Yes, well, I suppose according to the statistics over recent years we are seeing more overweight people (at least Americans) almost wherever we go, at home or on vacations.
My wife and I do eat more on cruises, especially considering that we generally have three table-served meals a day on cruises, less port days. We are not big fans of buffets or the all-you-care-to-eat concepts. You mention "junk" food. One thing we tend not do on cruises is eating snacks between meals, althought we do enjoy the HAL afternoon teas. We also always choose late seating dinners, so never feel the need to participate in late night special buffets, chocolate extravaganzas, etc.
I have been on a low carb (Atkins) regimen since January 2003. Since then I have cruised several times and have never gained any weight, even though I tried an occasional sugary dessert, and ate pizza throughout the Med cruise. The dining staff were more than willing to accomodate me - in fact, they seemed intrigued by the idea of butter, cream, etc, just no sugar or flour (to be quite simplistic about the whole thing). Once I explained the rules of the game, they checked every recipe and didn't put bread on my side of the table, even though I have never experienced cravings for foods I shouldn't eat. We noticed a heightened awareness of low carbing on each cruise on the part of the kitchen and have never been hungry or unhappy.
I do eat much more on a cruise than I do at home. I also eat things that I do not eat at home: Desserts, drinks, and types of food that I would rarely have such as venison or rabbit.
On the other side of the coin I expel more energy on a cruise than I do at home. Snorkeling, walking, swimming and being just relatively more active. Because of this it balances out and I really don't gain a large amount of weight. Yes, I probably do add one or two pounds but I have never had a situation where the dress shirt didn't fit on the second formal night or the belt had to be let out a notch.
Eating is a major part of the cruise experience and I enjoy it as much as I can and I'm not afraid to admit it.
Take care and Bon Appetit
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"There is a great difference between being well traveled and just having been to many places." ~Me
Just returned from cruising the Inland Passage of Alaska on the Oosterdam. Didn't gain a pound. I appreciated the portion sizes, they were not huge like some restaraunts. I ate three meals each day, but didn't have snacks between meals. We didn't drink a lot of alcohol or sodas. It was fun trying a lot of new foods. Did a lot of walking on deck and using the stairs!