For me, it’s practically an existential question:

Would I rather have a client stay as close as possible to their chosen diet?

Or would I like them to make consistently good food choices despite straying from their diet?

Ideally, I’d want high adherence and high quality rather than one or the other. But if I had to pick, the answer isn’t really obvious.

The argument for adherence is pretty simple:

Because every diet does the same basic thing—it gives you a way to eat less food—sticking to the diet over time guarantees weight loss.

The argument for quality is slightly more abstract:

A high-quality nutrition pattern will give you foods with lower energy density (fewer calories per unit of food) and higher nutrient density (more vitamins, minerals, and fiber).

That combination means you should feel full faster and thus eat less over time.

A new study from Stanford University researchers examines the separate and combined effects of diet quality and adherence.(1)

Background

This study is a secondary analysis of an earlier experiment named DIETFITS. (The acronym stands for Diet Intervention Examining The Factors Interacting with Treatment Success.) (2)

DIETFITS was a 12-month study in which volunteers were randomly assigned to either a low-carb or low-fat diet and extensively coached on how to make good food choices within that context.

The participants were not instructed to reduce calories, even though it was a weight-loss study.

After 12 months, the low-carb group lost an average of 6 kg (13.2 pounds), while the low-fat group lost 5.3 kg (11.7 pounds).

That brings us to the key question in this study:

How did adherence to the assigned diet interact with the quality of the diet to affect weight loss and other health outcomes?

How the study worked

The researchers reviewed the food records of 448 participants from the original DIETFITS pool—224 were assigned to the low-carb diet and 224 to the low-fat group.

Participants from each diet group were then classified into one of four subgroups based on diet quality and adherence to the assigned diet.

• High quality/high adherence (HQ/HA)

• High quality/low adherence (HQ/LA)

• Low quality/high adherence (LQ/HA)

• Low quality/low adherence (LQ/LA)

What the study found

The topline result is no surprise:

Participants in the high quality/high adherence (HQ/HA) subgroups cut the most calories: more than 900 Calories per day, on average, in both the low-carb and low-fat groups.

They also reduced their BMI by about 2.5 points and significantly reduced systolic and diastolic blood pressure.

You also won’t be surprised to learn that the low quality/high adherence (LQ/LA) subgroups had the worst results.

But that doesn’t mean they got no results after 12 months.

In the low-carb group, low-quality/low adherence (LQ/LA) participants still managed to eat less (they cut an average of 172 Calories a day) and reduced their BMI by 1.73 points.

Low-quality/low-adherence (LQ/LA) participants assigned to the low-fat diet reduced their BMI by 1.36 points.

If we frame this study as an Olympic event, the high-quality/high-adherence (HQ/HA) participants won gold medals, while low-quality/low-adherence (LQ/LA) participants didn’t make the podium. This is entirely predictable.

But who won silver, and who had to settle for bronze?

Takeaways

1. For weight loss, adherence to a diet appears to trump food quality.

The low-quality, high-adherence subgroups had the second-best results across the board.

They cut more than 800 Calories a day, reduced their consumption of both fat and carbohydrates (regardless of which diet they’d been assigned to), and reduced their BMI more than the overall average.

Their improvements in blood pressure also surpassed the average of all participants.

An important caveat: The study’s authors explicitly label both diets as “healthy.”

So, when we look at the weight-loss advantage of adherence to a diet, we’re not talking about blindly following something weird or extreme. (We’ll let you decide which popular diets deserve those descriptions.)

We should also repeat something we mentioned earlier: Participants were randomly assigned to the low-carb or low-fat diet.

If you’d been a participant, how closely would you adhere to a diet someone else chose for you? Especially if the assigned diet didn’t line up with your preferences.

That brings us to the other half of the adherence vs. quality question.

2. For optimal health, diet quality still matters.

Studies usually find an inverse association between healthy food choices and body weight.

That is, the higher the quality of an individual’s diet, the less likely they are to have obesity. (4)

The relationship wasn’t quite as clear in this study, although “high quality” and “low quality” were defined as above or below the median within this group of generally well-educated participants, all of whom received extensive coaching on making the best choices for their assigned diet.

For those assigned to the low-carb diet, both high-quality subgroups had significant improvements in fasting insulin concentrations. Fasting blood sugar was significantly lower in HQ/HA but not HQ/LA.

For those using the low-fat diet, both high-quality subgroups saw significant reductions in systolic blood pressure.

In other words, higher-quality diets provide unique health benefits compared to lower-quality diets.

Helping your health,

Michael Beiter

Certified Personal Trainer

PN L1 Sleep, Stress Management, and Recovery coach

References

1. Hauser ME, Hartle JC, Landry MJ, Fielding-Singh P, Shih CW, Qin F, et al. Association of dietary adherence and dietary quality with weight loss success among those following low-carbohydrate and low-fat diets: a secondary analysis of the DIETFITS randomized clinical trial. Am J Clin Nutr. 2024 Jan;119(1):174–84.

2. Gardner CD, Trepanowski JF, Del Gobbo LC, Hauser ME, Rigdon J, Ioannidis JPA, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018 Feb 20;319(7):667–79.

3. Htet MK, Fahmida U, Do TT, Dibley MJ, Ferguson E. The Use of Tablet-Based Multiple-Pass 24-Hour Dietary Recall Application (MP24Diet) to Collect Dietary Intake of Children under Two Years Old in the Prospective Cohort Study in Indonesia. Nutrients. 2019 Nov 27;11(12).

4. Asghari G, Mirmiran P, Yuzbashian E, Azizi F. A systematic review of diet quality indices in relation to obesity. Br J Nutr. 2017 Apr;117(8):1055–65.