For years, we have been told that weight loss is mostly a numbers game: eat fewer calories, lose weight, and your health will improve. The particular route you take is often treated as a matter of personal preference, rather like choosing between Google Maps and Waze.
A new randomised clinical trial suggests the route may matter more than we thought.
Researchers compared three very different eating patterns in people with obesity, prediabetes and fatty liver disease:
- A very-low-carbohydrate ketogenic diet
- A Mediterranean diet
- A very-low-fat, plant-forward diet
The clever part was that the researchers carefully adjusted each participant’s food intake so that everyone lost approximately the same amount of weight: around 10% of their starting body weight. In other words, this was not a competition to see which diet produced the greatest weight loss. It was designed to answer a much more interesting question:
If people lose the same amount of weight, does what they eat still make a difference?
The short answer? Yes, yes it does.
All three diets helped, or did they?
Fifty-five participants were randomly assigned to one of the three diets, and 42 completed the study. It took them around four to five months to lose the targeted 10% of their body weight.
All three groups experienced meaningful benefits. Body fat decreased, and muscle insulin sensitivity improved by approximately 50% across the board.
That is worth emphasising. Weight loss itself clearly mattered, regardless of which diet produced it. But the metabolic story did not end with the bathroom scale.
Participants following the ketogenic diet experienced greater improvements in several important markers of metabolic health, particularly those involving the liver and blood glucose regulation.
Liver fat decreased by:
- 67% with the ketogenic diet
- 43% with the Mediterranean diet
- 46% with the very-low-fat, plant-forward diet
That is a considerable difference, especially because each group lost virtually the same amount of weight. The ketogenic group also experienced a two- to three-fold greater improvement in liver insulin sensitivity, along with larger reductions in HbA1c, fasting glucose, insulin, fasting triglycerides and the amount of new fat being produced by the liver.
Despite the higher fat and saturated fat content of the ketogenic diet, there were no significant differences between the groups in LDL cholesterol, apolipoprotein B, 24-hour triglyceride concentrations, or blood pressure after weight loss.
The liver, it seems, was paying attention not only to how much food was arriving, but also to what was on the plate.
What happened to prediabetes?
Perhaps one of the most striking findings involved prediabetes.
After the intervention, the proportion of participants who no longer met the study’s criteria for prediabetes was:
- 50% in the ketogenic group
- 29% in the Mediterranean group
- 7% in the very-low-fat, plant-forward group
This was a small study, so we need to resist the urge to turn those figures into universal promises. Nutrition science has suffered enough from headlines that sprint several kilometres ahead of the evidence. Nevertheless, the results suggest that carbohydrate restriction may offer benefits beyond weight loss alone for people already showing signs of insulin resistance and fatty liver disease.
What’s really helping your liver?
When carbohydrate intake is high, glucose and insulin stimulate the liver to manufacture fat through a process called de novo lipogenesis. Elevated insulin also makes it more difficult for the body to release and use stored fat. In this study, the ketogenic diet produced the greatest reductions in glucose and insulin and increased the ratio of glucagon to insulin. Together, these changes appear to have reduced the production of new fat in the liver while encouraging the liver to break down and use the fat already stored there.
This matters because fatty liver is not simply an innocent passenger accompanying excess weight. It is closely connected to insulin resistance, type 2 diabetes and cardiovascular risk.
A liver carrying too much fat is rather like a kitchen cupboard into which someone keeps shoving groceries: eventually, the door will no longer close and the problem begins spilling into the rest of the house.
So, does the type of diet matter?
This was an unusually well-controlled nutrition study. The researchers supplied all meals and snacks, participants met with a dietitian every week, and reported adherence exceeded 95%. Weight loss was deliberately matched across the groups, allowing the researchers to examine the effect of diet composition more clearly.
That level of control is a major strength, although it also means the study was quite different from everyday life, where meals do not usually arrive in neatly labelled containers, and a dietitian does not appear every week to keep us on the straight and narrow.
The study was also small, only 42 participants completed it, and it lasted several months rather than several years. All participants had obesity, prediabetes, and fatty liver disease, so the findings cannot automatically be applied to everyone.
The study does not tell us that weight loss is unimportant. Quite the opposite: losing approximately 10% of body weight improved metabolic health in all three groups.
What it does tell us is that weight loss may not be the whole story.
Two people can lose the same number of kilograms while experiencing very different changes inside the liver, bloodstream and endocrine system. The scale may report the final weight, but it remains remarkably quiet about liver fat, insulin sensitivity and triglycerides.
For people with obesity, prediabetes and fatty liver disease, a well-formulated very-low-carbohydrate diet may therefore offer therapeutic advantages that extend beyond simply eating fewer calories and becoming lighter.
Perhaps it is time we stopped asking only, “Did the diet produce weight loss?” and started asking the more useful question:
What did it do to the person’s metabolic health?