Is mustard healthier than Mayo for a sandwich, why?

Mustard is far healthier that mayo...and not just for the fat content. Mustard is low in Saturated Fat, and very low in Cholesterol. It is also a good source of Dietary Fiber, Protein, Calcium, Iron, Magnesium, Phosphorus and Copper, and a very good source of Manganese and Selenium. It also tastes great and comes in a variety of flavours.

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Mustard is techically "healthier" calorically and fat-wise, for obvious reasons. Mustard is practically calorie free, and fat free, so if this is what you're looking for, then it's a better choice. However, don't forget to include some fat in your diet, because a little is necessary for proper nutrition and good health. If you're having lean deli turkey, there is no harm (it's healthy even) in having a tablespoon of light mayo. And, many will argue that mayo tastes much better, especially with turkey. But, if you're having a fattier meat, such as some ham, it's not a bad idea to have mustard, both for health reasons, and because most people will argue that mustard tastes better with ham than mayo.

Mustard is by far much healthier than Mayo, sure.. Mayo probably tastes better on some sandwiches. Also Mustard has Turmeric which is very good for the body.

Yes. Mustard has less fat. You can use FF Mayo if you like it better than mustard. I like Kraft FF Mayo.

Well, for one, Mayo is made with egg so Mustard has less cholesterol. Secondly, mayo that you buy in the store is packed with preservatives (not fresh). Cholesterol + Preservatives...YUMMM!

I guess mustard has less fat, I love spicy brown mustard on lots of food.

Yes, because mustard is fat free and mayo is full of fat.

there are no calories in mustard. no fats

Yes. Fat content, check out the labels.