The right fats help reduce age-related weight gain

The latest flurry of discussion in the nutrition world has been about fat - or rather, the different types of fats. As you probably know, for years the recommendation was for everyone to follow a low-fat diet: the less fat, the better, generally speaking. More recently we've discovered that some fats are in fact good for you: indeed, certain types of fats are considered to be protective against cardiovascular disease.

We also know that overweight and obesity are linked with increased cardiovascular and metabolic health risks. Might a person's consumption of various types of fats be associated with their body weight?

Authors affiliated with Harvard's T.H. Chan School of Public Health and the Brigham and Women's Hospital pored over the massive amounts of data associated with three major, long-term, and large-scale surveys of (largely white) American men and women: the Health Professionals Follow-Up Study (HPFS), the Nurses' Healthy Study (NHS), and the Nurses' Health Study II (NHSII) (J Nutr  https://doi.org/10.1093/jn/nxy183).

All three of these studies surveyed their participants every 2-4 years, with either a food frequency questionnaire that captured changes in dietary intake, an assessment of medical history, lifestyle factors, and other demographic information, or both. Together the three studies included over 289,000 men and women, 97% of whom were white persons of European descent.

The authors began their research with the data gathered for the 1986 survey for those in the Health Professionals Follow-Up Study and the 1991 surveys for NHS I and II. They assessed the dietary intake of every participant from each food frequency questionnaire conducted over the years and calculated the participants' intakes of a variety of types of fats as percentages of total caloric intake, which allowed the researchers to analyze the changes in participants' fat intake over time. At the same time the authors tracked changes in body weight for the same points in time as the food frequency questionnaires.

As noted, the researchers began their data analysis with either the 1986 or 1991 questionnaires (depending on when the three surveys implemented the most complete food frequency questionnaire) - and continued their analysis through 2011.

Those participants who reached 65 years of age at any point during the follow-up periods were removed from analysis to minimize confounding due to age-related weight loss. The authors also removed those who were diagnosed with cancer, cardiovascular disease, reported a Body Mass Index of over 30 (clinically obese), diabetes of any kind, and implausible or incomplete responses to food frequency questionnaires. The final analyses included just over 121,000 persons.

The authors assessed the association between intakes of types of fat and weight change over time, looking at total fat (from all sources) and breaking that out by animal versus plant source as well as saturated, trans, polyunsaturated, and monounsaturated fats as well as omega-3 and omega-6 fatty acids. Total carbohydrate and protein intake were assessed as well and the authors controlled for protein intake, cereal fiber, intake of fruits and vegetables, and other variables in their analysis.

Somewhat unsurprisingly, the authors found that those participants who increased their intake of saturated fats and trans-fatty acids gained an average of about 1.3 pounds every 4 years, while those who increased their intake of polyunsaturated fatty acids by about the same amount gained only about 1.2 pounds every 4 years. Consuming more monounsaturated fatty acids from animal sources was associated with a weight gain of about .6 pounds per year, but consuming more monounsaturated fatty acids from plant sources was not associated with weight gain at all.

What this means for you

The authors also looked at the consumption of red and processed meats along with other foods such as marine animal protein, nuts, and olive oil. For all three study groups, the intake of red and processed meats was positively correlated with higher intake of saturated and trans-fats, while consuming more nuts and olive oil were positively correlated with high intake of polyunsaturated fatty acids.

This is the drawback of studies like these: they show correlation, but not causation. That is, it's possible that the observed weight gain had more to do with an overall poorer quality diet (for example, a more Western-style diet with more fast food and sugar-sweetened beverages) than with saturated and trans-fat intake alone. Similarly, less weight gain might be due to a higher quality or more Mediterranean-style diet that is higher in plant-based fats. What we do know is that less saturated fats and more fats from plant sources are indeed better for you regardless of weight gain or loss, and for most people the easiest way to make that change is to follow a Mediterranean-style diet.

First posted: October 10, 2018