For every patient I have who comes to me believing that a low-carb diet is the way to be healthy or control weight, I have another who has been unable to shrug off the vestiges of the low-fat era, convinced that eating more fat will lead to heart disease or weight gain.
I remind them that the evidence strongly suggests that when you reduce something in your diet - such as carbs or fat - it's important to be mindful of what you replace it with.
For example, when the public was advised to lower fat decades ago, most people swapped it for foods high in sugar and white flour, rather than with fruits, vegetables and whole grains. Not ideal.
When it comes to weight loss, the past several years of research show that low-carb diets may have a slight short-term edge on average, but that neither can claim true superiority, especially given that about 95 percent of dieters end up regaining.
A decade ago, a group headed by nutrition scientist Christopher Gardner, professor of medicine at Stanford University, published a study comparing the Atkins, Zone, Ornish and LEARN diets among a group of 311 women.
After a year, average weight loss was only a few pounds, but when you looked beyond that, the difference between losers and gainers on each of the four diets was huge: some lost 55 pounds (25kg), others gained 10 to 20 pounds (4.5-9kg). Why?
Gardner's group decided to try to answer that question, focusing not on which diet was best, but which diet was best for whom.
Some preliminary research - including one study from Gardner's group published in 2012 - suggested that how someone's body responds to the hormone insulin may be key. And then there's our individual genetic makeup - could it predispose us to thriving on less fat, or less carbs?
I had been looking forward to hearing the results of Gardner's more recent DIETFITS study at a conference a few months ago.
When I did, what first struck me was how relatively "real world" this study was, designed to help participants stick to their randomly assigned plan, while following it in a sustainable way. I've had a few patients who went on months-long food benders after participating in highly restrictive diet studies, so I consider "sustainable" a critical detail.
Because there's no standard definition of "low-fat" or "low-carb," Gardner asked the 609 participants to aim for 20 percent carbs or fat, depending on their diet assignment. After a few weeks, they could adjust upward if needed.
"Please help define for the American public what is low-carb and low-fat, not just lower," he said to them. "What's the lowest you can tolerate so you can look us in the eye and say, at the end of this period, 'Yeah, I think that this place where I ended up is something I could do for the rest of my life.' "
In the end, "low-carb" meant about 30 percent carbs and 45 percent fat, while "low-fat" was about 29 percent fat and 48 percent carbs. Despite not being required to cut calories, participants were eating an average of 500 calories less, and what they were eating was high quality.
The low-fat group was encouraged to choose whole grains, a variety of beans and lentils, seasonal organic fruit, organic low-fat milk and lean meats. The low-carb group was pointed toward high-quality oils and fats, organic avocados, hard cheeses, nut butters, grass-fed meat and pasture-raised eggs.
"Everyone was supposed to have vegetables all day long as much as they could, have a salad every day, and no added sugar and as little refined flour as you could get," Gardner said.