Are diets worth the hype? A scientific review

Nutrition is an extremely complex subject and the goal of this review is not to not determine the “best” diet, but rather pull upon the current data of various diet types and draw common themes that, no matter what your eating habits are, you can apply.

One thing to be clear going forward, there is no perfect diet. In fact, one should first consider their lifestyle and pick dietary solutions to best match their individual needs and circumstances. This is for two reasons.

Firstly, to a large extent, nutrition is behaviour based. Despite this, many people first start by thinking about which diet is “best” and build their plan around that. Even though there are some key themes that will be touched on in this review that may benefit you, the main idea should be to reverse engineer instead the process by asking this question first: What are my current eating habits and lifestyle and what diet strategy can best match that?

Secondly, as you will see in the various diets, except for a few exceptions, all diet structures are effective in producing weight loss. Once this is established, you can best match the diet structure that best fits your needs.

In this review, we will be talking about the following diet types and their relevant changes and maintenance to body composition:

• Low-fat diets

• Low carbohydrate diets

• Ketogenic diets

• High-protein diets

• Intermittent fasting

Low-fat diets

Definition

A low-fat diet is defined as having 20-35% of the diet consisting of fats.

Consideration

This diet structure has the support of major health organizations due to the large evidence base, flexible macronutrients (macros) and does not limit certain foods (1). Fat is the most dense macronutrient providing the most amount of energy per gram of fat at 9 calories per gram. For reference, protein and carbohydrates have less than half that content, both being 4 calories per gram. Therefore, the logic behind the low-fat diet is twofold:

1. limiting the densest macronutrient will reduce the likelihood of overconsuming calories, and

2. because fat is limited, both carbohydrates and protein need to be increased.

A common theme in this review is that increased levels of protein, rather than the “diet” itself, is a primary driver of weight loss success.

To see the effectiveness of low-fat diets, a recent review analyzed 32 studies amounting to 54,000 subjects with a minimum duration of 6 months that reduced fat content but kept the other macronutrients similar and demonstrated a modest but consistent reduction in body weight, body fat and waist circumference (1). This demonstrates that simply decreasing fat content has a positive effect on weight loss due to less total energy intake being consumed.

​Key Point

A common theme in this review is that increased levels of protein, rather than the “diet” itself, is a primary driver of weight loss success.

Low carbohydrate diets

Definition

Diets with under 45% of total carbohydrates technically are classified as a low carbohydrate diets. This is because the acceptable macronutrient distribution range (AMDR) is between 45-65%. However, a better definition of having no more than 40% of the diet coming from carbohydrates is classified as low carb.

Consideration

Success is largely attributed to the higher protein intake as well as flexibility in food choices as this form of diet does not prohibit foods based on fat content (1).

Most studies examining this form of diet are on obese or overweight subjects. This is important because this population has a higher amount of energy to utilize which means

1. they can lose weight faster, and

2. they spare more muscle.

Analyses comparing low-carb to a control group showed small increases in favouring low-carb diets (1). Again, one should consider if this particular diet can work long term first, rather than focusing on the specifics.

Ketogenic diets

Definition

Ketogenic diets are a variation of low-carb diets where total carbohydrates are limited to either 50g or 10% of carbohydrates per day while keeping protein moderate and relying on fat to get most of the calories (1). Given the diet's very limited carbohydrate intake, ketones form as a result of amino acids being broken down into fuel. The ketones themselves are harmless and, contrary to popular belief, do not aid in satiety or diet success.

Considerations

Although this diet produces an altered state known as ketoacidosis, this process is harmless to the body and in regards to weight loss the diet itself performs similarly to others when calories consumed are matched (1). This goes against the common belief that ketosis plays a role itself in weight loss. The research does not support this claim and instead attributes the diet to its effectiveness in promoting a higher protein intake.

When comparing keto diets to non-keto diets, when all conditions were controlled for (calorie intake, exercise regimen, etc.) keto diets do not show a fat loss advantage. In fact, a longitudinal study comparing keto to high-carb diets found that at 6 months, the keto diet had a slight weight loss advantage, but at 12 months, the high-carb diet was shown to be superior largely because it is easier to adhere to (1).

High protein diets

Definition

Generally defined as exceeding 25% of total calories per day, high protein diets can be further classified as consuming between 1.2-1.6g/kg of body weight per day (1). For reference, the recommended daily allowance of protein, or RDA, is 0.8 g/kg per day.

A large amount of literature examining high protein intakes has drawn a relative consensus that eating double the RDA of protein consistently outperformed controls eating the RDA.

Considerations

The subject of protein and its impacts on body composition requires its own blog post, which will come out in a future post, but some key points will be talked about here.

Firstly, there does appear to be an upper limit where protein no longer provides additional significant benefits. Specifically, consuming triple the RDA (2.4g/kg per day) did not preserve lean mass to a significantly greater extent.

Secondly, a one-year crossover trial using resistance-trained subjects consuming either 3.3 or 2.5g/kg per day demonstrated no significant differences in body changes (1). This study also shows that eating a long-term, high-protein diet did not cause any adverse health effects as measured with clinical markers including a complete metabolic panel and blood lipid profile (1).

Intermittent fasting

Definition

Intermittent is an umbrella term categorizing three primary diet structures; alternate-day fasting, whole-day fasting and time-restricted feeding.

Considerations

Alternate day fasting

This is a 24-hour period of fasting, followed by a 24-hour feeding period. The rationale for this diet to work is that eating on the feeding days was often not enough to make up for the 24-hour fast and therefore produced a calorie deficit (1). Interestingly, this diet slightly outperformed daily calorie restriction after 6 months of unsupervised weight loss (1).

Whole-day fasting

Involves only a couple 24-hour fasts throughout the week, with the other days being at set at the individual’s maintenance calories. It’s important to understand that although this method has also been cited as effective for weight loss when compared with diets consuming the same amount of calories over 6 months, there was no difference between groups (1).

Time-restricted feeding

A form of intermittent fasting where a specific daily feeding window is given in the day. For example, a common window is between 3-8pm, but anytime can be given. Outside of this time, the individual fasts.

One of the largest systematic reviews ever published comparing time-restricted feeding to continuous energy restriction found that both diets result in relatively the same outcome in terms of body composition (1). In this regard, intermittent fasting does not result in better weight loss results versus traditional calorie restriction, debunking several myths about its touted “superiority”.

Takeaways

I hope you got a lot out of this blog as it set out to answer most major questions one would have about different diets. It also debunks some common myths, particularly surrounding ketoenergic and low carbohydrate diets, where they claim their superiority comes from its metabolic effects. This leads to two major themes I want you to take away.

Firstly, no matter what eating habits you follow, understand that it’s not the diet itself that is special but that it produces a sustained calorie deficit that you can incorporate into your lifestyle.

Secondly, high protein content was consistently shown to improve body composition both for muscle growth and fat loss, regardless of the diet. This is important to understand because if protein content is high enough, a myriad of nutrition plans can be followed to great effect!

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References

1. Aragon, A. A., Schoenfeld, B. J., Wildman, R., Kleiner, S., VanDusseldorp, T., Taylor, L., ... & Antonio, J. (2017). International society of sports nutrition position stand: diets and body composition. Journal of the International Society of Sports Nutrition, 14(1), 16. https://doi.org/10.1186/s12970-017-0174-y