The old adage “you are what you eat” implies that the key to good health is as simple as eating healthy foods, but this well-known piece of nutrition advice ignores that an individual’s response to food  -- whether it be storing fat, making insulin, or breaking apart carbohydrates -- depends not only on properties of the food but also on properties of the individual, including their DNA. Genes influence food preferences, nutritional requirements and even whether a food improves an individual’s health. This vast genetic variation within and between human populations argues against a “one-size-fits-all” diet. For optimal health, human diets need to get personal.

A new review paper by California Dairy Research Foundation executive director Gonca Pasin, Ph.D., and nutrition scientist Kevin B. Comerford, Ph.D., highlights the importance of a personalized nutrition approach. Bringing together results from 26 studies on the interaction of dairy intake and a small number of genetic variants, the authors have identified the potential for different sensitivities to the health effects of dairy foods based on an individual’s genetic make-up.

Not surprisingly, much of the review focused on interactions between dairy intake and the genetic variations that result in lactase persistence (LP). LP individuals continue to produce the enzyme lactase after infancy, allowing them to break down the milk sugar lactose in their small intestines.

“LP is a unique genetic variation in that it increases, rather than limits, the available number of dietary options that can be digested,” explained the authors. “This gain of function mutation has resulted in LP becoming one of the strongest positive selective pressures in human history and reinforces the importance of gene-dairy interactions in relation to human health.”

A major health outcome of interest is bodyweight -- considering the increased food options available to the lactase persistent individual, LP could be associated with higher body mass index (BMI). A handful of small studies suggest that is the case, noting a positive correlation between a particular gene variant for LP and BMI. However, the authors note that a large study of nearly 100,000 people found no evidence of a link between LP, dairy intake and measures of bodyweight. The discrepancy in findings speaks to the complexity of a nutrigenetic approach; understanding how a particular gene variant influences the diet must take into account many variables. In this case, Comerford and Pasin believe the different types of dairy foods consumed could help explain the conflicting results.

“Dairy is a complex food group, which contains many products that differ in their macronutrient, micronutrient and bioactive compound composition,” the authors said. Consumed in adequate amounts, any number of these compounds can interact with a person’s genetics to influence their health. Thus, even when focusing on just one food group, variation in food properties and their potential to interact with particular gene variants must be considered.

The importance of considering dairy’s different ingredients is underscored by the relationship between dairy intake, lactose digestion and markers of bone health. A study of over 250 post-menopausal Austrian women found that those with a limited ability to digest milk had lower measures of hip and spine bone mineral density compared to those with the greatest ability to digest milk. These findings were not attributed to greater calcium intake by LP individuals but rather to the synergistic actions of calcium with other dairy ingredients, such as bioactive casein proteins, phosphorus, and vitamin D. But don’t despair lactose intolerant individuals -- several other studies of post-menopausal European women found no association between LP and markers of bone health. Dairy intake may be important, but it is unlikely to be the only factor influencing bone health.

In addition to bodyweight and bone strength, LP can influence the types of foods and nutrients that are introduced into the gastrointestinal tract. For example, some individuals (those who are lactose persistent) break down lactose into glucose and galactose in the small intestine, resulting in a pleasant dairy-foods-digesting experience. Whereas in lactose intolerant individuals, lactose moves into the large intestine where it often causes digestive issues. Might these differences relate to risks of developing bowel diseases and even colorectal cancer? Interestingly, Comerford and Pasin reported that a meta-analysis of 80 studies on dairy intake and colorectal cancer actually found a protective role for dairy foods in both lactose persistent individuals and those who are lactose intolerant. Regardless of how people digest milk sugar, it appears that dairy foods offer some level of protection against colorectal cancer.

Many of these anti-cancer benefits may come from ingredients in milk besides lactose, including milk fat and bioactive proteins.

“It is also important to consider that genetic factors other than LP may be moderating the relationship between dairy intake and health outcomes,” explained the authors. Genetic variations in several different hormonal and metabolic genes, which have nothing to do with the lactase enzyme, may be able to influence both dietary intake and various disease states. Prime examples are genes that code for lipoproteins, which are proteins that carry fats throughout the body. Genetic mutations in the lipoprotein APOA2 gene, which codes for a particular lipoprotein that can help stabilize HDL cholesterol, was found to be associated with higher BMI in individuals consuming high-levels of dairy fats. However, there was no association between high-dairy fat intake and BMI in individuals without this lipoprotein mutation.

These types of findings are common in nutrigenetic studies, in which single gene mutations can potentially have major effects on diet and disease. Now just think, any two humans tend to differ by more than three million letters in their DNA code.

Far from “you are what you eat,” the studies reviewed by Comerford and Pasin, demonstrate how what “you are” is highly dependent on both what you eat and what gene variants you have. Eating more dairy foods may reduce cancer risk, decrease cholesterol and improve bone health for individuals with a particular set of gene variants, but have no effect on other individuals with different gene variants.

There is clearly a benefit to knowing which of these groups an individual fits in. Highly specific dietary advice has the potential to improve health outcomes, but Comerford and Pasin note, “the science of personalized nutrition is just scratching the surface of this potential." This review details only a few pages of the nutrigenetic story -- a story that keeps evolving and is different for everyone.

The California Dairy Research Foundation (CDRF) is an independent non-profit organization that leads and delivers research and science-based programs towards a more innovative and sustainable California and U.S. dairy industry.