What Can Your DNA Actually Tell You About Nutrition?

Nutrition advice is becoming increasingly personalized. We can measure body composition, blood biomarkers, aerobic fitness, glucose responses, dietary habits—and now, relatively easily, certain genetic variants.

That creates an obvious question:

Can your DNA tell you exactly how you should eat?

Not quite.

Nutrition genetic testing can provide useful information about biological differences that may influence how someone responds to nutrients, foods, caffeine, or other aspects of diet. But your DNA is not a meal plan, and it does not identify one perfect diet that will automatically work better than every other approach.

Your DNA is another layer of context—not a meal plan. Its value comes from interpreting it alongside your diet, health history, training, and goals.

When interpreted alongside your current diet, health history, training, goals, preferences, symptoms, body-composition data, and relevant bloodwork, genetics may help a Registered Dietitian ask better questions and make certain recommendations more individualized.

Here is what that means in practice.

What does nutrition genetic testing actually measure?

Humans share the overwhelming majority of their DNA, but small differences exist from person to person.

Many nutrition genetic tests look for specific variations in DNA known as single-nucleotide polymorphisms, or SNPs. These are differences at individual locations within the genome.

Some variants have been studied in relation to areas such as:

• caffeine metabolism and response
• folate metabolism
• lactose digestion
• lipid metabolism
• sodium and blood-pressure response
• appetite and satiety
• omega-3 metabolism
• vitamin and mineral status
• glucose regulation
• exercise and recovery characteristics

The test itself identifies which version of a particular genetic variant you carry. The important part comes afterward: determining whether that information is relevant enough to change what you actually do.

That distinction matters.

A genetic association can be scientifically interesting without being strong enough to justify a major change in someone’s diet.

Genetics can influence nutrition—but genes are not destiny

There are clear examples in human biology where genetics affect the way we process components of our diet.

Caffeine is a useful example.

The enzyme CYP1A2 is involved in caffeine metabolism, and genetic variation in the CYP1A2 gene has been studied extensively. Researchers have found differences in caffeine metabolism between genotypes, but studies examining whether those differences consistently translate into different exercise-performance responses have produced mixed results.

For example, a randomized trial in athletes found that caffeine’s effect on cycling performance differed substantially by CYP1A2 genotype. Other randomized studies have found that caffeine improved performance without a meaningful genotype-by-caffeine interaction. More recent research has also shown genotype-related differences in caffeine pharmacokinetics without finding that genotype clearly determined the performance response.

That is a good illustration of how we approach genetic results.

The biology may be real. The genetic variant may be relevant. But the practical recommendation still needs to account for the strength and consistency of the evidence—and for the individual sitting in front of us.

Someone’s actual caffeine intake, timing, sleep, anxiety, gastrointestinal tolerance, training schedule and subjective response may ultimately matter just as much as their genotype.

Can DNA tell you whether you should eat low-carb or low-fat?

This is where genetic testing is sometimes oversold.

One of the largest studies to directly examine this question was the DIETFITS randomized clinical trial conducted by Stanford researchers.

The study randomized 609 adults to either a healthy low-fat or healthy low-carbohydrate diet for 12 months. Researchers also tested a genetic pattern that had been hypothesized to predict whether someone would respond better to one diet or the other.

It did not.

There was no significant interaction between the tested genotype pattern and diet assignment for weight loss. In other words, participants whose genetic profile was predicted to favor one diet did not lose significantly more weight when assigned to that diet.

That does not mean genetics have nothing to do with nutrition or body weight.

It means something more specific: the genotype pattern tested in DIETFITS did not successfully tell researchers whether a healthy low-fat or healthy low-carbohydrate diet would produce greater weight loss for an individual.

That distinction is important whenever genetic research is discussed.

One study rarely answers the broad question, “Do genes matter?” Instead, it answers a much narrower question about a specific gene or genetic pattern, intervention, population and outcome.

Does genetic information make personalized nutrition more effective?

Research here is interesting—but still developing.

The large European Food4Me randomized controlled trial compared standard dietary advice with different levels of personalized nutrition.

Some participants received advice personalized to their current diet. Others received additional information based on phenotype, such as body measurements and biomarkers. Another group received dietary, phenotypic and genetic information.

Personalized nutrition overall resulted in better dietary changes than conventional advice. Participants receiving personalized advice improved several aspects of their diets, including saturated-fat and salt intake.

But importantly, the main trial found no evidence that adding phenotypic and genetic information made the intervention more effective than personalization based on diet alone.

Another analysis from the same Food4Me study found slightly greater improvement in Mediterranean Diet scores when genetic information was included, although the researchers described the clinical relevance of that difference as modest.

So the takeaway is not that genetic information is useless.

It is that personalization is bigger than genetics.

Understanding what someone currently eats, what they enjoy, what their schedule allows, what their goals are, and what changes they can realistically maintain can already produce meaningful personalization. Genetics can potentially refine that process, but it does not replace it.

So what is genetic testing actually good for?

The most useful genetic results tend to be ones that pass three questions.

1. Is there a meaningful biological relationship?

There should be credible evidence connecting the genetic variant with a nutrient, metabolic pathway, dietary response or other relevant outcome.

2. Is the finding actionable?

Knowing something about your DNA is only useful in nutrition counseling if there is a reasonable action to consider.

If a result does not change what we eat, monitor, measure or discuss, it may be interesting information—but not particularly useful information.

3. Does it make sense in the context of the individual?

This is the part that cannot be automated by a genetic report.

Imagine that a genetic result suggests someone may need to pay closer attention to a particular nutrient.

An RD still needs to ask:

• What does that person currently eat?
• Do they already consume plenty of that nutrient?
• Do they have relevant bloodwork?
• Are there medications, medical conditions or supplements to consider?
• Do they have symptoms?
• What are their goals?
• Would changing their intake actually improve something meaningful?

A genetic result becomes much more useful when it is combined with those answers.

What genetic testing cannot tell you

There are several limits worth understanding before getting tested.

It cannot identify your single “perfect diet.”

There is no established genetic formula that can look at your DNA and reliably prescribe the exact macronutrient ratio, foods or eating pattern that will produce the best health outcome for you.

It does not replace bloodwork.

Genetics may indicate a predisposition related to nutrient metabolism, but bloodwork can help tell us what is happening right now.

Your genes are relatively fixed. Your biomarkers, diet, training load, sleep, body composition and health status change over time.

Those are different types of information.

It does not replace your real-world response.

If your genetic report suggests one thing but your actual symptoms, performance, labs or dietary response suggest another, the real-world data still matter.

It is not a medical diagnosis.

Wellness-oriented nutrition genetic testing should not be treated as diagnostic testing for genetic diseases or inherited medical conditions.

Questions about hereditary disease, carrier status or clinically significant genetic risk belong with an appropriately qualified physician or genetic counselor.

Why interpretation matters more than the report itself

A long genetic report can create the illusion of precision.

Dozens of genes. Multiple categories. Color-coded results. Highly specific recommendations.

But more information does not automatically mean better decision-making.

The value comes from separating findings into categories:

• What is strongly supported?
• What is potentially useful but still uncertain?
• What is interesting but unlikely to change anything we do?

Then we combine those findings with the information we already have about the person.

At Custom Fit, that is the role of the Registered Dietitian.

Rather than treating genetic results as instructions, we use them as another piece of information within the larger nutrition assessment.

For one person, the most relevant finding might help guide a conversation about caffeine.

For someone else, the genetic report may lead us to look more closely at a nutrient alongside their food intake and laboratory values.

For another person, the most important conclusion may be that their existing nutrition priorities should not change.

That is still useful information.

The practical takeaway

Genetic testing is most valuable when it helps make an already thoughtful nutrition process a little more informed.

It can identify biological differences worth paying attention to.

It can generate useful questions.

It can provide additional context for nutrition, training and long-term health decisions.

But it cannot tell you everything about how your body will respond to food, and it should not override the fundamentals of good nutrition or the information we can observe directly.

Your DNA is one part of your physiology.

Your habits, environment, training, health markers, preferences and goals are part of it too.

The goal is not to build your entire diet around a genetic report.

It is to use the right data, in the right context, to make better decisions.

Save 30% on Genetic Testing

$349.30 with code GENETIC30
Regularly $499. Available now. Offer ends December 1, 2026.

Includes a simple cheek-swab test and a 30-minute results review with a Custom Fit Registered Dietitian. Complete the test at the studio or have a kit shipped to your home.

Purchase Genetic Testing → · Enter GENETIC30 at checkout.

Explore Genetic Testing at Custom Fit

References

1. Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018;319(7):667–679.
2. Celis-Morales C, Livingstone KM, Marsaux CFM, et al. Effect of personalized nutrition on health-related behaviour change: evidence from the Food4Me European randomized controlled trial. International Journal of Epidemiology. 2017;46(2):578–588.
3. Livingstone KM, Celis-Morales C, Navas-Carretero S, et al. Effect of an Internet-based, personalized nutrition randomized trial on dietary changes associated with the Mediterranean diet: the Food4Me Study. American Journal of Clinical Nutrition. 2016;104(2):288–297.
4. Guest N, Corey P, Vescovi J, El-Sohemy A. Caffeine, CYP1A2 Genotype, and Endurance Performance in Athletes. Medicine & Science in Sports & Exercise. 2018;50(8):1570–1578.
5. Carswell AT, Howland K, Martinez-Gonzalez B, Baron P, Davison G. The effect of caffeine on cognitive performance is influenced by CYP1A2 but not ADORA2A genotype, yet neither genotype affects exercise performance in healthy adults. European Journal of Applied Physiology. 2020;120(7):1495–1508.
6. Masters C, Ali A, Badenhorst C, Dickens M, Rutherfurd-Markwick K. The Effect of CYP1A2 Gene Polymorphisms on Caffeine Pharmacokinetics and Exercise Performance in Male Recreational Athletes. European Journal of Sport Science. 2026;26(7):e70203.