Hara hachi bu has an expiration date.
The Okinawan habit of stopping at 80 percent full is probably Japan’s best-known contribution to the longevity conversation. I wrote about it myself in Walking to 110. But carry that advice straight into your seventies and eighties and it flips on you. The most common nutrition problem among older Japanese today is eating too little, not too much.
One in five women over 65 is “undernourished”
Japan’s Ministry of Health, Labour and Welfare runs a National Health and Nutrition Survey every year. The 2023 edition, published in November 2024, found that 12.2 percent of men and 22.4 percent of women aged 65 and over fall into the “tendency toward undernutrition” category, defined as a BMI of 20 or below. The share is highest among people 85 and older, for both sexes, and it has barely moved in ten years.
One woman in five. Past 85, more than that.
BMI is your weight in kilograms divided by your height in meters, squared. A woman who is 5 feet 1 inch (155 cm) and weighs 106 pounds (48 kg) sits right at 20. You see that build everywhere in Japan, on the street and in day-care centers for seniors. It looks petite and healthy. On paper, it counts as undernourished.
Thin people die sooner than heavy people
Let’s check the risk of being thin against Japanese data specifically.
Researchers at Japan’s National Cancer Center pooled seven large domestic cohorts, 353,422 adults followed for an average of 12.5 years (Sasazuki S et al. Journal of Epidemiology. 2011;21(6):417-430). All-cause mortality was lowest at a BMI of 21 to 27. Compared with the reference band of 23 to 25, men with a BMI of 14 to 19 had 1.78 times the risk of dying, and women 1.61 times. Obese participants with a BMI of 30 to 40 came in at 1.36 (men) and 1.37 (women). In middle-aged and older Japanese, being thin raised the odds of death more than being obese did.
This is observational research, and I want to be careful with it. If someone lost weight because they were already sick and then died, thinness was the result, not the cause. That is reverse causation, and the authors adjusted for smoking and prior illness but could not rule it out entirely. Still, seven separate cohorts all pointed the same way. That is hard to wave off.
The finding is now baked into national policy. Japan’s Dietary Reference Intakes (2025 edition) set target BMI ranges by age: 18.5 to 24.9 for ages 18 to 49, 20.0 to 24.9 for ages 50 to 64, and 21.5 to 24.9 for everyone 65 and up. The floor rises as you get older, because the risk of frailty and disease from being underweight rises with age.
The “ideal weight” you had at 30 is the wrong target for 75.
So how solid is the science behind eating less?
There is real evidence that calorie restriction helps. The question is who it was tested on.
The American CALERIE trial randomized 218 healthy, non-obese adults and had one group cut calories by an average of 11.9 percent for two years. Blood pressure, LDL cholesterol, insulin sensitivity and other cardiometabolic markers improved significantly, and by more than the weight loss alone would predict (Kraus WE et al. The Lancet Diabetes & Endocrinology. 2019;7(9):673-683).
The participants were 21 to 50 years old.
No comparable trial has been run in people over 70. The traditional Okinawan diet was indeed low in calories, as Willcox and colleagues laid out in their review (Willcox BJ et al. Annals of the New York Academy of Sciences. 2007;1114:434-455). But that describes people who ate that way from childhood, not people who started cutting back at 80.
Here is my own opinion, clearly labeled as such: hara hachi bu is wisdom for your fifties. Past 65, the safer setting is “eat until full, and lean on protein.”
Just step on the scale
What makes undernutrition dangerous is that people don’t notice it. Appetite fades and you chalk it up to age. Your pants get loose and you’re pleased. By the time anyone looks closely, grip strength and walking speed have already slipped.
The fix is not complicated. Weigh yourself once a month under the same conditions. If you’re down 4 to 6 pounds (2 to 3 kg) over six months, that is not good news to celebrate. It is a signal to find the cause. Talk to your doctor. Dental or swallowing trouble, medication side effects, depression, cancer: unintended weight loss often has a treatable reason behind it.
If your BMI is under 21.5, drop whatever reason you have for eating less. Another bowl of rice, one more egg, a glass of milk. The protein target I wrote about in July, 1.2 grams per kilogram of body weight, collapses if the body weight itself keeps falling.
Hara hachi bu is a gift Japan gave the world. That is exactly why we owe the world an honest note about who it’s for, and when.
My book Walking to 110 covers the Okinawan diet and this age-based switch in its chapter on food. The Kindle edition is here.
References
- Ministry of Health, Labour and Welfare (Japan). National Health and Nutrition Survey 2023: Summary of Results. Published November 2024.
- Ministry of Health, Labour and Welfare (Japan). Dietary Reference Intakes for Japanese (2025 edition), report of the drafting committee. October 2024.
- Sasazuki S, Inoue M, Tsuji I, et al. Body mass index and mortality from all causes and major causes in Japanese: results of a pooled analysis of 7 large-scale cohort studies. Journal of Epidemiology. 2011;21(6):417-430.
- Kraus WE, Bhapkar M, Huffman KM, et al. 2 years of calorie restriction and cardiometabolic risk (CALERIE): exploratory outcomes of a multicentre, phase 2, randomised controlled trial. The Lancet Diabetes & Endocrinology. 2019;7(9):673-683.
- Willcox BJ, Willcox DC, Todoriki H, et al. Caloric restriction, the traditional Okinawan diet, and healthy aging. Annals of the New York Academy of Sciences. 2007;1114:434-455.