The best and worst diets of 2021 - as ranked by leading experts

Dr Nick Fuller
Leading Obesity Expert at the University of Sydney and founder of Interval Weight Loss.

“Going keto” and fasting might be popular, but mass adoration hasn’t saved the two diets from being condemned as the worst diets of the year.  

An annual evaluation of the most popular food regimes in the world, which favours diets with scientific evidence that are nutritionally balanced, has again condemned the Keto diet and the 5:2 regimes as the worst for your health.

Keto refers to eating fat and restricting carbs, while fasting involve cutting calories for a set period.

Science has proven that dieting contributes to weight gain and starting your new year with another diet will see you worse off in the long-term, not only from the weight regain you will experience, but also the serious psychological ramifications that come from dieting.

The five worst diets of 2021

1. Optavia

The Optavia diet is owned by meal replacement company Medifast, and prescribes pre-packaged products including shakes, bars, soups to help you achieve a significant calorie deficit by eating about 800 calories/day (compared to the Australian Dietary Guidelines which prescribes around 2,000 calories/day) and resulting short-term weight loss.

There is no question that a restrictive eating plan will result in short-term weight loss and other short-term health improvements. But eventually your body will fight the weight loss by shutting down – your metabolism will lower and your appetite will increase, so you regain the weight you lost.

Take-home message – Meal replacements are a short-term fix to your problem but they will not help you long-term and may even result in more difficulty losing weight down the track. 

2. The Keto Diet (and the Modified Keto Diet)

This high-fat, low-carb diet is designed to make your body enter a state called ketosis where it’s relying on fat rather than carbohydrates for energy.The increase in blood ketones (an alternative fuel source when blood sugar stores are depleted) is purported to suppress your appetite, leading you to eat less and thus weigh less. 

Sounds good, but studies have shown this does not work. 

The restrictive carb allowance equates to just one piece of fruit a day - radically below the recommended dietary intake – which means as well as not working, it is difficult to sustain. There are also serious long-term health implications of following such a high fat, low carb diet. While there's no evidence on keto specifically yet, we know that giving up fruits and wholegrain carbohydrates is the opposite of what we should be doing to prevent lifestyle disease such as cancer, type 2 diabetes and heart disease.

Take-home message – Kim Kardashian and Gwyneth Paltrow have certainly been a contributing factor to the popularity of the keto diet but it’s certainly one you should be avoiding in 2021.

3. The Fast Diet

Also known as the 5:2 diet, this involves eating normally for five days of the week then cutting your calories by 75% for two days of the week. This equates to just 600 calories for males on the two fast days and 500 calories for females – the same number of calories as two donuts. The lack of nutritional guidance on what to eat on the 5 days when you aren’t fasting and what constitutes a healthy eating pattern raises many red flags.

Take-home message – It’s an alternative way to cut calories in your diet if you follow it strictly but the science doesn’t prove it is any better than reduced calorie intake through conventional dieting. You will end up back where you started.

4. Paleo Diet

Advocates of the Paleo diet say we should be eating a diet based on the foods that were available during the Paleolithic period of more than 10,000 years ago. It requires you to cut out entire food groups like dairy and grains, not only making it an unrealistic diet to sustain but also dangerous and unhealthy due to the key nutrients you are missing. You will experience short-term and rapid weight loss due to a decrease in body water - just like with any diet that is low in carbohydrate. But there is no research to show it can deliver sustainable weight loss.

Take-home message – This is not a nutritionally balanced diet and those advocating we stay away from grains are simply wrong. History has proven that we were eating bread and grains more than 10,000 years ago.

5. AIP Diet

Also known as the Autoimmune Protocol Diet, this eating plan targets those with autoimmune disorders – for example, rheumatoid arthritis, psoriasis, inflammatory bowel disease - by eliminating foods that cause inflammation or irritate the gut. It requires you to eliminate foods for a month and reintroduce them one at a time to establish which ones might be problematic.

There’s a big focus on including organ meats like liver, grass-fed meat, bone broth and seafood, and an avoidance of veggies like tomatoes and potatoes, as well as eggs, legumes, nuts and seeds. Many of its advocates are Paleo enthusiasts.

Because you’re required to eliminate many food staples and entire food groups, not only is it restrictive and difficult to sustain, but also challenging to ensure you are meeting all your nutritional requirements.

Take-home message – If you suffer from an auto-immune condition, consult with an accredited healthcare professional, such as a dietitian.