Click any picture to load a larger one with more detail.
Before I dive into this topic, let me say that I am not an expert on anything, nor am I a novice. I'm going to write based on my copious amounts of firsthand experience and reach conclusions based on obvious indicators. I feel obliged to write that first because any time one asserts anything about Japan or Japanese people, there will be people with different experiences who will come forward to argue.
My experiences are based on living in Japan for 20 years, asking many people about their lives and habits, and working in a Japanese office for 12 years. My experiences are very far from limited, but they are also far from exhaustive. I don't want to get into quibbling matches with people who have anecdotal experiences which differ from mine. I'll grant that I am speaking generally and not about every individual in Japan. However, there are some things that are clear based on marketing as well as observation, and that is something of which I'm pretty confident about at this stage of the game.
I know how many snacks are put on sale and how much shelf space is devoted to them. I know that there are shops which specialize in selling nothing but snacks and that they are on offer in a greater variety of shops than other types of food. In fact, except for specialty shops, nearly any place which sells food will offer sweets and salty treats.
These things alone are strong indicators that Japanese people snack a fair bit. If they didn't, there wouldn't be more space devoted to candy, sugary beverages, alcohol, and salted junk food than there is to dairy products, rice, and fresh produce in some markets. Shops don't offer goods which don't sell well. In particular, supermarkets which have a narrow enough profit margin do not waste shelf space on goods that don't move, and all markets in my neighborhood devote at least both sides of one aisle to snacks. Most offer more than that.
It is important to separate Japanese snacking into at least two separate categories. The first is otsumami, or snacks that are marketed and consumed with beverages. Those foods aren't always consumed with alcohol, but they often are. There's a survey on "What Japan Thinks" which says that it is unclear if snacks like cookies, cakes, etc. that are designed to be served with drinks like tea are included in the category of "otsumami", but my experience makes me believe otherwise (note that this is a speculation on the part of the translator, not a part of the survey itself).
A snack bar at Inokashira park with familiar snacks on offer including hard candy, milk and black sugar caramels, "Shigekix" gummi candies and Mentos.
Based on my experience with the way in which foods are marketed (and I've read a lot of labels by now), manufacturers only consider food that is to be served with booze or soft drinks to be otsumami. I have never seen a sweet snack labeled as such and often see foods which are salty with such labels. Mainly, savory items like dried fish, nuts, chips, pretzels, rice crackers (sembei), etc. are placed in the otsumami sections of various shops.
When speaking with Japanese people about their consumption of such snacks, those that are labeled otsumami appear to be much more popular than other types. Most of my students, for instance, go home and have beer and nosh on sembei or chips. Far more of them will buy potato chips or something like them than they will sweets. That's not to say they never buy sweets or eat them, but they seem to snack on them far less often in the evenings at home. Japan is a country of voracious drinkers, though not voluminous. A great many adults unwind at night with a can or two of beer and some salty thing or another on the side.
In regards to sweets, there is another helpful survey on "What Japan Thinks". I mention these surveys mainly because they are scientific and objective ways of measuring snacking habits (rather than speculation based on anecdotes). Note that 25% people say they eat sweets everyday and about 75% eat them multiple times a week. This rather flies in the face of any notion that the Japanese are not a country of snackers, a supposition which is often made to explain how the Japanese stay thin, or that they aren't very keen on sweets. Certainly, they do snack, and they like sweets.
My experience has been that sweets are consumed mainly on the job when one becomes tired. In essence, they're seeking a sugar rush, but it also probably has something to do with the fact that people work late hours and eat lunch around noon. Around 4:00 pm, many Japanese offices have "tea time" and people will snack and sweets are fairly common. Often, salespeople or those who travel for their type of work will bring back boxes of "omiyage" (souvenirs) to share with coworkers. Omiyage are almost always food items.
Bean cakes, sembei, cookies, and chocolates are popular choices as souvenirs. It's one of the reasons Nestlé Japan makes so many regional KitKats. They know people who travel for a day to some city on business might find them appealing to bring back and share with coworkers. Regional KitKats are, in part, exploitation of the business culture and the near-obligation to bring back snacks for coworkers. It helps that, at 800 yen ($9.20) a box, they're relatively cheap.
The main difference between the way Japanese folks snack and Western ones do is in volume. In my experience, most Japanese young women working in offices eat sweets or snacks nearly everyday, but in very small quantities. One small cookie with a cup of tea or one big or a few small rice crackers tend to be the extent of such snacking. That being said, I have known people who will go to a hundred yen shop and buy cheap salted snacks and eat the whole bag at night with beer. And trust me when I say they are just as slender as most Japanese people.
I don't want to weigh in (no pun intended) on the chorus of explanations about why Japanese people tend not to be fat, but I can provide some anecdotal experiences that may relate to this point. One of my students has told me that she has consumed copious amounts of beer and food on multiple occasions. And when I say "copious", I mean she eats enough for 3 people and drinks enough for 2 burly men. Sometimes she does this, but at other times she doesn't eat anything much at all, especially after a big night of boozing and binging.
Japanese people quite often spend entire days (or weekends) sleeping all day in an off and on fashion to recover from a long week of overtime. In fact, it's quite amazing to me that they can sleep so much. On those days, my student will eat one rice ball (onigiri), drink some bottled tea, and consume nothing else. She sleeps so much that she simply doesn't remember to eat. My guess is that her eating habits on such days even out her higher calorie consumption on days when she eats a lot such that she doesn't gain weight. A day where she eats 5000 calories isn't of much consequence when she eats about 250 the following day, especially when her job requires her to be on her feet all day.
Are there Japanese people who never touch a sweet and never snack? Sure. Are they common? I don't believe so. I think that like many Western folks, there are people who eschew sugar or fried foods in the interest of pure health concerns, but they are even fewer and far between here than in Western countries where people are even more fanatical about health in some ways due to the high incidence of obesity and more extreme and malleable notions of what constitutes "healthy" eating.
Lots of booze, but only edamame (beans) as a snack.
Japanese folks tend not to be extremists about food and seem to practice moderation in nearly all things, though some do go overboard with alcohol consumption. I've never known a teetotaler here, for instance. People only give up alcohol for medical reasons and alcoholism is defined medically, not psychologically, in Japan. The idea that one should give something up in an effort to preempt potential health issues (rather than deal with an actual present one) is not very common here, so deciding not to eat sugary treats or salty snacks isn't something most people are going to do unless a doctor tells them to quit. Considering the fact that most Japanese people visit the doctor at the drop of a hat, I'm guessing they figure any incipient problems will be caught and proper advice will be forthcoming.
So, yes, Japanese people stay thin and they snack. They eat between meals, but usually not more than once or twice per day (tea time and with drinks in the evening) and in quite small portions. Frankly, I feel this is a pretty sensible way to approach food on the whole, and does more for ones health than tossing all treats out the window in an act of radical diet extremism.