“Does the ketogenic diet really work?”
“Fasting or a low-fat diet: which makes it easier to lose weight?”
There are so many diet methods out there that it’s hard to know which one to pick, isn’t it?
This time, I’ll compare three popular methods, “ketogenic (keto),” “intermittent fasting,” and “low-fat dieting,” in depth based on scientific data, looking at
fat-burning effect, ease of sticking with it, safety, and cost.
I’ve put together information to help you judge which one “suits you,” so please use it as a reference.
- 1 Before Comparing Keto, Fasting, and Low-Fat: A Review of the Basics of the 3 Diet Methods
- 2 Comparing Fat-Burning Effects With Scientific Data
- 3 Comparing Ease of Sticking With It, Safety, and Cost
- 4 How to Choose the Right Diet for You: A Guide by Type
- 5 Common Rules for Avoiding Failure With Keto, Fasting, or Low-Fat
- 6 Frequently Asked Questions About Comparing Keto, Fasting, and Low-Fat Diets
- 7 The Conclusion From the Scientific Data: Fasting Is the Strongest Choice for Losing Weight Healthily Over the Long Term
Before Comparing Keto, Fasting, and Low-Fat: A Review of the Basics of the 3 Diet Methods
What Is the Ketogenic Diet (Keto)?
The ketogenic diet is a way of eating that keeps carbs very low (20 to 50 g or less per day) and makes fat the main source of energy.
When you restrict carbs drastically, your body breaks down fat to produce “ketone bodies.”
Ketone bodies are like a “backup fuel” made from fat when sugar runs short.
By entering a state called “ketosis,” in which these ketone bodies are used as energy by the brain, muscles, and organs, your body burns fat efficiently.
- Foods you can eat: meat, fish, eggs, cheese, butter, avocado, nuts, leafy greens, and so on.
- Foods to avoid: rice, bread, noodles, sugar, fruit, root vegetables, and anything else high in carbs.
Unlike a “relaxed carb restriction” where you just eat a little less rice, keto’s defining feature is cutting carbs to the point of eating almost no staple foods at all.
What Is Intermittent Fasting?
Intermittent fasting (IF) is a way of eating that sets a window of time when you can eat (the eating window) and a fasting period.
The most common form is “16:8 fasting (16 hours of fasting and 8 hours of eating),” and it doesn’t restrict what you eat.
The idea is that creating periods of hunger switches your body into fat-burning mode.
For detailed instructions, also read 【The Complete Beginner’s Guide】Types, benefits, and how to do intermittent fasting.
What Is a Low-Fat Diet?
A low-fat diet is a way of eating that keeps fat intake to 20 to 30% or less of total calories.
Fat has more than twice as many calories per gram as carbs or protein, so the underlying idea is that cutting it makes it easier to keep your total calories down.
In Japan, too, the idea that “cutting back on oil helps you lose weight” has been widespread for many years, and the basic diet centers on foods like chicken breast, tofu, vegetables, and white rice.
It often requires counting calories, and it’s characterized by many restrictions on the types of food you can eat.
Comparing the 3 Differences to a Car’s Gasoline
The differences between the three are easier to picture if you compare your body to a car and food to gasoline.
Keto switches the type of fuel from “sugar” to “fat,” changing how the engine burns fuel in the first place.
Fasting keeps the same type of fuel but sets “times for filling up,” making the car use the reserve fuel in the tank (body fat).
Low-fat cuts back on “oil,” the most energy-dense part of the fuel, reducing the amount you put in.
All of them share the same goal of “taking in less energy than you use”; what differs is the route you take to get there.
Comparing Fat-Burning Effects With Scientific Data
Keto Has the Edge for Short-Term (3 to 6 Month) Weight Loss
For short-term weight loss, multiple studies show the ketogenic diet producing the largest effect.
| Method | Short-term (3 to 6 months) weight loss | Long-term (1 year or more) |
|---|---|---|
| Ketogenic | Tends to lose 2 to 5 kg (4 to 11 lb) more | Converges to about the same as low-fat |
| Fasting (IF) | Equal to calorie restriction | High adherence, with long-term effects |
| Low-fat | Standard weight-loss effect | Hard to stick with; many drop out |
A study published in 2020 in the BMJ (British Medical Journal) reported that while the ketogenic diet produces greater weight loss than other diets in the short term,
over the long term of a year or more, its effect settles to about the same as a low-fat diet.
Also, the sudden drop in weight during the first few weeks of keto is said to be related not only to fat loss, but also to losing the water that was stored in the body along with sugar.
Be careful not to assume “I’ve lost this much fat” based only on the number on the scale.
Fasting Excels at Reducing Visceral Fat
When you compare not just body weight but the effect on “visceral fat,” which carries high health risks, fasting shows especially strong results.
A 2019 review paper in the NEJM reported that fasting has wide-ranging effects on improving visceral fat, blood sugar, blood pressure, and cholesterol.
Keto also has an effect on visceral fat, but the big difference is that fasting can improve metabolism itself without changing what you eat.
Visceral fat is fat deep in your belly that works like a “wallet you can dip into right away,” and it’s said to be used before subcutaneous fat.
When you create periods of hunger, your body first draws energy from this wallet, which is probably why fasting is well suited to reducing visceral fat.
For details on fasting and scientific research, also see 【Revealed by the Latest Research🔬】A thorough explanation of the health benefits of fasting proven by medical journals such as NEJM.
The Real Problem With Low-Fat Diets: Fat Wasn’t the Villain After All
The idea that “fat = weight gain” was once mainstream, but recent research has overturned that conventional wisdom.
Fat is an essential nutrient for making hormones, forming cell membranes, and absorbing fat-soluble vitamins (A, D, E, and K).
Restricting fat drastically can actually lead to hormonal imbalances, weakened immune function, and emotional instability.
Also, be careful: if you make up for the fat you cut with carbs like bread and sweets, your blood sugar can actually become more unstable.
What to Know When Reading Research Data
Diet research results are almost always presented as the “average” of many participants.
Even if keto produced a few more kilograms of weight loss on average, that group includes people who lost a lot and people who barely changed.
In addition, most studies run for a few months to about a year, and in many cases we don’t know what happens when people continue for 10 or 20 years.
It helps to think of research data as a map for figuring out “which method might suit you,” not as the answer itself.
Most important of all is actually trying it and checking with your own body how you feel and how easy it is to keep up.
Comparing Ease of Sticking With It, Safety, and Cost
Ease of Sticking With It: Fasting Has an Overwhelming Advantage
The most important factor in diet success is “whether you can keep it up.”
- Keto: Strict carb restriction makes eating out and socializing difficult. “Keto flu” (fatigue and headaches) may appear at first. Relatively high dropout rate
- Fasting (IF): No restrictions on what you eat. You just manage when you eat. Once you’re used to it, it becomes a natural rhythm
- Low-fat: Requires counting calories and checking food labels. Sticking with it long term takes strong willpower
With fasting, you only manage “when you eat,” not “what you eat,” so you can keep it up without losing much of the enjoyment of food.
That’s what leads to its high adherence rate.
Comparing Safety
The three methods differ in terms of safety.
| Method | Main precautions | Long-term safety |
|---|---|---|
| Ketogenic | Possible strain on the kidneys and rise in LDL cholesterol | Little long-term data |
| Fasting (IF) | Hunger and mild headaches in the first few days | Safety confirmed in many studies |
| Low-fat | Risk of essential fatty acid deficiency; harder to feel full | Safe if done properly, but sticking with it is a challenge |
When practiced properly, fasting can be said to be the method with the most robust safety research of the three, and one that healthy adults can easily practice.
If you stay on keto, some people see their LDL cholesterol rise significantly, so it’s reassuring to check with a blood test every few months.
With low-fat, be careful not to cut out the fats your body needs, such as those from oily fish, nuts, and olive oil.
That said, with any of these methods, people with chronic conditions or who take medication should consult a doctor before starting.
Comparing Cost
The ketogenic diet relies heavily on relatively expensive foods such as meat, fish, avocados, and nuts, so food costs tend to go up.
With fasting, you just change the time window in which you eat the same foods you’ve always eaten.
If anything, you eat fewer meals, so food costs often go down.
Low-fat sits in the middle because you cut out certain foods, but buying specialty products drives the cost up.
Look not only at money but also at the cost of “effort.”
Keto and low-fat tend to take up a fair amount of time every day for choosing ingredients, planning menus, and counting calories.
With fasting, eating fewer meals also means less shopping, cooking, and cleaning up, which is a nice bonus.
Meals with family and friends are another big factor in how easy a diet is to stick with.
With keto or low-fat, you inevitably end up making separate meals from your family or hunting for things you can eat at social gatherings more often.
With fasting, as long as you match your eating times, you can share the same dishes, so you can minimize the impact on your social life.
| Method | Food costs | Effort | Meals with family and friends |
|---|---|---|---|
| Ketogenic | Tend to go up | Tends to take up time for choosing ingredients, planning menus, and counting calories | More often making separate meals or hunting for things you can eat at social gatherings |
| Fasting (IF) | Often go down | Less shopping, cooking, and cleaning up | As long as you match your eating times, you can share the same dishes |
| Low-fat | In the middle (buying specialty products drives it up) | Tends to take up time for choosing ingredients, planning menus, and counting calories | More often making separate meals or hunting for things you can eat at social gatherings |
How to Choose the Right Diet for You: A Guide by Type
Based on the comparisons so far, let’s sort out which method suits which kind of person.
Who Keto Is Suited For
- People who don’t mind changing what they eat from the ground up
- People who want to see a change in weight in a short time (with a goal within three months)
- People who prefer meals centered on fat and protein rather than carbohydrates
- People who rarely eat out and can cook for themselves
If you choose keto, fatigue and headaches are common during the first one to two weeks, so I recommend starting during a period when work isn’t busy.
Who Fasting (IF) Is Suited For
- People who don’t want to change what they eat and want to enjoy eating out too
- People looking for a method they can keep up long term without spending money
- People who want to improve not just their weight, but also their physical condition, focus, and mental health
- People who have been dieting for a long time and have repeatedly regained weight
With fasting, rather than aiming for 16 hours right away, starting by leaving 12 hours between dinner and the next morning makes failure less likely.
Who a Low-Fat Diet Is Suited For
- People whose doctor has told them to limit fat (for example, due to dyslipidemia)
- People who like a Japanese-style diet (centered on rice, vegetables, and fish)
- Meticulous people who can carefully manage calories
If you choose low-fat, the key to sticking with it is cutting back on fried foods and fatty cuts of meat while keeping a moderate amount of fat from fish.
With any of these methods, “whether you can actually keep it up” is the ultimate deciding factor.
Rather than which one is “correct,” try choosing based on which one you “can keep up.”
If You’re Unsure, Start With “Fasting + Food Quality”
If you can’t decide, a realistic approach is to first set your “eating times” with fasting, and then gradually rethink what you eat.
For example, simply setting an 8-hour eating window, cutting back on sweet drinks and sweet pastries, and limiting fried foods to a few times a week lets you gradually bring in the good points of keto and low-fat.
Rather than demanding a perfect score from one method, keeping up a “70-point” habit for a long time will end up changing your body much more.
It’s a relief to think of the three methods not as rivals, but as “tools” you can combine.
When things don’t go well, instead of blaming the method itself, look back on “whether it fit my lifestyle” and adjust.
Common Rules for Avoiding Failure With Keto, Fasting, or Low-Fat
Get Plenty of Protein and Vegetables
Whichever method you choose, eating less makes it easy to fall short on protein, vitamins, and minerals.
If you don’t get enough protein, you can lose muscle along with fat, which may actually make it easier to gain weight.
At each meal, aim for about a palm-sized portion of meat, fish, eggs, or soy products, and about two handfuls of vegetables.
Track Not Just Your Weight, but Also Your “Waist” and “How You Feel”
Body weight can quickly shift by 1 to 2 kg (2 to 4 lb) just from water going in and out, so if you let the daily numbers jerk you around, you won’t be able to keep going.
Measuring your waist once a week at the same time and jotting down how you feel, such as your sleep, fatigue, and focus, makes it easier to see the real changes.
If your weight has stalled but your waist is shrinking, you can take it as a sign that you’re burning fat properly.
Don’t Cut Back Too Much Too Fast
Losing a lot of weight in a short time makes it easier to lose muscle, or to binge on the rebound and regain weight.
Aiming to lose slowly, at around a few percent of your body weight per month, should be easier on both your body and your mind.
Don’t Forget Sleep and Hydration
It’s said that when you’re short on sleep for a while, hormones that increase appetite go up, making you more likely to crave sweet or greasy foods.
No matter how carefully you plan your meals, if you keep staying up late, it’ll be harder to see results.
Also, during fasting and in the early stages of keto, your body tends to lose water easily, so take frequent sips of water or tea.
Who Should Consult a Doctor Before Starting
If any of the following apply to you, don’t start on your own judgment; consult a doctor first.
- You use diabetes medication or insulin injections
- You have kidney, liver, or heart disease
- You’re pregnant or breastfeeding
- You’ve experienced an eating disorder in the past
- Children who are still growing, or older adults with reduced strength
In particular, if people who take medication suddenly eat less, it can lead to trouble such as blood sugar dropping too low.
Frequently Asked Questions About Comparing Keto, Fasting, and Low-Fat Diets
Yes, you can.
Keto and fasting go well together, and some studies show that ketone production tends to be boosted during fasting, increasing the fat-burning effect.
However, starting both at the same time puts a heavy load on your body, so I recommend getting used to fasting alone first and then adding keto.
The biggest difference is “whether you change what you eat or when you eat.”
Keto requires fundamentally changing the types of food you eat, while fasting works just by managing your eating window without changing what you eat, so it has less impact on your daily life.
Yes, it does.
In fact, using fasting to stabilize blood sugar and metabolism is especially meaningful after 40, when insulin sensitivity tends to decline with age.
It’s said that it’s precisely during times of major hormonal change that fasting has the greatest effect in resetting your internal environment.
For more on women’s hormones and fasting, also read 【A Must-Read for Women】Women should start 16-hour fasting at 12 hours: the right way to do it in tune with your hormonal balance and menstrual cycle.
Of the three methods, fasting (IF) combined with adequate protein intake and light exercise is said to make it easiest to lose body fat while maintaining muscle mass.
With both keto and fasting, there’s a risk of losing muscle if you don’t get enough protein, so it’s important to eat enough protein within your eating window.
The Conclusion From the Scientific Data: Fasting Is the Strongest Choice for Losing Weight Healthily Over the Long Term
When you compare the three methods using scientific data, each has its strengths and weaknesses.
If you’re chasing only short-term weight loss, there are situations where keto has the edge.
But when you add up long-term adherence, safety, cost, the effect on visceral fat, and how easily it fits into your life,
fasting (intermittent fasting) is the most balanced option.
“No food restrictions,” “no cost,” and “no loss of the enjoyment of food”: of the three diet methods, only fasting has all three.
No eating plan “means anything if you can’t keep it up.”
In that respect, fasting can be called the most realistic way to maintain a healthy body shape over the long term.
Of course, borrowing a few of the good points of keto or low-fat and adding them to fasting is also a perfectly good choice.
Start by leaving 12 hours between tonight’s dinner and tomorrow morning, and see for yourself how your body changes.
If you want to know how to start fasting, be sure to begin with 【The Complete Beginner’s Guide】Types, benefits, and how to do intermittent fasting.
If you’d like to keep up fasting more effectively, another option is to add a supplement that supports fasting.
▼ Related Articles
- 【The Complete Beginner’s Guide】What Is Intermittent Fasting? Types, Benefits, How to Do It, and Why You Might Not Be Losing Weight, Explained Simply
- 【Flip the Fat-Burning Switch】Does 16-Hour Fasting Produce Ketones? How It Works, Tips to Boost Them, and Precautions
- 【Which Makes Weight Loss Easier?】Fasting vs. Calorie Restriction Compared with Scientific Data: Who Each Suits and How to Combine Them
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