A Dietitian’s Perspective on Popular Diets: Keto, Intermittent Fasting and More

It seems like pretty much every few years there’s another popular diet that’s tried by the masses. Keto, intermittent fasting and believe it or not, even eating a vegan diet was considered a trend few years ago. So, what’s the deal with all of these diets – are they healthy? Worth the hype? And should you try them?

I’m going to break down some of the top most popular diets and what to know about them: what they are, what the research says, and my opinion on them as a dietitian. But, before I do that it’s important for me to give this disclaimer:

Everyone is different and has different needs. If you, or someone you know, tried any of these diets and had a different experience than what you read here, then that’s great. You know your own body and mindset best. Please keep in mind that this is only general perspective and advice. 

Also, this perspective is intended from a general nutrition standpoint and does not (and cannot possibly) address different medical conditions. While I try to touch on certain medical conditions that these diets are indicated for, I’m sure there are specific situations that will still be left out. So, please keep in mind that if you have been personally told by a licensed healthcare provider (not Instagram or not your cousin’s boyfriend’s friend) to try any of these diets for a specific health issue, then again, they/you know your body best.

That said, let’s get to it!

Let’s be real: I could dedicate an entire blog post to each of these diets. There’s just so much to say. But, in efforts to keep it concise (let’s be real, who wants to read that much?!) and keep it in all in one place, I thought it would be easiest to break down and summarize each of the diets in the same blog post.

So, there’s a lot more that could be said. This is really just the tip of the iceberg. But, IMHO, this is a great summary of the most up to date, evidenced-based research, coupled with my opinion and experience as an RD in private practice.

And lastly, let me say this: despite the fact that I’m an all foods fit dietitian and clearly not a fan of anything that adds rules to what you eat, I tried to look at this in the most open minded way possible. We’re all different and have different needs, and the research definitely reflects that. So I promise, unless I specifically state that it’s my opinion, consider this your unbiased review. 

The ketogenic diet

What it is:

In short, the ketogenic diet is a very low carb, very high fat diet. It focuses on eating about 20-50 grams of carbohydrates per day (which, again, is very low).

The diet is typically made up of about 75% fat, 20% protein and 5% carbohydrates. Compare this with the AMDR recommendations, which are: 20-35% fat, 10-35% protein and 45-65% carbohydrates.

In a 2,000 calorie diet, that would equal about 44-77 grams of fat and 225-325 grams of carbs. On a keto diet, that’s about 166 grams of fat and 25 grams of carbs. Drastic difference!

Quick refreshers: carbs are 1 of 3 macronutrients found in virtually everything we eat, but found mostly in grains, fruit and dairy (read more about carbs here!).

The idea of a ketogenic diet is that instead of using glycogen (stored carbs) for energy, the body will instead use ketones (stored fat).

What the research says:

The keto diet was originally developed for drug-resistant epilepsy in the 1920’s. While it became popular again over the past few years, it’s still considered the recommended medical nutrition therapy in treating seizures. Additionally, studies show that keto may be beneficial for those with type 2 diabetes, but research on this is limited.

Other than epilepsy, no benefits have been found. In fact, most studies show that the keto diet simply doesn’t provide enough carbs for our brains to function, which need about 130 grams per day. Finally, most studies have found it difficult to maintain long term.

My take:

We need carbs to function. Our brain can manage if it has to, but our red blood cells still need those 130 grams of carbs to function properly. All in all, I’m not a fan of any diet that drastically reduces or eliminates any food group. From a pure mindset perspective, carbs are needed with meals to provide satisfaction.

If you’ve been following me for a while you likely know this, and if not, you’ll see this theme often: I believe that our diets should be realistic and enjoyable in order to be sustainable long term. Unless the keto diet is medically indicated (epilepsy or possibly type 2 diabetes), I don’t think it’s a sustainable, realistic or most importantly, enjoyable, diet long term. 

Intermittent fasting

What it is:

There a few different methods of intermittent fasting but, in a nutshell, the diet consists of fasting (going without food or eating less than 500 calories) for entire days or periods of time. 

The 4 main types:

Time restricted eating: fast for 14-16 hours each day

5:2 diet: fast for 2 full days per week

Eat-Stop-Eat: complete a full 24-hour fast once or twice a wee

Alternate-day fasting: modified fasting (around 500 calories) every other day

What the research says:

I was actually surprised to see that the research does show possible benefits on heart health, blood sugar and anti-aging. But, here’s the thing: the long term research on IF is extremely limited. In fact, most research has only been done on mice. And it’s hard to translate research on mice to humans.

Research has found for the most part, that IF – no matter how it’s done –  is not sustainable and difficult to stay on long term. Plus, it’s been shown to disrupt our metabolism. Remember, metabolism equals the breakdown of food in the body. So, it makes sense that if we go long periods of time without eating and not having food to break down, our metabolism gets disrupted. 

My take:

In general, I’m not a fan. If more studies showed conclusive, positive benefits, then maybe I’d consider it. But factually, most positive benefits of IF are unfortunately just anecdotal.

But there is one caveat: I’ve heard from many people that time restricted eating has helped them to stop mindless snacking at night. This is great, and if that works for you, awesome! But, as part of my work in helping clients create a healthy relationship with food, I like to teach them how to prevent mindless snacking overall – instead of simply replacing it with another rule.

I’m also not a fan of anything that tells you when to eat. Instead of eating based on your hunger, IF can lead you to eat based on your IF clock and teach you ignore your hunger cues fully. Plus, being overly hungry makes it easy to overeat the next time you do eat. And, even if you get used to the hunger, it means living life based on your IF clock. What happens if you get invited to a dinner outside of your IF eating window?

I don’t think IF is sustainable. Going long periods of time without eating and having to base your meals off your IF clock just seems completely unrealistic to me. Remember, I believe that in order to be sustainable, a diet must be both realistic and enjoyable. To me, IF is neither of these things.

Vegan diet

What it is:

There are many types of vegan and vegetarian diets. I’m not at all suggesting they’re all the same – they’re in fact very different – but in order to share as much information as possible, I’ll lump them together here and explain the differences

Vegan: excludes all animal products – no meat, fish, eggs or dairy. In some cases, no honey

Vegetarian: excludes all animal protein; includes eggs and dairy

Lacto-vegetarian: excludes meat, fish and eggs; includes dairy

Ovo-vegetarian: excludes meat, fish and dairy; includes eggs

Lacto-ovo vegetarian: excludes meat and fish; includes dairy and eggs

Pescatarian: excludes meat, dairy and eggs; includes fish

FWIW: there’s no reason to memorize or strictly classify yourself by any of these labels if you don’t want to. I just wanted to show how many different ways there are to follow a vegetarian diet – it’s definitely not one-size-fits-all.

What the research says:

Again, vegan and vegetarian diets are different. I don’t mean to lump them into the same category and totally appreciate that they’re not the same But, for brevity’s sake, here’s what the research shows.

Both vegan and vegetarian diets are helpful for the environment. They help to reduce waste, run-off and pollution. Plus, the obvious: they’re ethical and prevent animal cruelty.

Because these diets include nutrient-dense foods like vegetables, fruit and whole grains, they often lead to eating more complex carbs, fiber and antioxidants. And, a diet rich in complex carbs, fiber and antioxidants has been proven time and time again to decrease the risk of many diseases.

So in general, these can be super healthy diets. But, here’s what’s important to note: while both vegan and vegetarian diets can be done in healthy ways, they are not inherently healthier than a non-vegan or vegetarian diet. Basically, your diet can be comprised of mostly cookies whether you eat a vegan diet, vegetarian diet or neither. Sounds fun, but not the most nutrient-dense.

Finally, the one downside research has found is that these diets may lead to a higher risk of vitamin B12 deficiency, since vitamin B12 is found mostly in animal foods.

My take:

I 100% support following a vegan or vegetarian diet for the ethical and/or environmental reasons above. In fact, I respect it a lot. If you’re following either of these diet for those reasons, then great. As long as you still focus on getting on a variety of foods and food groups in your diet, they can both be super healthy lifestyles to live. 

(And, there are still many ways to ensure adequate protein on either diet – check out my list of the top plant-based proteins here!)

But, my one caution is this: please make sure you’re following these diets for the right reasons. As a dietitian, I’ve seen many clients who transition from an eating disorder to a vegan diet because it offers a less conspicuous set of rules to follow (spoiler: that is NOT what these diets are for).

If you’re following either of these diets for restriction or weight loss purposes, I’d highly caution against that. If you genuinely enjoy eating foods like meat, dairy or eggs, it is not in your best interest to place unnecessary rules and restrictions on what you can and can’t eat. 

Remember: vegan, vegetarian and non-vegan and vegetarian diets can all be made as nutritious or not-nutritious as you want. You have my full support if you choose to follow them for ethical and environmental reasons, but, any diet can be made as nutritious or not as you want it to be.

Whole30

What it is:

Whole30 is a diet that’s intended for 30 days. It’s said to emphasize whole foods and eliminates sugar, alcohol, grains, dairy, legumes and soy. It’s somewhat similar to the paleo diet but, unlike paleo, Whole30 does not even allow sweeteners like honey or maple syrup.

A breakdown of foods not allowed on Whole30:

• Sugar, including maple syrup, honey, agave, coconut sugar, date syrup, monk fruit, stevia and any artificial sweeteners or sugar alcohols.
• Grains of all kinds, includ
  ing wheat, rye, barleys, oats, corn, rice, bulgur, sprouted grains, quinoa, amaranth and buckwheat.
• Legumes, including all beans, peanuts, and anything with soy (including soy lecithin in the ingredient list).
• Dairy, including cow, goat or sheep’s milk products like milk, cheese, yogurt, sour cream and ice cream.
• Alcohol, not even when used for cooking.
• Carageenan, MSG and sulfites.
• Baked goods with approved ingredients. From the Whole30 website: “these are the same foods that got you into health-and-craving trouble in the first place—and a pancake is still a pancake, even if it’s made with coconut flour.”

What the research says:

There actually really isn’t much research about Whole30. The only thing I was able to find was a quote from the founders who claim that the majority of participants lose weight during those 30 days.

Because Whole30 is similar to the paleo diet, and there is research on the paleo diet, I can compare it to that:

Research has shown that the paleo diet may lead to eating more whole foods overall, which can benefit our health and microbiome. Unsurprisingly though, research has also not found it beneficial to exclude nutritious foods like grains and pulses – both of which contain fiber and essential micronutrients.

Finally, unsurprisingly, research has found the paleo diet difficult to maintain long term. Because Whole30 is even more restrictive than paleo, I would assume this means Whole30 is also difficult to maintain long term (although of note, it is only intended for 30 days).

My take:

I respect the fact that this diet is only intended for 30 days. I was once talking with one of the founders, Melissa Hartwig, about it, and even she claimed she couldn’t follow this diet for more than 30 days! I think it’s often falsely assumed that Whole30 is a diet to attempt to stay on for life.

That said, I just can’t get behind a diet that’s so restrictive. This diet excludes even super nutritious foods like whole grains, beans and soy. Yes, you may temporarily “feel better” while you experience less bloat (higher fiber foods can often cause bloat as they take longer to digest – this is not a bad thing), but overall, there is no benefit to excluding these nutritious foods.

On top of the fact that this diet excludes some super healthy food, it also just doesn’t sound fun. I can’t imagine needing to so strictly read labels and carefully eat – even for just 30 days. Do I think “feeling good” for 30 days is worth missing out on eating with friends, social events, pizza, cookies, pasta and enjoying life? Absolutely not. Plus, what happens when those 30 days are up?

Weight Watchers

What it is:

Weight Watchers – now known as WW – has been around for a long time, with periodic updates. Right now, when you join, you complete a personal assessment that learns about your eating habits and lifestyle. Then, based on your answers, you’re matched with a color that helps guide the correct food choices for your goals. As always, WW is a system that uses points to assess food.

When Weight Watchers rebranded to WW, it also tried to take some of the focus off of weight loss. Likely, WW realized that there was a big shift in the wellness world to focus more on overall health and less on weight loss and it followed suite.

According to its website, WW now assigns every food and beverage a point value with its SmartPoints system. For support, participants are able to access the app, in-person workshops, or help via chat or phone.

What the research says:

In general, research has shown that participants do lose weight on Weight Watchers (because the rebrand occurred in late 2019, most research has been done on Weight Watchers and not on WW). Associated with this weight loss is often lowered total cholesterol and increased HDL (or good) cholesterol.

But, and it’s a big but: the weight loss is typically short term. In fact, most research has not shown sustained weight loss past 12 months.

This is in line with research that shows that diets don’t work in the long run. Why? Put simply, because anything that has you track, log or count what you eat has been shown to not be sustainable long term. 

My take on it:

Despite very clearly being a diet, I’ll be honest with you: I’m not 100% anti WW. It’s cost-effective and studies have proven it to be cost-effective in populations that need it. While there’s a lot I don’t agree with about WW, I think it’s unfair – and even elitist – to simply say no to it. Many people and populations don’t have access to individual dietitians or nutrition education. If WW can help teach what a balanced diet looks like to those populations, then I support it.

That said, I would not recommend it to anyone struggling or who’s struggled with their relationship with food (which is the population I personally work with). The WW system inherently labels foods as good or bad – a concept that I adamantly do not support. Plus, the research shows it doesn’t even “work” long term.

So, I’m not totally anti WW, but for my private practice and audience on social media in general, it’s a no from me. I’d rather you learn healthy eating and food choices based on your internal cues versus earning them via an app, tracking and labeling. It’s not sustainable to have to track your food long term, it’s definitely not enjoyable, and I don’t know about you, but I’d much prefer to just eat the damn pizza than first have to see how many points it is.

All in all, again, the important thing to remember is that there’s truly no one size fits all. While I don’t believe in restrictive or short term diets, for almost each of these diets, I’ve identified a population that they might work for. We’re all different and we all have different needs.

That said, you’ve likely noticed a common them in my take on these diets: anything that causes you to label foods, restrict foods or entire food groups, or choose what and when to eat based on the diet’s “rules” is not something I support.

The main thing that a diet like that does is distract from listening to your own body, hunger and fullness cues, and cravings. And, in my experience as a dietitian, learning to listen to your body in that way is the most satisfying, enjoyable and ultimately, sustainable way that you can eat.