By Robby Berman on September 10, 2023 — Fact checked by Kelsey Costa, MS, RDN
- 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 range.
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 Guidelines 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.
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How bad are carbs, really?
There is a fear that carbohydrates are detrimental to health. As a result, low carb diets have become popular, especially for weight loss. In this Honest Nutrition feature, we explain what carbohydrates are, look at healthy vs. unhealthy options, and discuss whether a diet rich in carbs is actually harmful.
Carbohydrates are one of the three macronutrients that occur naturally in plant foods, including peas and beans, nuts and seeds, grains, dairy and dairy products, fruits, and vegetables.
The other two macronutrients are dietary fats and proteins.
Carbohydrates are considered essential nutrients for the body to function properly by several global health organizations, including the World Health Organization (WHO). Their primary role is to provide energy to all cells in the body. In the absence of carbohydrate intake, the body will use an alternative fuel source called ketones.
The word “carbohydrate” is an umbrella term that describes various types of sugar-containing molecules present in foods.
Generally, there are three types of carbohydrates: sugars, starches, and dietary fiber.
It is possible to classify them further as simple or complex carbs, depending on the number and type of sugar molecules — such as glucose — that each structure contains.
Simple carbs
Also called “simple sugars,” “sugars,” or “saccharides,” these carbohydrates contain between one and 10 sugar molecules and are present in fruits, vegetables, and dairy products. Those with one or two sugar molecules are called monosaccharides and disaccharides, respectively, while those containing up to 10 sugar molecules are called oligosaccharides.
Lactose — the main sugar in animal milk — is a disaccharide comprising the monosaccharides glucose and galactose.
Oligosaccharides, however, are mid-length prebiotic carbohydrates that are in fiber-rich foods and human milk.
Complex carbs
Complex carbohydrates are made up of polysaccharides, which are longer, intricate chains of sugar molecules. Complex carbs include both starches and dietary fiber.
Starches are the storage carbohydrates in peas and beans, grains, and vegetables, and they provide the body with energy.
Dietary fiber, or roughage, is the indigestible part of plants — in whole grains, fruits, vegetables, nuts and seeds, and legumes such as peas and beans — that supports good gut health.
Carbohydrates often get a bad rap due to the association of their excessive consumption with weight gain, obesity, metabolic syndrome, and diabetes.
This phenomenon, which some researchers call “carbotoxicity,” promotes the idea that the excessive consumption of all types of carbohydrates favors the development of chronic diseases.
For this reason, many low carbohydrate diets have become popular among people interested in losing weight or managing blood sugar levels. They are even in favor among seasoned athletes.
However, several other studies have demonstrated that the quality of carbohydrates that people consume is as important as the quantity.
This finding suggests that rather than all carbs being “created equal,” some options are better than others for health.
‘Unhealthy’ carbs
Carbohydrates that people may consider unhealthy because they are less nutritious include:
- refined carbohydrates, such as polished rice and flour
- sugar-sweetened beverages, such as sodas and juices
- highly processed snacks, including cookies and pastries
According to existing research, a diet with a higher intake of these types of carbohydrates and fewer of the more nutritious options can increase markers of inflammation and perpetuate hormonal imbalances in people with polycystic ovary syndrome (PCOS).
The excessive consumption of simple added sugars is also linked with an increased risk of insulin resistance, non-alcohol-related fatty liver disease, heart disease, stroke, diabetes, and cancer.
However, studies distinguish that added sugars and simple sugars that occur naturally in foods may not have the same negative effects.
A 2018 study even suggests that natural sources of sugar, such as honey, may be effective in reducing blood sugar levels and lowering the risk of developing type 2 diabetes.
Emerging research continues to shine a light on the adverse health effects of these so-called unhealthy carbohydrate foods.
Experts recommend eating a balanced diet that consists primarily of nutritious foods and includes these types of carbohydrates only in moderation.
‘Healthy’ carbs
More nutrient-dense sources of carbohydrates that people typically see as healthy include:
- fruits, such as bananas, apples, and berries
- nonstarchy vegetables, such as spinach, carrots, and tomatoes
- whole grains, such as whole grain flour, brown rice, and quinoa
- peas and beans, such as black beans, lentil peas, or garbanzo beans
- dairy and dairy products, such as low fat milk, and yogurt
Research has linked diets rich in these complex carbohydrates — such as the Mediterranean diet — with anti-inflammatory benefits, lowered insulin resistance, and a reduced risk of chronic diseases.
The researchers attribute many of these benefits to the dietary fiber content of complex carbohydrates.
For instance, the dietary fiber in whole fruits improves long-term weight management and supports regular bowel movements and healthy aging.
Furthermore, boosting the quality of the diet by including more complex carbs and dietary fiber can lead to improvements in some of the effects of PCOS, such as insulin resistance and elevated androgens.
A 2020 review found that the dietary fiber in whole grain foods confers several health benefits, including a reduced risk of heart disease, gut disorders, cancer, and diabetes.
The glycemic index (GI) and glycemic load (GL) are two measures that people have used to establish the quality of carbohydrate foods and categorize them as “healthy” or “unhealthy.”
The GI is a measure of the blood sugar-raising potential of a single carbohydrate food compared with pure glucose.
Low GI foods, which primarily consist of complex carbs, have minimal effects on blood sugar levels. They include whole grains and nonstarchy vegetables. High GI foods include potatoes and foods with added sugars.
Likewise, people use the GL to assess how much a particular meal is likely to increase blood sugar levels.
Although people have used both the GI and GL for decades to guide meal planning and manage blood sugar levels for people with diabetes, the science is inconclusive.
Many studies suggest that an increased intake of low GI foods improves health outcomes, but other studies demonstrate that differences in daily glucose tolerance and individual responses are responsible for blood sugar levels rather than the GI of the foods themselves.
A food’s GI may, therefore, not be a direct predictor of an individual’s glycemic response.
Differences in glycemic response between individuals make it challenging to determine which carbs are truly the healthiest, since even whole grains may not be a consistent and reliable measure of GI and GL.
Despite the popularity of low carbohydrate diets, they are not suitable for everyone, and some populations still benefit from a carbohydrate-rich diet.
For example, endurance athletic performance becomes compromised on a low carbohydrate diet, and a high carbohydrate intake remains the most evidence-backed choice for elite athletes.
Among members of the general population with high carbohydrate intake, significant reductions in blood sugar levels — potentially promoting the remission of prediabetes — occur when the daily intake of carbohydrates is reduced.
Therefore, for populations that consume 65–75% of their daily calories from carbohydrates, experts recommend reducing carbohydrate calories to 50–55% of daily intake and increasing protein.
A carbohydrate restriction of 45% or less of daily calories is more effective for short-term blood sugar control, but it may be unsustainable and does not provide greater long-term results than a range of 50–55% of daily calories from carbohydrates.
Before making changes to their diet, people should speak with a doctor or registered dietitian to determine their specific carbohydrate needs to optimize their health outcomes.
Carbohydrates are an essential macronutrient, providing the body with energy and dietary fiber to support good health.
Excessive consumption of carbohydrates is associated with weight gain and an increased risk of the development of chronic diseases, such as heart disease and diabetes.
Despite their bad rap, however, carbohydrates offer many health benefits when a person frequently consumes sources of complex carbs and dietary fiber in favor of refined carbs and sugar-sweetened beverages.
Also, the ideal diet varies among individuals. For example, a carbohydrate-rich diet optimizes athletic performance.
However, nonathletic populations that consume 65–75% of their daily calories from carbohydrates see the greatest reduction in blood sugar levels when they reduce their calorie intake from carbohydrates to 50–55% of their daily energy intake.
Carbohydrates are not bad when people manage the amount and types that they consume and tailor these to their specific needs.
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Carbohydrates: Are they really essential?
Carbohydrates are a type of macronutrient, and they provide the body with energy. Whether they are essential to the diet remains a highly debated topic. In this Honest Nutrition feature, we define the term “carbohydrates,” discuss whether they are essential in the diet, and explain what happens when people restrict them.
To address whether carbs are essential, it is essential to explore the role of carbohydrates — specifically glucose — in the body and diet.
Carbohydrates are one of the three main classes of macronutrients, alongside proteins and fats. They provide energy to the body, which breaks them down into glucose.
There are three types of carbohydrates: starches, simple sugars, and dietary fiber. However, people may use the term carbohydrates loosely, and incorrectly, to define just starchy foods.
Using this term to refer solely to starchy foods such as bread, rice, and potatoes may lead to an incomplete understanding of the role of carbohydrates in the diet and the body.
Furthermore, carbs are present in a wide variety of foods. Here are some dietary sources of carbs:
| Food group | Carbs per serving, in grams | Examples |
| grains | 15 | quinoa, sweet potato, and rice |
| legumes | 17 | pinto beans, black beans, and lentil peas |
| dairy | 11 | cow’s milk, yogurt, and cheese |
| fruits | 15 | cherries, raspberries, and stone fruit |
| vegetables | 5 | Brussels sprouts, bok choy, and cabbage |
These numbers show that nonstarchy vegetables provide about one-third the amount of carbohydrates compared with grains, legumes, and fruits. However, they are not completely devoid of carbs.
Given that food sources of carbohydrates also provide essential micronutrients — such as B vitamins for energy production and disease-fighting phytochemicals — these foods remain important in the diet.
However, the diversity of food sources of carbohydrates means that higher intakes of starchy foods specifically may not be necessary.
The body breaks down carbohydrate into its major energy-producing form — glucose.
Glucose then undergoes a series of enzyme reactions that generate the energy-producing substrates pyruvate and lactate in a process called glycolysis.
Under normal metabolic conditions and oxygen availability, pyruvate is the main precursor to energy production in the mitochondria of the cell, and the body consistently produces small amounts of lactate via the lactate shuttle.
However, when an oxygen debt occurs — such as during intense physical activity — lactate, in the form of lactic acid, becomes the primary substrate that the body uses to provide energy to muscle cells.
In the mitochondria, the further processing of pyruvate produces acetyl-coenzyme A (acetyl-CoA), a two-carbon compound that combines with carbohydrate-derived oxaloacetate to form citrate and start the tricarboxylic acid (TCA) cycle.
Amino acids and fatty acids also produce some acetyl-CoA.
The TCA cycle, which people may also refer to as the Krebs cycle, generates carbon dioxide and adenosine triphosphate (ATP) — a signaling molecule and the “energy” that enables muscles to contract.
Each glucose molecule goes through two rounds of the TCA cycle, producing ATP and carbon dioxide. This gas enters the blood and leaves the lungs during exhalation.
A steady supply of glucose is necessary to maintain oxaloacetate levels and regular functioning of the TCA cycle.
When dietary carbohydrates are restricted, the TCA cycle is diminished, and ketone production becomes predominant for energy needs.
The body then relies on ketogenic amino acids and fatty acids to produce acetyl-CoA. However, instead of entering the TCA cycle, acetyl-CoA substrates are metabolized to form the ketone bodies acetoacetate and beta-hydroxybutyrate.
The production of ketone bodies takes place in the liver, but these molecules enter the circulation and travel to the brain, heart, kidney, and skeletal muscle to supply energy.
This alternative metabolic pathway for energy production in the body has motivated many low carbohydrate diets as an approach to reduce the chronic disease risk associated with the excess intake of dietary carbohydrate, particularly that of refined carbohydrates and added sugars.
Two of the groups of people most likely to restrict carbohydrates are athletes and those trying to lose weight.
Athletes
Keto adaptation, which refers to the metabolic adjustment to using fat as the predominant energy source when carbohydrates are restricted, can take as little as 5–6 days.
A 2018 study involving weightlifting athletes demonstrated that a low carbohydrate ketogenic diet reduced body fat without compromising lean muscle mass and powerlifting performance.
However, many studies in endurance athletes have revealed that this dietary approach lowers peak power and impairs exercise performance.
For this population, therefore, keto adaption may not be the optimal dietary choice. However, more long-term studies are necessary to confirm this.
Several scientific studies even advocate for high carbohydrate diets for endurance and elite athletes.
Weight loss
When appropriate, weight loss improves metabolic health and reduces obesity-related comorbidities.
Weight loss is often a major motivator for those following low carbohydrate, high fat (ketogenic) diets, and based on the carbohydrate-insulin theory, carbohydrate restriction does offer metabolic advantages.
However, although carbohydrate restriction is beneficial for some people — with a ketogenic diet promoting weight loss and a reduction in blood sugar markers — it can increase the risk of heart disease and the levels of LDL cholesterol, the so-called bad cholesterol.
Furthermore, many researchers have disregarded the carbohydrate-insulin theory as too simplistic given the lack of evidence to demonstrate that a high carbohydrate intake is fattening for most people.
In fact, the restriction of dietary fat leads to more significant body fat loss than carbohydrate restriction in people with obesity.
Instead, overall caloric intake — regardless of the percentages of carbohydrates, protein, and fat intake — appears to matter more for long-term diet adherence and weight loss.
Glycemic index and glycemic load are predictive measures of how a carbohydrate food or combination of foods may affect insulin and blood sugar levels.
For instance, foods with a low glycemic index, which include complex carbohydrates such as quinoa, should have less effect on blood sugar levels than high glycemic index foods such as simple sugars.
However, this system is inconsistent and does not account for individual differences in tolerance to carbohydrates and glucose. These differences help determine exactly how much foods raise blood sugar by in some people.
Individuals may react differently to carbohydrates in the diet, depending on their glucose tolerance, which doctors base on the results of fasting blood sugar, glycated hemoglobin, and oral glucose tolerance tests.
For those with impaired glucose tolerance or insulin resistance, which will include people with prediabetes and hormonal imbalances such as polycystic ovary syndrome (PCOS), carbohydrate restriction is beneficial for improved metabolic health.
It is important to monitor which carbohydrate foods spike blood sugar levels in these individuals because glucose tolerance may also vary from day to day and depending on the time of the day.
Carbohydrates are a macronutrient that provides the body with energy. They occur naturally in a variety of foods, including grains, legumes, fruits, vegetables, and dairy. Despite this, many people use the term carbohydrates to describe starchy foods and simple sugars.
In the body, carbohydrates generate pyruvate, acetyl-CoA, and oxaloacetate in the tricarboxylic acid cycle, playing a key role in energy production.
In the absence of carbohydrates, the body uses ketogenic amino acids and fatty acids to produce ketone bodies for energy production. This process is called keto adaptation.
Carbohydrates are not the only source of energy production in the body, but they provide supportive essential nutrients for energy production, such as B vitamins.
A low carbohydrate diet may improve metabolic markers in people with glucose intolerance and insulin resistance, but it may not be appropriate for other groups of people, such as endurance athletes.
