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What the Science Says About Weight Loss and Blood Sugar Control (and What It Doesn’t)

How ketogenic diets are defined in clinical research

In clinical research, ketogenic diets follow strict carbohydrate limits. In the randomized trial conducted by Westman and colleagues, participants assigned to the keto group consumed fewer than 20 grams of carbohydrates per day. Importantly, researchers did not prescribe calorie restriction, and participants were encouraged to eat until satisfied.

By contrast, the comparison group followed a low–glycemic index diet that provided approximately 55% of total daily energy from carbohydrates, alongside structured calorie reduction. Therefore, the two groups differed not only in carbohydrate intake but also in dietary philosophy.

Meanwhile, the meta-analysis defined ketogenic diets as very-low-carbohydrate diets providing 50 grams of carbohydrates per day or less, or roughly 10% of total energy intake. All included trials lasted at least 12 months, which allowed researchers to assess longer-term outcomes rather than short-term metabolic shifts.

Because of these definitions, many real-world “keto” approaches do not meet research standards. Consequently, outcomes reported online often diverge from those seen in clinical trials.

Effects on blood sugar control in type 2 diabetes

The 24-week randomized trial by Westman et al. focused on adults with obesity and established type 2 diabetes. Participants were randomly assigned to either a ketogenic diet or a low–glycemic index diet, and both groups received identical levels of education, supplements, and exercise guidance. As a result, differences in outcomes could be more confidently attributed to dietary composition.

Among participants who completed the study, hemoglobin A1c decreased by an average of 1.5 percentage points in the keto group. By comparison, A1c fell by 0.5 percentage points in the low–glycemic index group. Therefore, glycemic improvement was approximately three times greater in the keto group over the six-month period.

Weight loss followed a similar pattern. Participants following the ketogenic diet lost an average of 11.1 kilograms, whereas those in the comparison group lost 6.9 kilograms. Notably, this difference occurred despite the absence of calorie limits in the ketogenic group.

Medication use changed substantially as well. Specifically, 95.2% of participants in the ketogenic group reduced or eliminated diabetes medications, compared with 62.1% in the low–glycemic index group. In several cases, participants discontinued insulin entirely under physician supervision.

Importantly, the authors cautioned that carbohydrate restriction can rapidly lower blood glucose. Therefore, they emphasized that dietary changes of this magnitude are “quite powerful” and require careful monitoring, particularly for individuals using insulin or sulfonylureas.

Long-term ketogenic weight loss outcomes

While short-term trials offer insight into early metabolic changes, long-term data provide essential context. For this reason, the meta-analysis by Bueno and colleagues examined 13 randomized controlled trials lasting 12 months or longer.

Across these studies, participants assigned to ketogenic diets lost an average of 0.91 kilograms more than those assigned to low-fat diets. Although this difference appears modest, long-term weight loss is notoriously difficult to sustain. Consequently, even small average differences can be meaningful at the population level.

At the same time, both dietary approaches resulted in weight loss. Therefore, ketogenic diets did not dramatically outperform low-fat diets, but they showed a consistent advantage in average outcomes.

Lipid profiles and blood pressure: mixed effects

The meta-analysis also examined cardiovascular risk markers. Compared with low-fat diets, keto diets were associated with higher HDL cholesterol levels and lower triglyceride levels. These changes are often considered favorable.

However, LDL cholesterol increased on average in keto diet groups. In contrast to improvements in triglycerides and HDL, this rise in LDL represents a potential concern, particularly for individuals with elevated cardiovascular risk.

Blood pressure outcomes were less pronounced. Diastolic blood pressure declined slightly more in keto groups, while systolic blood pressure showed no meaningful difference between diets. Therefore, blood pressure effects appear modest and variable.

ketogenic diet: Adherence and variability over time

Across both studies, adherence emerged as a key limitation. In the long-term trials included in the meta-analysis, carbohydrate intake in ketogenic groups increased over time. As a result, many participants no longer met ketogenic thresholds by the end of follow-up.

Moreover, individual responses varied widely. Some participants experienced substantial improvements in weight, glycemic control, and lipid profiles. Others showed minimal change or unfavorable shifts in LDL cholesterol. Consequently, ketogenic diets do not produce uniform outcomes across individuals.

Interpreting the evidence responsibly

Taken together, these studies do not support claims that ketogenic diets are universally superior or inherently harmful. Instead, the evidence suggests a more nuanced conclusion.

On one hand, ketogenic diets can meaningfully improve glycemic control and support weight loss, particularly in people with type 2 diabetes. On the other hand, LDL cholesterol increases, adherence challenges, and medication risks require careful consideration.

Therefore, ketogenic diets function best as one dietary option among several, rather than a default recommendation.

What this evidence means in practice

In practice, ketogenic diets may offer benefits for individuals seeking improved blood sugar control, especially when medical supervision guides medication adjustments. They may also support modestly greater long-term weight loss compared with low-fat diets.

However, lipid monitoring remains essential, particularly for LDL cholesterol. Furthermore, long-term sustainability should be evaluated honestly, since adherence often declines over time.

Overall, these studies support individualized decision-making rather than one-size-fits-all dietary guidance.

References

Westman EC, Yancy WS Jr, Mavropoulos JC, Marquart M, McDuffie JR.
The effect of a low-carbohydrate, ketogenic diet versus a low-glycemic index diet on glycemic control in type 2 diabetes mellitus. Nutrition & Metabolism. 2008;5:36. doi:10.1186/1743-7075-5-36.

Bueno NB, de Melo ISV, de Oliveira SL, da Rocha Ataide T.
Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British Journal of Nutrition. 2013;110:1178–1187. doi:10.1017/S0007114513000548.

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