In the developed world, the biggest change in eating habits in the next 100 years will come from increasing knowledge about the connections between diet and health.
In 1900, we knew next to nothing about nutrition; all there was to go on was folklore and common sense. (Some of this, admittedly, was good stuff: Use a little wine for thy stomach's sake, as Paul the Apostle advised Timothy.) Now, we have crude data based on studies of large--and sometimes not so large--numbers of cases. The results are more ambiguous and oracular than anyone likes to let on.
By 2100, we will know a great deal more about diet and health--knowledge that will almost certainly have to do with genetic susceptibilities. Today's general scare about fats and salts will be replaced by specific know-how about what individuals can and can't eat. As a result, people will be eating more foods that we currently think of as bad for us. We may also, one hopes, be eating less of the bad food that we now consume: industrially raised meat, vegetables grown for looks rather than taste, food produced with hormones, pesticides and other chemicals.
As the "natural" becomes ever harder to find, people will value it more. The supermarket idea--that everything ought to be available all the time, irrespective of geography and season--may lose a little of its grip as people become more attentive to real food. On the other hand, restaurants will be increasingly driven by pure fashion. The previous hot restaurant in London did Peruvian-Japanese fusion. The latest does "Modern Ottoman." Why? No reason. The next will be Louisiana Retro, Tasmanian-Nigerian fusion, Icelandic, whatever. Some of the fashions will be, from a gastronomic point of view, welcome; others won't.
Eating habits will become more faddish. The rich are always strange about food, and as more people get rich, more will want to enact psychodramas in relation to what they consume. This is something that only the unneedy can afford to do. It cannot be a coincidence that as the Western world has grown wealthier, it has grown increasingly obsessed with being thin. This kind of thinness is a form of artifice, a contemporary equivalent of medieval Japanese courtiers' dying their teeth black in order to be fashionably similar to the emperor. It would be a welcome thing if this mania were to subside, but I can't see any reason to expect that to happen.
As for the rest of the world, well, it would be nice if fewer people were dying of hunger in 2100 than are doing so in 2000. A realization that global hunger is already a solvable problem would be a start. There might be a glimmer of hope for the hungry if the well-fed were to ask ourselves, in the words the late writer Primo Levi took as the title of a book: If not now, when?
John Lanchester's writing on food includes "A Debt to Pleasure: A Novel" (Henry Holt).