Does it matter how you lose weight?
Provided you have it to lose, intentional weight loss can improve your physical health.
When you lose weight through improved nutrition and added exercise (as opposed to unintentional weight loss, due to disease or deprivation), you expect to lower your risk of cardiometabolic problems like type 2 diabetes and heart disease.
And, although it probably wouldn’t be a front-of-mind concern, you’d expect to have a healthier microbiome, with more good bacteria doing… whatever good bacteria are supposed to do.
But does losing weight with one type of diet produce better results than losing it with another?
A new study tackles that question with fascinating—though far from definitive—results.(1)
How the study worked
Researchers at the University of Bath in the UK recruited healthy, non-obese adults for a 12-week study.
The participants were randomized to one of three diet groups:
Control
Low sugar
Low carb (ketogenic)
Here’s a quick look at the three diets:
Participants
The study began with 60 volunteers: 20 were randomized to the control group, 18 to low sugar, and 22 to the low-carb keto diet.
The keto group, which had the most participants, also had the most dropouts, with eight. Three quit immediately, with all of them offering non-diet-related reasons (work, sports, family).
The control group had the next-highest attrition rate, with seven bailing out before the end of week 12.
The low-sugar group retained 16 of its original 18 participants. The two who left cited work and travel as the reasons.
Of the 15 participants who dropped out, only one cited their group assignment as the reason. That person was assigned to the control group. According to the researchers, the individual was “unwilling to meet [the study’s] sugar intake target.”
Detail you need to know
The researchers’ primary goal was to see if the low-sugar or low-carb diet would lead to a reduction in physical activity, compared to the control diet.
But they didn’t tell the participants that. Nor did they emphasize exercise or body weight in any of their instructions. They wanted to see if participants changed their activity levels organically in response to their assigned diet.
What they found was… the type of diet didn’t seem to matter.
Overall, physical activity energy expenditure was pretty much the same in all three diet groups.
What the study found
Ketogenic diet
The researchers used urine tests to confirm that participants in the low-carb group were, in fact, eating very little carbohydrate.
That led to massive energy deficits at first—close to 700 Calories a day, on average, in the first four weeks. After 12 weeks, most were still eating a little less than they needed to maintain their weight.
That led to significant weight loss. On average, participants in the low-carb keto group lost 3.8 kg (8.4 pounds) in three months. One extreme outlier lost 5 kg (11 pounds).
However, there was a potentially negative change to the keto dieters’ microbiome.
Fecal tests showed a reduction in Bifidobacterium, a genus of microorganisms associated with lower inflammation and risk of infection.
It’s not clear if that change would have long-term health consequences, or if it could be alleviated by taking a probiotic supplement.
Low-sugar diet
Reducing sugar to 5 percent of the diet led to an average energy deficit of 256 Calories per day and weight loss of 1.5 kg (3.3 pounds).
Interestingly, the weight loss was entirely fat. With the keto group, the loss was in the expected range: 75 percent fat, and 25 percent lean mass.
And here’s a fun detail:
For every 1 percent reduction in daily calories from sugar, participants reduced total food consumption by 17 Calories.
In other words, if you habitually get about 20 percent of your total daily energy from sugar (as did the control group in this study), and reduce that to 10 percent, you would cut about 170 Calories per day.
By contrast, participants in the control group actually gained a little weight from their high-sugar diet. Worse, they lost a bit of lean mass, which means the added weight was all fat.
Takeaways
1. None of the groups consumed an optimal amount of protein.
The researchers set protein at 15 percent of total calories for all three groups. We understand why: if the groups consumed different amounts of protein, it could have affected the results.
The study used 15 percent protein for the keto group, so it made sense to use the same level for the control and low-sugar groups.
But in real life, you’d want at least 1.2 grams of protein per kilogram of body weight, and 2.2 grams per kilogram at the high end. That would not only support lean body mass but also improve satiety.
2. Participants in this study may not resemble you.
The average BMI of participants in this study was slightly below 25, which means they wouldn’t be classified as overweight, much less obese.
Not only were they relatively lean, they were quite active. The individuals who finished the study averaged close to 10,000 steps per day.
On top of that, the 45 volunteers who completed the study appear to have adhered closely to their assigned diets for 12 weeks.
In my wildest dreams, I can’t imagine having virtually all my clients follow their meal plans to the letter at the same time.
So while the study’s results are interesting, keep in mind that the participants who stuck it out for three months were healthy, fit, and perhaps uncommonly motivated and disciplined.
We have no idea how the same diets would affect clients who are heavier, less active, or otherwise unlike the study’s participants.
3. For exercise adherence, having less food appears to be better than way less food.
As we noted earlier, the researchers set out to discover if a low-sugar or low-carb diet would affect physical activity levels.
Previous studies from the University of Bath research teams found that participants were less active when they skipped breakfast or did alternate-day fasting.(2)(3)
But when they merely ate less food, without skipping meals or otherwise restricting when they could eat, they maintained their customary activity levels.
As with so many aspects of nutrition, the only way to find out if these results apply to you is to try it, learn from it, adjust as necessary, and repeat.
Closing Thoughts
This study adds another layer to our understanding of the relationship between diet, physical activity, and weight loss. While both low-carb and low-sugar diets led to some degree of weight loss, the differences in fat versus lean mass loss and the impact on gut health suggest that no one-size-fits-all approach exists.
Ultimately, the best diet is one you can stick with that aligns with your goals, preferences, and lifestyle. As fascinating as these results are, your journey to better health will always be uniquely yours. Experiment, adapt, and find what works for you—and remember that small, sustainable changes often have the biggest impact over time.
Michael Beiter
Personal Trainer
Nutrition, Sleep, Stress Management, Recovery Coach
References
Hengist, Aaron, Russell G. Davies, Jean-Philippe Walhin, Jariya Buniam, Lucy H. Merrell, Lucy Rogers, Louise Bradshaw, et al. 2024. Ketogenic Diet but Not Free-Sugar Restriction Alters Glucose Tolerance, Lipid Metabolism, Peripheral Tissue Phenotype, and Gut Microbiome: RCT._ Cell Reports. Medicine_ 5 (8): 101667.
Betts, James A., Judith D. Richardson, Enhad A. Chowdhury, Geoffrey D. Holman, Kostas Tsintzas, and Dylan Thompson. 2014. The Causal Role of Breakfast in Energy Balance and Health: A Randomized Controlled Trial in Lean Adults. The American Journal of Clinical Nutrition 100 (2): 539–47.
Templeman, Iain, Harry Alex Smith, Enhad Chowdhury, Yung-Chih Chen, Harriet Carroll, Drusus Johnson-Bonson, Aaron Hengist, et al. 2021. A Randomized Controlled Trial to Isolate the Effects of Fasting and Energy Restriction on Weight Loss and Metabolic Health in Lean Adults. Science Translational Medicine 13 (598): eabd8034.
