This is question for which the answer should be obvious, and the motives of any “doctor” that says otherwise questioned deeply. To suggest that DNA can predict a perfect diet is only slightly more absurd than suggesting DNA sequence data (alone) can predict one’s likelihood of suffering from any particular complex disease in their lifetime. Despite what many have claimed the nature vs. nurture question is not at all settled and environmental factors still play a huge (and highly variable and mostly still unknown) role in the disease process (from onset to severity). The fact that environmental factors are complex, variable, and mostly unknown is why we hear so much more about DNA which is simple, and increasingly totally known. In no sense does that make it any more or less important however. In the case of diet there is another massively confounding and contributing factor which is the impact of a person’s gut microbiome on weight management. The role of the gut microbiome, it’s importance, and mechanisms of action are still mostly unknown as research in this area is still in its infancy.
At this point in time, and possibly forever, DNA based diet advice is worthless and I would be willing to go so far as to call it a scam or a hustle. The scam artists offering these services are exploiting the ignorance of the general population for their own profit. Personalized nutrition can work, but DNA sequence information as an input to a personalized nutrition/diet plan will give zero additional value at present. This could change in the future, though with complex processes, be it disease or weight, the influence of any one component (in this example DNA sequence) becomes more and more diluted. Therefore it is possible, likely even in my view, that there may never come a time when DNA sequence data is a useful input to a weight management plan because its overall impact on the process of weight management is simply too tiny to be of consequence in comparison to all the other inputs. Unfortunately these other inputs are complex, variable, and still mostly unknown (sound familiar?)