteens), mainly because there is no shortage of food places in Singapore. Having said that, many of the local type places serve local food (Asian, Singapore/Malaysia style - in the US it would be Penang or Chinese Malaysian style restaurants). However it seems you're after Western food. There's quite a few Western restaurants around. However from the sounds of what you are after, the closest I can think of is a sandwich chain called O'Brien's. Try this website:

http://www.obriensonline.com/ASIA/Food/docs/food.pdf

I don't know if there's one around the Orchard area although the concierge at the hotel where I presume you will be staying should be able to help if you tell them you want O'Brien's or something similar to that.

As for American style restaurants in the vicinity of the Marriott and the Hilton, the Hard Rock cafe (easy three minute walk from the Hilton) and Dan Ryan's (seven minutes from the Hilton) should fit the bill.

If you'll be at a place where you can do some light cooking, Jason's Supermarket in the basement of Orchard Towers (catecorner from the Hilton and close to the Orchard Hotel) has a fairly large expat friendly selection of food products and has the highest probability of stocking the low carb and low calorie foods you're looking for.

In conclusion this shouldn't be a big issue. If you can stand going to restaurants all the time, finding cooperative chefs (especially along either side of Orchard Road between the Hilton and the Centrepoint Shopping Centre) shouldn't be a problem.