How might carb and fat consumption affect longevity in men vs women?

• When it comes to longevity, consuming too little in the way of carbohydrates and fats can shorten one’s lifespan, according to a new Japanese study.
• The study finds that men who eat too few carbohydrates daily may increase their risk of dying, while women who consume insufficient quantities of fat may do the same.
• The researchers studied people in Japan, so the findings may or may not apply as well to Western populations.

A new study from Nagoya University Graduate School of Medicine in Japan finds that going to extremes with carbohydrates and fats can shorten one’s lifespan. However, the hazard differed for men and women. All the study participants were in fit condition at recruitment.

The study found that men who ate too few carbohydrates significantly increased their risk of all-cause mortality. At the same time, women who consumed too little fat had a marginally higher risk of all-cause and cancer-related mortality.

The authors of the study paint a complex picture of healthy eating in terms of carbohydrates and fats, overall suggesting that going to any extreme may negatively affect longevity.

The study appears in The Journal of Nutrition.

When it came to carbohydrate consumption, the researchers found that, in the study cohort, men who got fewer than 40% of their daily calories from carbohydrates were at a significantly higher risk of all-cause mortality.

For women, by contrast, those who got more than 65% of their calories from carbohydrates were at a higher all-cause mortality risk.

The researchers found no appreciable difference between the effect of consuming minimally processed carbohydrates versus refined carbohydrates.

Regarding dietary fat, men who got more than 35% of their calories from any kind of fat were at a higher risk of cancer and cardiovascular mortality.

In men, when the quality of fat intake was examined, no clear association was observed for saturated fat intake. However, consuming less unsaturated fat was associated with a higher risk of all-cause and cancer-related mortality.

For women, consuming more fats — particularly saturated fats — decreased their risk of all-cause and cancer mortality.

The study involved 34,893 men and 46,440 women, ranging in age from 35 to 69 years. The average body mass index (BMI) for men was 23.7, and for women 22.2, within the healthy rangeTrusted Source.

According to cardiology dietician Michelle Routhenstein, who was not involved in the research, “[t]his study suggests that low carbohydrates in diet and low-fat weight loss diets for women can decrease longevity.”

Clinical nutrition epidemiologist Prof. Linda Van Horn, who was also not involved in the study, expressed concern that Americans may take the wrong message from its findings.

“It does not suggest anything about [fad diets],” said Dr. Van Horn, “nor should these studies [be] conducted using different methods across different populations and mostly not in the United States with its high rate of obesity, intake of ultra-processed foods and generally low nutrient adequacy.”

In the United States, nearly one in three, or 30.7%, of adults qualify as having overweight, and two in five, or 42.4%, qualify as having obesity.

Dr. Van Horn added: “The U.S. Dietary GuidelinesTrusted Source take all of these considerations into account, and are far more reliable than attempting to extract meaningful applications from this cohort study with a mean BMI of 23.7 in men and 22.2 in women!”

She also expressed concern that some deaths described in the study may represent “poverty and inadequate nutrient intake overall, and are unlike the U.S. population.”

“Some sources of saturated fat include red meat, coconut oil, butter, palm oil, and full-fat dairy,” said Routhenstein. Prof. Van Horn also notes that “saturated fats are derived from animal sources: butter, cream, bacon, processed meats.”

“Some sources of unsaturated fats,“ Routhenstein pointed out, “include avocados, olives, pecans, and pumpkin seeds.” Prof. Van Horn added corn oil to the list, as well as nuts and seeds in general.

If men require at least 40% of their calories from carbohydrates to avoid reducing their longevity, why might that be, and why might they struggle to obtain the necessary carbs?

Prof. Van Horn suggested that “[i]n this study, [this is] likely due to poor dietary quality, poverty-related lack of adequate healthy care, smoking, [and] alcohol.“

“Diets low in carbohydrates, lack dietary fiber, and nutrients such as magnesium, potassium, vitamin C, and B vitamins, which are essential for our bodies to thrive. When we lack these protective nutrients, it can increase the risk of some cancers.”

– Michelle Routhenstein

The study suggests a shortfall in bioactive dietary components may be at play. Specifically, the authors mention fiber, heme iron, vitamins, minerals, branched-chain amino acids, fatty acids, and phytochemicals as being in short supply.

The authors of the study also mention that a diet lacking in plant sources — particularly when animal products make up the difference — has been seen to encourage inflammatory pathways, cause more rapid biological aging, and produce oxidative stress.

Prof. Van Horn suggested that given female participants’ “low BMI, it may be likely that they eat less sugar and drink less alcohol, and thus consume higher percent fat compared to the men.“

“But […] this is all speculative because these questions are not addressed in the paper,“ she cautioned.

Routhenstein noted the need in women for “a certain amount of fat in order to produce adequate hormones like estrogen, which are cardioprotective.”

The authors themselves do not speculate on this, but note that the intake of saturated fat was inversely linked to mortality risk only among women.

The study supports the need for further research. For people in the U.S. and other Western countries, a similar study done with a more locally representative population may provide more actionable findings that consider the local dietary and health landscape.