The Effects of Milk Protein Versus Soy Protein on Essential Omega-3 Fatty Acid Metabolism

Dairy products provide macronutrients, vitamins, and minerals that are important for health and development. Despite this, many Canadians now eat fewer dairy products and are instead consuming more non-dairy alternatives, such as soy. Some of this is because of misconceptions about the effects of dairy on health. However, the study investigators hypothesize that, compared to soy, dairy may actually improve a person's health due to its effects on the enzymes in the body that make omega-3 fatty acids, such as EPA and DHA. The goal of this study is to examine omega-3 metabolism in men and women consuming either milk or soy protein. Briefly, this study will determine if the consumption of milk or soy protein differentially impacts blood EPA and DHA levels, the synthesis of EPA and DHA, fatty acid oxidation, and omega-3 fat absorption from the gut. It is anticipated that the findings from this novel and innovative research project will provide high quality evidence to support a previously unappreciated protective effect of dairy for human health and development.

Direct comparisons between dairy and non-dairy plant-based alternatives on metabolic processes in humans are limiting. The study investigators recently completed a cross-sectional analysis in over 1000 young Canadian adult men and women and reported that the consumption of soy beverages/foods was associated with a reduction in estimated fatty acid desaturase pathway activity compared to milk and dairy food consumption (PMID: 34444977). The fatty acid desaturase pathway controls the endogenous synthesis of beneficial long-chain omega-3 polyunsaturated fatty acids (n3-LCPUFA), such as eicosapentaenoic and docosahexaenoic acids (EPA and DHA), from the essential fatty acid, alpha-linolenic acid (ALA). Epidemiological and clinical studies have reported that higher levels of EPA and DHA in the body are associated with reduced CVD risk. Thus, this prior work identified a previously unrecognized cardio-protective role for dairy that could help to counter consumer misconceptions about dairy increasing cardiovascular disease (CVD) risk.

The primary outcome of this research proposal is to determine if the consumption of dairy protein supports n3-LCPUFA synthesis compared to soy protein. The investigators hypothesize that study participants consuming dairy protein will show a greater conversion of ALA into EPA and DHA via the fatty acid desaturase pathway compared to study participants consuming soy protein. To test this hypothesis, the investigators will conduct a randomized, parallel-arm clinical trial in healthy adult men and women who are consuming either a diet containing milk protein isolate (and no soy) or a diet containing soy protein isolate (and no dairy).

Additional secondary outcomes of interest to further advance understanding of how milk and soy proteins affect acute and chronic ALA metabolism will include:

1. Whole blood gene expression analysis of fatty acid desaturases and elongases to determine if milk and soy proteins influence n3-LCPUFA synthesis through a transcriptional mechanism.
2. Breath sample analysis to determine if milk and soy proteins influence ALA degradation through β-oxidation.
3. An oral fat tolerance test (OFTT) to examine if milk and soy proteins modify ALA absorption from the gut.

Eligibility Criteria:
Inclusion Criteria:

* Healthy adult men and pre-menopausal women
* Must be able to consume soy and dairy beverages and foods
* Sufficient level of education to understand study procedures and be able to communicate with clinical staff

Exclusion Criteria:

* Diabetic
* Body mass index greater than 35 kg/m2
* Pregnant or lactating
* Neuropsychiatric disorders (major depression, schizophrenia)
* Digestive disorders
* Inflammatory disease
* Cardiovascular disease
* Use of omega-3 supplements within the last 3 months
* Regular consumption of oily fish
* Allergies/sensitivities to dairy and/or soy food products
* Use of lipid-lowering or digestive medication