Prebiotic and probiotic foods

Nutritional genomics is a new field in nutrition research which is dedicated to understanding how nutrition can affect gene expression and how genes determine our response to our nutritional choices. For example, there is a genetic deficiency called phenylketonuria, known as PKU for short. In this condition, there is an excess of phenyl ketone in the blood and it spills into the urine, the name which literally means phenyl ketones in the urine.

What is this disease? It is a failure to metabolize correctly the amino acid phenylalanine. This failure is a result of a mutation in the gene that codes for an enzyme that is essential for the normal metabolism of that amino acid. The mutation in the gene results in the formation of an inactive enzyme. Without a noramlly functioning enzyme, a newborn baby with an inactive (defective) enzyme accumulates phenylalanine in the blood. This results in a lot of the substance ending up in the urine (hence, the name). Unfortunately, a lot of it also ends up in the brain, where it causes severe neurological disorders and profound mental retardation. Fortunately, doctors in most states are now mandated by law to test the blood of newborns for phenylalanine. If the concentration is too high (over 20 mg per 100 ml of blood), the baby is immediately put on a phenylalanine-free diet, for at least 6-7 years. After that period, the brain circulation is mature enough to exclude phenylalanine so that toxic amounts cannot accumulate any more.

Another example of nutritional genomics is the relative deficiency of folic acid (a B vitamin) in pregnancy. If the mother does not supplement her diet with folic acid, there is a chance that the baby will be born with spina bifida, a serious neurologic disorder. To be sure, only a small minority of mothers have a problem with their folic acid metabolism. But rather than try and ferret them out, it is simply cheaper to supplement every pregnant mother with folic acid.


Enter the brave new world of genomics

The conditions I described (PKU, spina bifida) are classical examples of nutritional remedies for genetic diseases. Both the diseases and the remedies were discovered before the human genome was elucidated. But, now, we have extremely powerful tools to investigate the structure of our genomes in minute detail. This allows us to make correlations between a certain type of mutations, called SNP for Single Nucleotide Polymorphisms, and subtle nutritional consequences. For instance, one such mutation is correlated with high levels of homocysteine in the blood. High levels of this amino acid are known to correlate with increased incidence of cardiovascular disease.


The commercial angle

Almost inevitably, entrepreneurs have identified an opportunity to make money in this arena. They believe some consumers will want to be able to tailor their diets to their specific genetic makeup? And, they believe many will be willing to pay to obtain the information needed to do this. Indeed, several small companies are already advertising their nutrigenomic services. You send them a swab of your buccal mucosa (cells scraped from the inner surface of the mouth) and a fee of a few hundred dollars, and they will analyze 10-15 of your genes and give you nutritional advice “tailored just for you.” Voilá—you are on your way to everlasting health!

But, hey, wait, not so fast. You may have noticed that I used the word “correlate” in describing the homocysteine relationship to cardiovascular disease. This is because no interventional studies have been done to show that altering our diet to reduce homocysteine accumulation actually does anything at all to reduce your cardiovascular disease risk.

My skepticism about the commercialization of nutritional genomic therapy is based on 2 reasons:

  • Just finding a molecule that happens to rise in concentration does not mean that it caused the disease. It could be that the molecule rose as a result of the disease itself or it rose incidental to some other event that caused the disease but also caused the molecule to rise. It is an important distinction. Nutritional therapy to lower a molecule that was incidental, and not causal, to the disease would not do anything to prevent or treat the disease. There are many examples of correlations that simply did not pan out as therapeutic options.
  • Most diseases are not caused by a single gene (like the case of PKU), but by multiple genes. So, even if the gene mutation that causes an increase in homocysteine had something to do with heart disease, changing our diet to reduce its accumulation is unlikely to make much of a difference. To make a real difference, we’d have to make global changes in our lifestyle. Stop smoking, control your weight, eat vegetables and fruits, exercise—these are much simpler, much cheaper, and most important, much more effective means to keep our hearts healthy.

So in summary, don’t get taken by any commercial claims for “genetically tailor-made diets”—at least not yet. They are simply not yet ready for prime time.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.