While nutrition has made significant strides in recent years, such as recognizing the harm trans fats cause, it’s surprising that even within the academic field there’s still debate about the most efficient diet. Spoiler alert: any of them can work, as long as a diet suits your preferences and provides more consistency.
In other words:
“Do we need diet books and programs that tell us to eat fruits and vegetables, consume refined sugar and alcohol in moderation, or nutrition coaches who scare their social media audiences with lists of allowed and forbidden foods?”
– What nonsense, pseudoscience and other absurdities [1]
The answer to both questions is, of course, a resounding no. However, this hasn’t stopped a growing number of self-proclaimed health experts from pointing the way — often with a link to buy their consultancy or products — to healthy eating.
In recent years, we have increasingly seen claims on social media such as that the low-carb diet is ideal for weight loss, that intermittent fasting is the best way to live long, that the carnivore diet can cure psychiatric disorders, and that the vegan diet can save the planet and even relieve migraines.
While I would love to go into the details behind each of these claims, the focus of this article is primarily on a not-so-new industry trend: diets based on your genetics, derived from DNA testing. In short, nutrigenetics
How should these tests work?
Typically, the consumer orders the test online, collects a saliva sample, and sends it to the company. The company extracts DNA from the sample and analyzes it for genetic variants associated with increased risk of disease, regulation of specific traits, or health conditions. Some of the most common services offered by these companies include genetic kits related to physical fitness (such as athletic performance and injury risk), pharmacogenetics (personalized drug treatment), and nutrigenetics (weight management, food intolerances and sensitivities).
According to a judgement The nutrigenetic tests published in 2020 are intended to serve as a compass, helping users make informed and healthier decisions.
“Standard dietary guidelines are based on the average population and are used to prevent deficiencies, not to optimize personal fitness levels, health and well-being. You are unique, your diet should be too.”
– NutriGenetix
However, this claim blurs the line between science and marketing.
For example, NutriGenetixA company that markets these tests and develops personalized food shakes based on the genetic results—what I would call a double win—claims that its tests are based on nutrigenetics. This emerging field studies how genetics influence the processing and metabolism of various nutrients.
They promise that their test results:
- Discover your ability to use, process and absorb different nutrients (such as caffeine and sodium) based on your genetic variations.
- Please indicate your specific needs for different micronutrients (vitamins and minerals).
- Assess how many endogenous antioxidants you can produce.
- Determine which macronutrient (fat, protein, or carbohydrate) is likely causing weight gain.
- Detecting food allergies.
There is some truth to these claims, at least for caffeine, the metabolism of which is primarily influenced by an enzyme called cytochrome P-450. Genetic variations in this enzyme can alter its activity, speeding up or slowing down metabolism. A systematic review published in Nutrients investigated how single nucleotide polymorphisms (SNPs) [2] impact on habitual caffeine use and the ergogenic and anxiogenic effects of caffeine. The review found strong evidence that SNPs in the three genes are associated with habitual caffeine use.
Although there is substantial evidence on how genetic variability affects caffeine metabolism, this does not necessarily mean that other substances are similarly affected by genetic polymorphisms associated with their metabolism. By assuming that genetics are the basis for personalized nutrition, these nutrigenetic tests often fail to distinguish between well-established research and preliminary studies that require replication and quality evaluation. Nutrigenetic testing sales often make hasty generalizations, draw conclusions from a small or unrepresentative sample, and make false analogies where two situations share superficial similarities but have significant and relevant differences.
What is the evidence for weight loss?
To answer this question, a randomized clinical trial is needed published in Nature communication tested the following hypothesis: Would participants who were fed a diet matched to their genotype (fat- or carbohydrate-responsive) lose more weight over 12 weeks than those who were fed a diet that did not match their genotype?
Participants were between 18 and 75 years old, non-smokers, overweight (BMI between 27 and 45 kg/m²) and free of conditions or medications that affect body weight or metabolism. They were stratified by genetic predisposition based on the characterization of 10 SNPs, favoring diets rich in carbohydrates or fats.
Of the 145 participants recruited and randomized, 16 were lost to follow-up, and 7 were excluded due to genotyping errors or missing weight data. Therefore, the final analysis included 122 participants, who were randomly assigned to a high-fat or high-carbohydrate diet and divided into four analysis groups:
- People who react to fat are given a high-fat diet
- People who respond to fat and are given a high-carbohydrate diet
- Carbohydrate respondents fed a high-fat diet
- Carbohydrate respondents fed a high carbohydrate diet
The average age was 54.4, with a BMI of 34.9, classified as grade 1 obese, predominantly female and white, with a higher prevalence of a fat-responsive genotype. Both diets were designed to create a 750-kcal calorie deficit, which reduced weight by just over a pound per week. The only difference between the diets was their nutritional composition; the high-carbohydrate diet had 20% more calories from carbohydrates, and the high-fat diet had 20% more from fat.
During the 12-week intervention, volunteers participated in weekly group sessions on a variety of eating topics, from meal planning to behavior changes. Participants were instructed to weigh themselves daily and send photos of their weight to the interventionists before each session. Although the sessions were initially planned to be held in person, most were conducted remotely due to the COVID-19 pandemic.
Results
- Weight loss did not differ between genotype-matched and non-genotype-matched diets. Weight change in fat respondents was similar on both the high-fat and high-carbohydrate diets. The same pattern was observed in people who responded to carbohydrates.
- There were no significant differences in body fat and anthropometric measurements, such as hip and waist circumference, between genotype-concordant and genotype-discordant diets.
- Changes in hunger, satisfaction with the intervention, and food desires and preferences did not differ between matched and nonmatched diets.
Based on these findings, the authors wrote
“We found no difference in WL [weight loss] between individuals on the genotype-concordant vs. genotype-discordant diet.”
Among the most obvious limitations of the study are three key issues:
- There were challenges in the participants’ adherence to the diet. 39% of the participants following the high-carbohydrate diet and 66% of the participants following the high-fat diet followed the recipe and diet preparation they were supposed to follow.
- The sample size was small, limiting the detection of small but significant clinical differences, such as body fat.
- The authors acknowledge that the genetic algorithm used to classify individuals as fat or carbohydrate respondents was based on retrospective and modest studies that could lead to false-positive stratification.
Finally, there are two points to consider, although they do not change the results found. First, the “high fat” diet contained more carbohydrate than fat, suggesting that participants with a fat-prone genotype did not receive an adequately tailored intervention. Second, there is a potential conflict of interest. Two of the authors are shareholders and employees of Weight Watchers, which does not currently offer or believe in the value of Weight Watchers genetic testing for weight loss.
Does this mean the findings are unreliable? Not necessarily. However, these factors underscore the need for caution in interpreting the results and emphasize the importance of awaiting future studies to confirm or refute the validity of these findings.
The bottom line is that while nutrigenetic companies present themselves as scientific, their aggressive marketing is filled with anecdotal consumer experiences. We currently have no robust evidence to support the effectiveness of nutrigenetic testing for weight loss. Until new evidence emerges, I recommend not spending your hard-earned money on these products that, at best, promote weight loss not by offering customized “foods” to help you lose or prevent weight gain, but by simply imposing a calorie-restricted diet.
[1] This insight is described by the authors, microbiologist Natália Pasternak and journalist Carlos Orsi.
[2] SNPs are genetic variations in a single base pair in the DNA sequence.
Sources: The Personalized Nutrition Study (POINTS): Evaluating a Genetically Informed Approach to Weight Loss, a Randomized Clinical Trial. Nature Communication DOI: 10.1038/s41467-023-41969-1.
Genetics of caffeine consumption and responses to caffeine. Psychopharmacology. DOI: 10.1007/s00213-010-1900-1.