If you've spent any time reading about healthy aging, you've probably seen the debate.

Keto or Mediterranean?

One diet cuts carbohydrates dramatically. The other emphasizes vegetables, fruits, whole grains, legumes, olive oil, fish, and other minimally processed foods.

Both have passionate supporters.

And now, a new randomized clinical trial has added fuel to the conversation.

The study found that a very-low-carbohydrate ketogenic diet produced greater improvements in several metabolic measures than a Mediterranean diet or a very-low-fat, plant-forward diet in adults with obesity, prediabetes, and fatty liver disease.

That sounds like a clear victory for keto.

But it isn't.

The study tells us something much more interesting:

The amount of weight people lost mattered—and the composition of the diet appeared to matter too.

But that still doesn't answer the much bigger question:

Is keto the best diet for healthy aging and longevity?

Based on this study, we simply don't know.

And that's exactly why it's worth looking beyond the headlines.

What Did the Study Actually Test?

The randomized clinical trial included adults with obesity, prediabetes, and hepatic steatosis, commonly described as fatty liver.

Participants were assigned to one of three eating patterns:

• A very-low-carbohydrate ketogenic diet
• A Mediterranean diet
• A very-low-fat, plant-forward diet

Researchers provided the food and designed the diets to produce approximately 10% weight loss.

That detail is extremely important.

The researchers weren't simply asking:

“Which diet causes the most weight loss?”

Instead, they wanted to understand what happens to the body when people achieve a similar amount of weight loss while eating diets with very different amounts of carbohydrates and fat.

And all three groups benefited.

The weight loss itself produced substantial improvements in metabolic health across the groups.

That is the first lesson worth remembering.

Weight loss itself mattered.

The keto diet wasn't the only diet that helped.

The Mediterranean and plant-forward groups also experienced meaningful metabolic improvements.

Where Did Keto Come Out Ahead?

The differences appeared when researchers looked more closely at certain aspects of metabolic health.

The ketogenic group experienced larger improvements in several measures, including:

• Liver fat
• Liver insulin sensitivity
• HbA1c
• 24-hour blood glucose
• 24-hour insulin levels
• Hepatic fat production

The study found that muscle insulin sensitivity improved by approximately 50% across all three groups, while improvement in liver insulin sensitivity was approximately two to three times greater with the very-low-carbohydrate diet.

The ketogenic diet also produced larger reductions in liver fat.

That is particularly interesting because the participants already had metabolic problems—including prediabetes and fatty liver.

In other words, this wasn't a study of healthy people looking for a trendy weight-loss plan.

It was a study of people who already had metabolic health challenges.

And in this particular population, the composition of the diet appeared to influence some outcomes beyond weight loss itself.

The Prediabetes Finding Is Interesting Too

Another notable finding involved prediabetes.

According to the study's accompanying research report, approximately half of the participants in the ketogenic group no longer met the criteria for prediabetes after the intervention, compared with about 29% in the Mediterranean group and 7% in the very-low-fat plant-forward group.

That is an impressive difference.

But again, context matters.

This does not mean:

“Keto cures prediabetes.”

It means that, in this controlled research setting and in this particular group of participants, the ketogenic diet produced stronger improvements in several metabolic outcomes.

That's an important finding.

It is not the same thing as a universal recommendation.

So, Did Keto Beat the Mediterranean Diet?

In some metabolic measures, yes.

But that is not the same as saying:

“Keto is the healthiest diet.”

The trial was designed to answer a specific scientific question.

It looked at what happened when adults with obesity, prediabetes, and fatty liver achieved approximately 10% weight loss through different dietary patterns.

It did not determine which diet:

• Helps people live longer
• Prevents dementia
• Prevents heart attacks
• Prevents strokes
• Preserves independence in older age
• Is easiest to maintain for 10 or 20 years
• Produces the best quality of life
• Is best for every adult over 55

Those are different questions.

And they require different research.

This Is Where Diet Headlines Can Get Misleading

Health headlines often turn complicated research into a competition.

Keto beats Mediterranean.

Mediterranean beats keto.

Carbs are bad.

Fat is bad.

This is the one diet you need after 50.

Real nutrition science is rarely that simple.

A diet can produce a particular benefit without being the best choice for every person.

For example, a person with significant insulin resistance and fatty liver may respond differently to a very-low-carbohydrate diet than someone who is already metabolically healthy.

Someone else may find a Mediterranean-style eating pattern much easier to sustain.

Another person may have medical conditions, medications, food preferences, cultural traditions, or nutritional needs that make one approach more appropriate than another.

The best diet is not necessarily the one that wins a short-term study.

It may be the one that improves your health and that you can realistically continue.

What About the Mediterranean Diet?

The Mediterranean diet shouldn't be dismissed simply because keto performed better on several metabolic measures in this trial.

The Mediterranean eating pattern has been extensively studied and is characterized by foods such as:

• Vegetables
• Fruits
• Legumes
• Whole grains
• Nuts and seeds
• Olive oil
• Fish and seafood
• Moderate amounts of dairy and other animal foods

It is also a dietary pattern—not a rigid menu.

Its strength is that it can provide a broad range of nutrient-dense foods while emphasizing minimally processed ingredients.

And for many people, sustainability matters just as much as the short-term response.

A diet that looks excellent on paper but is impossible for someone to maintain is unlikely to become a successful long-term health strategy.

What This Study Really Teaches Us About Weight Loss

Perhaps the most important finding is not “keto versus Mediterranean.”

It's this:

Losing excess weight can substantially improve metabolic health.

All three groups achieved approximately 10% weight loss, and all three experienced improvements.

That matters.

For someone with obesity, prediabetes, and fatty liver, even moderate weight loss can have meaningful metabolic consequences.

The researchers found improvements in insulin sensitivity across the groups, while the ketogenic diet produced additional benefits in several measures of liver metabolism.

So we should not lose sight of the bigger picture while debating which diet performed best.

The first win was achieving meaningful weight loss.

The second question is whether the way you achieve that weight loss provides additional benefits—or creates other challenges.

Why This Matters After 55

Nutrition can become more complicated as we get older.

At 25, losing weight may be the primary goal.

At 65, the goal may need to be broader.

You may want to reduce excess body fat while also:

• Maintaining muscle
• Getting enough protein
• Preserving bone health
• Supporting heart health
• Maintaining energy
• Getting enough fiber
• Protecting physical function
• Avoiding unnecessary weight loss
• Supporting independence

This is one reason we should be cautious about applying a diet study to everyone over 55.

A diet that helps one person lose weight and improve blood sugar may not automatically be the right choice for another person who is already losing muscle or struggling with appetite.

Healthy aging is not just about weighing less.

It is also about staying strong, capable, nourished, and independent.

What About Protein?

Protein deserves special attention as we age.

When someone loses weight, we don't want all of that weight loss to come from muscle.

Preserving lean mass becomes increasingly important for maintaining strength, mobility, balance, and independence.

That means a weight-loss plan for an older adult should consider more than calories or carbohydrates.

It should also consider whether the person is getting adequate protein and resistance exercise.

The exact amount of protein someone needs can vary based on age, body size, activity level, kidney function, medical conditions, and other factors.

That's another reason individualized nutrition matters.

What About Fiber?

This is another consideration when evaluating restrictive diets.

Many high-fiber foods—such as beans, lentils, whole grains, fruits, and some vegetables—are limited on strict ketogenic diets because they contain carbohydrates.

That doesn't automatically make keto unhealthy.

But it does mean someone following a very-low-carbohydrate approach needs to think carefully about where their fiber and micronutrients are coming from.

A diet shouldn't simply remove foods.

It should replace nutrients thoughtfully.

What About Kidney Health and Medications?

This is particularly important for older adults.

People over 55 are more likely to have existing health conditions and take medications.

A major dietary change can affect blood sugar, blood pressure, fluid balance, appetite, and other aspects of health.

Someone taking diabetes medication, for example, may need medical guidance when making significant dietary changes because blood glucose can change as carbohydrate intake and body weight change.

Likewise, people with kidney disease or other medical conditions may have specific nutritional considerations.

This is why “try keto” is not a complete piece of health advice.

The better question is:

“Would this approach be appropriate for me?”

Is Keto Good for Fatty Liver?

This new study provides an interesting answer—but only within its specific context.

Participants with fatty liver experienced greater reductions in liver fat on the very-low-carbohydrate ketogenic diet than on the other diets when weight loss was approximately matched.

That suggests that diet composition may influence liver metabolism beyond weight loss itself.

But it doesn't mean everyone with fatty liver should immediately start a ketogenic diet.

Fatty liver can have different causes and can occur alongside other medical conditions.

If you have been diagnosed with fatty liver, discuss your options with your healthcare professional rather than choosing a diet based solely on a headline.

So, Which Diet Is Better?

The honest answer is:

It depends on the person and the goal.

If the goal is improving certain metabolic outcomes in someone with obesity, prediabetes, and fatty liver, this trial suggests that a very-low-carbohydrate ketogenic diet may offer advantages over the Mediterranean and very-low-fat plant-forward diets at a similar level of weight loss.

But if the question is:

“Which diet will help me live the longest?”

This study cannot answer that.

If the question is:

“Which diet will best protect my brain?”

This study cannot answer that either.

And if the question is:

“Which diet can I follow comfortably for the next 10 years while maintaining muscle, energy, social connection, and quality of life?”

Again, this trial wasn't designed to answer that.

That's why we need to be careful about turning one study into a universal diet recommendation.

The PrimeSpan Approach: Personalized Nutrition for Healthy Longevity

Instead of asking:

“What is the best diet?”

Try asking:

“What nutritional strategy is best for me?”

That question takes the conversation to a much more useful place.

Your ideal eating pattern may depend on:

Your metabolic health
Do you have prediabetes, diabetes, insulin resistance, or fatty liver?

Your cardiovascular health
What are your blood pressure and cholesterol levels?

Your muscle mass and strength
Are you maintaining the physical function you need?

Your medical conditions
Do you have kidney disease or another condition that affects your nutritional needs?

Your medications
Could a major dietary change affect your treatment?

Your preferences
Can you actually enjoy the foods you're eating?

Your lifestyle
Can you maintain the diet when traveling, eating with family, or attending social events?

Your ability to sustain it
Can you realistically follow this eating pattern months and years from now?

That is the foundation of personalized nutrition for healthy longevity.

You Don't Have to Choose a Diet Team

One of the biggest problems with nutrition culture is the idea that you have to pick a side.

Team Keto.

Team Mediterranean.

Team Low-Fat.

But your health doesn't need a team jersey.

You can learn from different dietary patterns without turning them into identities.

This study gives us useful information about how carbohydrate intake may affect metabolic health in a specific group of people.

The Mediterranean diet gives us a well-established pattern built around a wide variety of nutrient-dense foods.

Other approaches may work well for other people.

The goal isn't to win a diet argument.

The goal is to build a way of eating that supports your health.

What Should You Take Away From This Study?

If you remember only three things, remember these:

1. Weight loss itself matters.

All three diets produced meaningful metabolic improvements when participants achieved approximately 10% weight loss.

2. Diet composition can matter too.

The ketogenic diet produced larger improvements in several measures of liver and glucose metabolism in this particular population.

3. Keto has not been proven to be the best diet for longevity.

The study was designed to examine short-term cardiometabolic responses to weight loss in a specific group.

It did not measure lifespan, dementia, long-term cardiovascular events, disability, or decades-long adherence.

So don't let a headline make the decision for you.

Healthy Longevity Is Bigger Than a Diet

Food matters.

But healthy aging is not created by one diet alone.

It is shaped by the combination of nutrition, movement, sleep, strength, cardiovascular health, metabolic health, social connection, and the habits you can maintain over time.

That's why PrimeSpan approaches nutrition as part of a bigger healthy-longevity picture.

You don't need to find the perfect diet.

You need to understand what your body needs, what your health goals are, and what you can realistically sustain.

The best diet for longevity may not be the one that wins the internet debate.

It may be the one that helps you become healthier, stronger, and more capable—and that you can continue living with.

Because healthy aging isn't about following the most extreme plan.

It's about building a healthier life you can actually live.

Frequently Asked Questions

Is keto better than the Mediterranean diet?

A new randomized clinical trial found that a very-low-carbohydrate ketogenic diet produced greater improvements in several metabolic measures than Mediterranean and very-low-fat plant-forward diets in adults with obesity, prediabetes, and fatty liver disease who achieved approximately 10% weight loss. However, the study does not establish that keto is better for longevity or healthier for everyone.

Did keto cause more weight loss?

The study was designed to compare the metabolic effects of approximately 10% weight loss achieved through three different dietary patterns. The key finding was that the ketogenic diet produced additional improvements in several measures of liver and glucose metabolism even when weight loss was matched.

Is keto good for prediabetes?

This trial found greater improvements in several measures of glucose metabolism with the very-low-carbohydrate ketogenic diet. Approximately half of participants in the keto group no longer met the criteria for prediabetes after the intervention, compared with approximately 29% in the Mediterranean group and 7% in the very-low-fat plant-forward group.

However, these results came from a specific study population and should not be interpreted as a guarantee that keto will reverse prediabetes in everyone.

Is keto good for fatty liver?

In this trial, the ketogenic diet produced greater reductions in liver fat and greater improvements in liver insulin sensitivity than the comparison diets at approximately matched weight loss.

That does not mean keto is automatically the right treatment for every person with fatty liver. Individual medical circumstances should be considered.

Is the Mediterranean diet still healthy?

Yes. This study does not show that the Mediterranean diet is unhealthy or ineffective. Participants following the Mediterranean diet also experienced meaningful improvements with weight loss. The study simply found greater improvements in certain metabolic measures with the ketogenic diet in this particular population.

Is keto the best diet for longevity?

We don't know. This trial did not measure lifespan or long-term outcomes such as dementia, cardiovascular events, disability, or quality of life. Therefore, it cannot establish which diet is best for longevity.

What is the best diet after 55?

There is no single diet that is best for every adult over 55. Nutrition should take into account metabolic health, cardiovascular risk, muscle preservation, protein and fiber intake, kidney health, medications, medical conditions, personal preferences, and the ability to sustain the eating pattern.

A Note About This Research

This article discusses findings from a randomized clinical trial published in Cell Metabolism in August 2026. The study examined the cardiometabolic effects of approximately 10% weight loss achieved through three dietary patterns in adults with metabolically unhealthy obesity, including prediabetes and hepatic steatosis.

The findings should not be interpreted as proof that a ketogenic diet is the best diet for everyone, or that it improves longevity. The study examined specific metabolic outcomes in a specific population.

Anyone considering a major dietary change—particularly people with diabetes, kidney disease, cardiovascular disease, or those taking medications—should discuss their individual situation with a qualified healthcare professional.

Research Source

Petersen MC, Smith GI, Farabi SS, Palacios HH, Shankaran M, Hellerstein MK, Patterson BW, Klein S. Effect of diet macronutrient content on the cardiometabolic response to weight loss: A randomized clinical trial. Cell Metabolism. 2026. DOI: 10.1016/j.cmet.2026.07.020.