Research Review: Diet vs. exercise for weight loss | Precision Nutrition

Research Review: Diet vs. exercise for weight loss

By Helen Kollias


As a PNer, you know that you need both diet and exercise to lose weight. But if you had to choose one, which do you think would yield the best results?

In the exercise corner

I confess I’m rooting for exercise. I think our bodies were made to move. Until very recently, people moved constantly: they hunted, fished, farmed, walked or did some sort of manual labor for hours a day.

Meanwhile, I think dieting without exercise is usually disastrous! With a caloric deficit and no exercise, people lose muscle, which leads to their metabolism slowing down. To keep losing weight, they need to cut calories even more. They end up with minimal muscle and the dreaded “skinny-fat”.

Exhibit A, below: supermodel Kate Moss at 170 cm (5’7”) and 47.2 kg (104lb) with a BMI of 16.3.

So I’m backing exercise.

Kate Moss, who might consider adding some lean mass
Kate Moss, who might consider adding some lean mass

In the diet corner

The pro-dieter camp may argue that diet is key and no reasonable amount of exercise can trump gallons of cola and pounds of cheese puffs. Indeed, it’s very difficult to get enough exercise to offset a ridiculously high calorie intake. Thus, a “good diet” with caloric restriction makes way more sense than exercise.


Looking at both of these arguments it seems pretty clear: a lot of exercise in the face of bad nutrition is a losing battle, while strict dieting with no exercise looks just as bleak.

Research question

Researchers Stephen Ball and Anne Bolhfner also wanted to know whether diet or exercise alone would be best for weight loss. They compared weight loss between two groups of women:

  • A diet-only group doing Weight Watchers
  • An exercise-only group signed up at a fitness centre with an exercise program.

Here’s the link to to the full study:

Comparison of a Commercial Weight Loss Program to a Fitness Center. Journal of Exercise Physiology 11:3 (June 2008). (PDF)

Before we jump to the findings, let’s look at the study in more detail:



The study participants were what I’d consider the typical “Weight Watcher” enrollees: sedentary, overweight and female. They were:

  • All women
  • Overweight as determined by a BMI>25 (average BMI was actually 30.2)
  • Sedentary (less than 60 minutes of total exercise per week!)
  • Average age of 31.7 years
  • 80.2 kg (176.4 lb)
  • 162.9 cm (5′ 4”) tall
  • 40.4% body fat (32.4kg or 71.3 lb of fat)

The experiment

Researchers assigned the 48 participants randomly to either the Weight Watchers group or the Fitness Centre group and then monitored them for 12 weeks. Participants’ body weight, body fat percentage (using Bod Pod), fat mass, and fat free mass (aka lean mass) was measured before and after the 12 week intervention.

The researchers also looked at the women’s fat type and blood chemistry. Intra-abdominal fat (or visceral fat) and lipoproteins have been linked to each other [1], heart disease [2] and diabetes. Using computer tomography (CT) – very fancy – the researchers determined the area of intra-abdominal fat (cm2). Lipoproteins measured were total cholesterol, HDL and triglycerides.

In my opinion, intra-abdominal and lipoproteins measures are better indicators of overall health improvement compared to body fat percentage and total body weight. When it comes to body fat, the amount is important. But where the fat is located in the body is also significant. Intra-abdominal fat is more dangerous than fat under the skin (aka subcutaneous fat). Just like real estate, it’s location, location, location.

Group #1 – Weight Watchers

What is Weight Watchers? The Weight Watchers website [] has a list of what makes their system different from other diets. This includes (drum roll please):

  1. An integrated approach emphasizing good eating choices, healthy habits, a supportive environment and exercise.
  2. A plan that allows you to eat what you like, with an emphasis on nutrition and advice on staying satisfied by choosing the foods you enjoy.
  3. A sensible plan to help you lose weight at a healthy rate plus the knowledge and info you need to help you keep it off for good.
  4. A time-tested approach informed by analyzing years of scientific studies.
  5. Flexible food plans that can adapt to any lifestyle or unique needs.

“Even though WW (Weight Watchers) promotes exercise,” notes the website, “the major component of the program is calorie restriction.” In other words, diet is the primary focus of the WW approach.

While there is no structured exercise in the program, Weight Watchers does have ongoing support with weekly weigh-ins and counselling sessions for the participants – that helps to keep people on track.

Group #2 – Fitness Center

The other half of the participants received a membership to Gold’s Gym and followed Gold’s “Quick Start” exercise program. However, the exercise program wasn’t well defined in the paper so I decided to e-mail the authors and ask.

Yup, you can do that. Researchers are real people! In fact, most scientists are more than happy to discuss their findings. We’re all a bit passionate/obsessed/nuts about what we do. If you read a study and want clarification about a specific point, send the author an e-mail. You’ll be surprised how often you get a response. (Just don’t ask them to summarise their entire paper… that’s a faux pas.)

Here’s how Dr. Stephen Ball described the exercise program to me:

The fitness center group received 3 personal training sessions to help get them started on their program. The program consisted of a combination of resistance training (8-10 exercises, approx 10 reps to failure, 2-3 sets per exercise with approximately 1 minute rest between sets) 3 times a week, and cardio exercise (approximately 30 minutes per session) at least 3 times per week.

Researchers didn’t monitor the exercise sessions. Why not? Too much trouble? Not enough graduate students to go around? Nope. Ball and Bolhfner wanted a real world study. As Ball explained, “We purposely didn’t perform the exercise sessions in our lab nor were we hard on the subjects to complete the exercises. They were on their own, just as they would be if they joined a fitness center.” Most people don’t have a person in a lab coat holding a clipboard looking over their shoulder at the local Y, so these subjects didn’t either. Probably, said Ball, the results would have been a lot better for the exercise group if researchers had monitored the exercise sessions. (This is why we at PN focus on accountability – it’s a lot harder to waffle and wuss out when other people are watching.)


After 12 weeks:

  • The Weight Watcher group lost 4.1 kg (9.0lb)
  • The Fitness Centre group only lost 1.3 kg (2.7 lb)

This “between-group” difference is statistically significant — meaning that the Weight Watcher group clearly lost more weight than the Fitness Centre group.

Here’s something else that’s fascinating: There was no difference in body fat percentage between groups after 12 weeks. Could the weight loss difference between groups be because of loss of fat free mass?

Nope. While both groups lost fat free mass (lean body mass) over the 12 weeks – not good – there was no difference in fat free mass between groups.

Both groups lost intra-abdominal fat, but there no difference between groups – no “winner” in this round. This is important, because health-wise decreases in intra-abdominal fat are associated with improved cholesterol, HDL and triglycerides [1]. But in this study there was no improvement in cholesterol, HDL or triglycerides despite the loss of intra-abdominal fat.


There is no clear winner – they both did badly. If you just want to lose weight and don’t care if it’s muscle or fat, then Weight Watchers will help you with that – for the first 12 weeks anyway. In the long term, you’re probably setting yourself up for trouble. Eventually your metabolism will slow down.

The take home message: a combo of both diet and exercise will be the most beneficial and if you want to succeed you need to commit to the long term.


Click here to view the information sources referenced in this article.

Learn more

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They’re probably better than 90% of the seminars we’ve ever attended on the subjects of exercise and nutrition (and probably better than a few we’ve given ourselves, too).

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