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  • Writer's pictureDaniel Rockman

Popular diets - what does science say?

Updated: Jan 6, 2022

Low fat, no-fat, full cream, low-carb, high-protein, paleolithic... I just want a diet that tastes like real diet.

With so many popular diets being advocated for online, it can be hard to know what really works and what is being sold like snake oil. With something as life-changing as diet - remember that diet affects everything from your mood and energy levels to your life expectancy - you want to know the truth.

Luckily for us, we have science to separate the snakes from the salad, so to speak. A review published in January 2020 looked at the current scientific evidence for popular diets. This study was better than just a dick measuring contest between each group of zealots, the study summarises each diet, the alleged weight loss mechanisms and the effectiveness of each one based on the best scientific studies we currently have.

The diets were broken down into 3 categories:

1. Diets based on the manipulation of macronutrient content (i.e., low-fat, high-protein, and low-carbohydrate diets).

2. Diets based on the restriction of specific foods or food groups (i.e., gluten-free, Paleo, vegetarian, vegan, and Mediterranean diets).

3. Diets based on the manipulation of timing (i.e., fasting).

Macronutrient based diets

The manipulation of macronutrient intake is intended to aid weight loss, alter metabolic pathways and change the gut microbiome to reduce fat storage.

Low fat diets appear to be more beneficial for fat loss than low carbohydrate diets when protein and calories are equal. A low carb diet may be better for those with pre-diabetes or diabetes type 2 because carbs are believed to stimulate the release of insulin.

The ketogenic diet (low carb, high fat) has been shown to promote weight loss and reduce appetite. There were, however, commonly reported side effects such as constipation, headaches, muscle cramps and weakness. This diet may also not help or even worsen lipid profiles (like cholesterol).

In the long term, both high-carb and low-carb diets have an increase in mortality. The least risk is observed when carbs make up around 50% of your total calorie intake. Or in other words, being an extremist one way or the other is associated with a higher risk of dying than striking a moderate balance.

Finally, high-protein diets, in which ≥20% of energy is derived from protein, appear to offer advantages regarding weight loss and body composition in the short term. High-protein diets increase satiety and energy expenditure, leading to weight loss. Unfortunately, this effect doesn't appear in diets lasting over a year and is associated with higher intakes of saturated fats raising LDL levels (the bad fats).

The typical macronutrient content of popular diets.

So, to summarise the macronutrient-based diets: High-protein, low-carb diets are suggested for their short-term benefits to weight loss. The study recommends these should be considered more of a "jumpstart" diet rather than a diet for life. In the long term, it appears that the type of macronutrients don't seem to matter as much as the total number of calories consumed.

Food group restriction diets

Above is a summary of the food groups allowed in diets from the restriction category

Plant based diets

These have been associated with protection against chronic diseases, such as CVDs, hypertension, type 2 diabetes, and some cancers, but it is not clear if this is due to a reduction of animal products or an increase in fruits and vegetables. They can also lead to nutritional deficiencies in protein, iron, zinc, calcium, and vitamins D and B12 due to the limited intake of animal products.

Plant based diets have been shown to reduce body weight, likely due to the low calorie density nature of the commonly eaten foods. Long term studies are required as some studies showed no weight loss. Adopting an effective plant based diet requires counselling and nutritional supplementation.

Paleolithic diets

Paleo is based on everyday foods that mimic the food groups of our pre-agricultural, hunter–gatherer ancestors, including meat, nuts, eggs, healthy oils, fresh fruits and vegetables. The diet claims to help optimise health, minimise risks for chronic disease, and result in weight loss.

The diet is high in protein (20–35% of energy) and moderate in fat and carbohydrates (22–40% of energy). The paleo diet features a higher content of unsaturated fatty acids, antioxidants, fibre, vitamins, and phytochemicals as well as a lower sodium content that works together to promote health benefits. There are multiple beneficial metabolic effects which are well studied.

Consistent weight loss has been found in both the short-term and the long-term, however, low adherence, poor palatability and high costs are commonly reported drawbacks. More research is required to support some of the popular claims and there is a potential deficiency risk that includes vitamin D, calcium, and iodine.

Gluten-free for weight loss

Gluten is a protein found in cereals such as wheat, rye, barley, and oats. Studies have shown that gluten cannot be completely digested, triggering an intestinal inflammatory response in susceptible individuals.

It is well established that for those with celiac disease, wheat allergy and non-celiac gluten sensitivity, avoiding gluten is a valuable treatment, however, there is not much evidence on gluten-free diets for weight loss, despite the growing market of gluten-free products. Some studies suggest that gluten may stimulate unfavourable metabolic pathways, but the few studies performed don't show a significant weight loss effect either way.

Mediterranean diet

The Mediterranean diet is a balanced diet characterised by high consumption of vegetables, fruits, legumes, whole-grain cereals, seafood, olive oil, and nuts. Red meat, dairy and alcohol are recommended in moderation. The Mediterranean diet is rich in plant-based foods, having high levels of antioxidants and dietary fibre, and low glycemic load compared with other diets.

The Mediterranean diets association with weight loss is similar to other diets in both the short and long-term. The Mediterranean diet has shown a reduction of inflammatory markers cardiovascular risk factors (including insulin resistance in type 2 diabetics) and mortality. Of note is that these health benefits were present even without weight loss. The Mediterranean diet has been considered a healthy eating pattern due to the high nutritional quality of its food composition.

Diets based on food timing

Intermittent fasting (IF)

IF consists of abstaining from food and caloric beverages for a certain period of time, alternated with normal eating. Several variations of IF differ in length and frequency of the fasting cycles. IF can also be combined with other diets.

The idea of IF is to improve major physiological health indicators including greater insulin sensitivity, reduced blood pressure, body fat, glucose, atherogenic lipids, and inflammation. Studies in animals have found the potential to delay aging by up to 30%, although these results remain controversial.

After 12-24h of fasting, the body switches to a "ketogenic metabolic mode" using non-hepatic glucose, fat-derived ketone bodies, and free fatty acids as energy sources. This means the carbs which were stored in the liver have all been used up, and the body is now burning the much slower source of energy, fat in the body, as well as a small storage of carbs in the muscles.

The average reported weight loss in cohorts with overweight and obesity has ranged between ∼4% and 10% over dieting periods of 4 to 24 weeks. However, these studies often didn't have a control group to compare against, so it is hard to decide if these results were due to the altered metabolic pathways from fasting or just a calorie restriction.

Ramadan has been studied frequently, a month in which healthy Muslims fast for 12-16h per day. The results on weight loss showed some positive and some negative results, as well as showing that after a few weeks post-fasting period, the weight was often regained.

In summary: while in rodents there is a significant bout of evidence for the benefits of IF, humans require further investigation. Furthermore, it has been suggested that IF does not produce superior weight loss in comparison with continuous calorie restriction plans and that hunger can lead to overeating. Adverse effects of fasting include fatigue, weakness, and headaches. It is also important to emphasise that fasting diets might be harmful to specific populations such as children, the elderly, and underweight individuals.

It is important to remember that dieting isn't just a biochemical process, it is strongly influenced by human behaviour and environmental factors. To learn more on the factors that influence success of a weight loss diet, check out my article here (coming next week).

In conclusion

Popular diets often stem from personal impressions and published books as opposed to scientific research, so studies like these are important to further our understanding of how what we eat affects our body. These diets promote varying degrees of weight loss with small differences, however, show underwhelming results after a significant amount of time has passed.

In the short term, a low carb, high protein diet appears to be a successful jump-start weight loss strategy. Long term, the Mediterranean diet's high quality foods appears to be a healthy, successful strategy.

There is no one size fits all diet which will be successful for everyone. Anyone who claims to have found "the secret to activating our ancestor's long lost starvation mode" or "the perfect diet" is probably trying to sell you something.

Personally, I suggest trying out these diets for yourself, then take what works and throw away the rest. When going for weight loss, it is important to lose weight quickly as this boosts motivation so try going for intense bursts of dieting interspersed with periods of maintaining that weight loss. Attempting any of these diets will also make you more mindful of the foods that you are regularly eating.

To read more on the factors that affect the success of a diet, click here

For a more in-depth description of everything I've written here, check out the published journal article here.

Stay strong.

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