“Cutting Carbs and Fats Is Wrong?” Doctor Debunks Popular Diet Myths

Western diets are not necessarily suitable for Japanese people

“Build muscle to boost your metabolism and you’ll lose weight.”
“If you skip the rice, yakiniku won’t make you fat.”
“Zero-carb beer and distilled spirits won’t lead to weight gain.”

These are commonly believed diet truths—but in reality, they’re all wrong. These methods won’t necessarily help you lose weight, nor will they guarantee you won’t gain it.

It may be a harsh truth for those dieting ahead of summer, but Japanese people are physiologically more prone to gaining visceral fat. Not only that, they also tend to have a harder time building muscle, and their basal metabolism tends to drop in summer, making weight loss even more difficult.

Despite this, most of the diet advice circulating is based on Western-origin methods. Diet fads that become popular overseas are often introduced in Japan without considering physiological differences. While the information itself may be accurate, many of these methods simply don’t apply to the Japanese body type.

According to Dr. Masako Okuda, a physician and occupational health specialist who has treated over 300,000 patients, many people end up gaining weight rather than losing it because they follow diets that don’t match their body type.

Low-carb diets, strength training, cutting out oils, banana breakfasts. There’s no shortage of popular methods said to help with weight loss. In her book published in May, titled “If You Quit This, You’ll Lose Weight,” Dr. Okuda spotlights common “NG habits” (practices to avoid) drawn from these trends. At the end of the book, she even ranks the top 7 worst diet habits.

With clear medical explanations, she shows why these seemingly healthy practices can backfire and emphasizes that the fastest path to weight loss is cutting out the inefficiencies in your current approach.

Is it true that Japanese people can’t lose weight through strength training!?

These days, low-carb diets and strength training are all the rage. For example, some people believe that if you raise your basal metabolism through strength training, you’ll lose weight or as long as you skip the rice, yakiniku won’t make you fat, so they hit the gym on weekends, cutting out white rice and enduring zero-carb beer.

However, according to Dr. Okuda, all of these are diet methods that should be avoided (NG habits) when considering the Japanese body type. She explained three such NG habits, including the above.

◇ “If I increase my basal metabolism through strength training, I should lose weight.”

“You often see articles online claiming that if you build muscle through strength training, your basal metabolic rate will increase and fat will burn more easily.

It’s true that building muscle increases your basal metabolic rate — there’s no mistake in that. However, that doesn’t necessarily mean you’ll lose weight.

This is related to the Japanese body type, which has difficulty building muscle.

Human muscles are composed of two types of fibers: red fibers, which provide endurance, and white fibers, which provide explosive strength. Muscles are made up of a mix of both.

Which type dominates differs by race. For Japanese people, 70% are red fibers. In contrast, Europeans and Africans are said to have 70% white fibers. It’s a stark contrast.

And the type of muscle that grows with strength training is the white fiber — the type that Japanese people only have 30% of.

Furthermore, even if you work hard to build muscle, the increase in basal metabolism is minimal. From that perspective, the dieting method of building muscle to raise basal metabolism and lose weight must be said to be highly inefficient for Japanese people.”

So, what is an efficient form of exercise for dieting?

As mentioned earlier, East Asians including Japanese people are fundamentally prone to accumulating visceral fat. Taking this body type into account, aerobic exercise — not strength training — is more suitable.

“First, there are two types of fat that accumulate on the body: visceral fat and subcutaneous fat, and they have different characteristics.

Compared to subcutaneous fat that accumulates under the skin all over the body, visceral fat that accumulates around the abdomen is seen as dangerous fat that negatively affects health, such as causing lifestyle diseases. Moreover, when people gain weight, it tends to start with visceral fat.

Being prone to accumulating visceral fat is a major weakness for Japanese people, both in terms of health and dieting. That said, when you lose weight, visceral fat is the first to go — so there is a positive side too.

When it comes to exercise, subcutaneous fat can be reduced to some extent with strength training (anaerobic exercise), but visceral fat can only be reduced through aerobic exercise. It doesn’t decrease with strength training. That’s why, if a Japanese person is exercising for the purpose of dieting, aerobic exercise is the only choice. I hope people will remember this.”

Is cutting carbs really the right approach?

◇ If there is no white rice, yakiniku won’t make you fat

Nowadays, carbohydrates are seen as the enemy of dieting. Since white rice contains carbohydrates, many tend to think that as long as they stop eating white rice, even yakiniku is fine—but yakiniku contains a lot of fat. There’s no way eating it won’t make you gain weight.

“In the past, it was common to refrain from consuming fats such as meat, fried foods, and dairy products when dieting. However, some time ago, carbohydrate restriction began to be promoted, and the claim that carbohydrates are more problematic than fats spread.

Because some carbohydrates are converted into fat through insulin secretion, voices arose saying carbohydrates should also be limited, but the claim that carbohydrates are more problematic than fats went too far, and it seems to have been replaced by an extreme idea like ‘If you watch your carbohydrate intake, you don’t need to worry about fats.’

Fats are basically lumps of oil. To burn 1 gram of fat requires consuming 9 kilocalories. In contrast, carbohydrates require less than half that—only 4 kilocalories. It is clear that fat intake should be limited. On top of that, rather than just feeling safe by limiting fats alone, I think a balanced approach is to pay attention to limiting carbohydrates as well.”

By the way, many people think that if beer is zero carbs, it won’t make them fat and drink it every night, but alcohol has calories (one can of beer (350ml) is about 100 kilocalories). Naturally, consuming too much will make you gain weight. People tend to focus too much on carbohydrates, but the enemy of dieting is not carbohydrates alone.

◇ Eating the same amount of calories in summer as in winter

The hot summer is just around the corner. Some people might excuse themselves by saying, “Summer burns more energy, so it’s okay,” and try to maintain their stamina by eating foods like eel or tonkatsu (fried pork cutlets).

However, this is a big mistake. For Japanese people, the basal metabolic rate fluctuates with the seasons. Since basal metabolic rate is about 8% lower in summer compared to winter, if you eat the same amount of calories in summer as in winter, you will definitely gain weight.

“There is data showing that the basal metabolic rate of Europeans remains constant throughout the year, but for Japanese people, it changes seasonally. This is probably because the body has adapted to the humid Japanese summer, with a higher basal metabolic rate in winter and a lower one in summer.

In winter, when the temperature drops, the body needs to burn energy internally to maintain body temperature and produce heat, which raises basal metabolic rate. In contrast, in hot summer, the body can maintain temperature without burning much energy, so basal metabolic rate decreases.

In other words, since calorie consumption is lower in summer compared to other seasons, eating the same amount will cause weight gain.”

Because calorie consumption is lower in summer, appetite naturally decreases. This is a well-designed bodily mechanism, but some people misunderstand a drop in appetite as a bad thing.

“You’ll see articles that say if you don’t eat, you’ll suffer from summer fatigue, lose strength, and weaken immunity, so you need stamina foods. But doing that just leads to weight gain. When your strength is down, you should rest your body.

You don’t need to be as active as in other seasons. Of course, hydration and replenishing vitamins and minerals (which the body cannot store) is necessary, so consciously eating vegetables is enough.

For healthy people, it’s natural to lose appetite in summer. For those aiming to diet, it’s even better to just let your body take care of it naturally.”

Restricting calorie intake also leads to better health

Many people struggling with obesity are unaware of their own incorrect eating habits, and “I worry that they might even be avoiding facing the truth,” says Dr. Okuda.

“When I ask patients why they can’t lose weight, many blame lack of exercise even though overeating is the real cause. Others list trendy foods like bananas or avocados and say, ‘I eat these because they’re good for dieting, but I still can’t lose weight.’

No matter what food you eat, you can’t lose weight just by eating it. The reason they can’t lose weight lies in their eating habits, but they refuse to look at that. It’s very unfortunate.”

For Japanese people trying to diet, aerobic exercise (as mentioned earlier) and practicing a diet that prevents gaining weight in terms of food are important. It’s crucial to firmly grasp the golden rules of dieting.

Dr. Okuda’s golden rules of dieting suited for Japanese people are: First, reduce calorie intake; next, reduce fat; and then reduce carbohydrates.

“The first step to a diet that prevents gaining weight is to reduce calorie intake. Ultimately, no matter what you eat, if your calorie consumption is less than your calorie intake, you will gain weight.

This was demonstrated by an American nutrition expert who experimented on himself.

He limited his daily calorie intake to 1,800 kcal—half the average American intake—and ate only junk food for 10 weeks straight.

After 10 weeks, he lost 12 kg. Despite eating only junk food, his weight dropped, his cholesterol and blood sugar levels decreased, he woke up feeling refreshed in the morning, was full of energy, and felt as if he had been reborn.

Since this was a medically supervised experiment, ordinary people shouldn’t imitate it, but it clearly shows that calorie intake is the most important factor in losing weight, and that reducing calories improves health.”

Why is it preferable to reduce fat before carbohydrates?

“As mentioned in ‘If you skip rice, yakiniku won’t make you fat,’ there are various opinions on whether to reduce fat or carbs first.

But race differences play a big role here, related to how insulin works. Recent research has revealed which should be limited based on these differences.

Japanese and other East Asians naturally secrete only half to a quarter the amount of insulin compared to Europeans. However, East Asians’ insulin works more effectively—it is more sensitive. But when Japanese people become obese, their insulin sensitivity decreases, so resolving obesity is also important for this reason.

Based on these differences, research shows that Europeans, who secrete a lot of insulin but have poor insulin sensitivity, do better on a low-carb diet. Conversely, East Asians, who secrete less insulin but have high insulin sensitivity, do better on a low-fat diet.

The reasons are still under investigation, but here is my hypothesis:

As I mentioned earlier, some carbohydrates convert to fat, but since insulin works very well in Japanese people, it processes carbs efficiently and they are less likely to convert to fat. Therefore, even if Japanese people consume the same amount of carbs as Europeans, fewer carbs turn into fat. So, it’s thought that restricting fat is more effective for weight loss than restricting carbs.

Therefore, the golden rules of dieting are first reducing calorie intake, then fat, then carbs. If you keep this in mind when reviewing your diet, I hope it helps.”

Masako Okuda. Graduated from the Graduate School of Medicine, Kyoto University. Physician specializing in internal medicine. Holds a PhD in Medicine from Kyoto University. During her doctoral studies, she engaged in basic research. While contemplating the nature of life and health, she was drawn to the philosophy of preventive medicine and has examined and treated more than 300,000 patients in health screening and medical checkup institutions. She has also translated overseas medical literature and textbooks. Concurrently serving as an industrial physician for an airline company, she continues comprehensive medical care including stress management. Her publications include: “How Different Japanese People Are from Westerners: The Japanese Body Type” (Kodansha Bluebacks), “The Fastest Way to Lose Visceral Fat” (Gentosha Shinsho), “If You Stop Doing This, You’ll Lose Weight” (Toyo Keizai Shinposha), “Super-Translated Youjo-kun: Building a Body That Doesn’t Get Sick” (Discover Twenty-One).

■ Purchase Dr. Masako Okuda’s new book If You Stop Doing This, You’ll Lose Weight here: