Adherence to a diet is crucial (Johnston et al., 2014; Freedhoff and Hall, 2016; Bray and Siri-Tarino, 2016). An objective way to measure ketogenic adherence is via ketone levels (Nymo et al., 2017; Cipryan et al., 2018). If ketones increase, we know that the person is restricting carbohydrates. If ketone levels fall, then participants are probably not adhering to the diet.

Show Scientific Quotes

“Because different diets are variably tolerated by individuals, the ideal diet is the one that is best adhered to by individuals so that they can stay on the diet as long as possible.” - Johnston et al., 2014 “ … no diet has yet been shown to be uniformly easier to stick with than another in the long run.” - Freedhoff and Hall, 2016 “[Adherence] is likely a function of psychological issues (e.g., frequency of dietary counseling, coping with emotional eating, group support) rather than macronutrient composition, per se ... Being conscious of one’s behaviors, using social support, confronting problems directly, and using personally developed strategies may enhance long-term success ....” - Freedman et al., 2001

Studies with normal weight, diabetic, or obese participants

After doing a systematic search of the ketogenic literature, we identified ten suitable studies (read more about our inclusion and exclusion criteria). You can see data from these ten studies in the line chart below.

Eight groups never reached ketosis above 0.5 mmol/l. Four were in borderline ketosis (0.3–0.49 mmol/L BHB) by the end of the study. Only two groups were clearly in ketosis at one point during the study.

An interesting trend is that ketone levels decrease over time.

Figure: every line represents a group of people who were aiming to achieve ketosis in various studies. The solid blue lines were in ketosis at least once during the study. Dotted grey lines were in borderline ketosis. Dotted red lines were not in ketosis at the end of the studies. We chose studies that reported BHB levels in the blood. Every line represents one keto group from a study. Note that every study aimed for a carb level that would lead to ketosis (≤70 g per day).

These data suggest that people, primarily diabetics and the obese, struggle to adhere to the ketogenic diet.

Note: some of our reviewers have pointed out that decreasing BHB levels are expected, because they are an adaptation to the diet. They are not an indication of poor adherence. This observation is currently anecdotal and we could not find any published data on this adaptation.

Though, it should also be said that most keto scientists use ketone levels as an objective marker of adherence (see scientific quotes below). Sci-Fit is currently writing an article on this topic.

Show Scientific Quotes

“The data indicated imperfect compliance with the prescribed carbohydrate restriction, but the mean carbohydrate intake of the HPLC group was less than 40 g/day, an intake likely to maintain ketosis. The 13 week serum β-hydroxybutyrate concentrations support this: 2.28 ± 0.34 versus 1.0 ± 0.12, for the HPLC and LF groups, respectively (p = 0.002).” - Krebs 2010 “Compliance to dietary treatment was also confirmed by a change in plasma ketones between diets. VLCARB produced higher plasma levels of ketones (β hydroxybutyrate and acetoacetate) than the VLF or HUF diet treatments (P < 0.01), indicating adherence to a very low carbohydrate intake during the study (Figure 3). Despite continued apparent compliance to the diet plasma ketones declined with time.” - Noakes 2006 “Objective measures of compliance Diet: Participants met every week for an individual 20 min consultation with a dietician, to review their food records. Urine acetoacetic acid concentration was also measured weekly, using Ketostix reagent strips. Participants who were not ketotic on more than one occasion were considered not compliant and were excluded from the analysis. Concentration of plasma β-hydroxybutyric acid (β-HB) in the fasting state was also measured with a Ketone Body Assay Kit (Mark134, Sigma-Aldrich, St Louis, MO, USA) at baseline, day 3, 5 and 10% WL and weeks 9 and 13.” - Nymo 2017 “Compared to T0 (127.0 ± 57.6 μ mol/L), total serum ketones increased 4-fold during the intervention (561.6 ± 317.9 and 489.5 ± 285.9 μ mol/L for T3 and T6, respectively; P < .001). (…) taken together with the food record, results are evidence that participants were adherent to the dietary recommendations and markedly reduced carbohydrate intake.” - Ballard 2013 “Although compliance with the diets was assessed primarily by dietary records, these data are supported by more objective measures. For example, the average 3-month weight loss in the low fat diet group (∼4 kg) is what would be expected for individuals decreasing their daily caloric consumption by about 400 kcal (28), approximately the restriction these women reported making. In addition, there was a significant correlation between reported changes in caloric intake and weight loss (r = 0.41; P < 0.001). Finally, the presence of measurable ketonemia and ketonuria in the very low carbohydrate group is consistent with severe carbohydrate restriction and was not seen in the low fat dieters.“ - Brehm 2003 “To maintain significant plasma ketosis requires very limited carbohydrate intake, and this measure has been the hallmark of adherence to these diets.” - Brehm 2005 “All subjects in the HFMP group demonstrated β - hydroxybutyrate concentrations above 0.20 mmol·L –1, indicating compliance with the HFMP diet.” - Fleming 2003 “Dietary analysis, compliance, and physical activity On the basis of the results of the food records, the participants showed good compliance with the prescribed diets.” (...) During the initial stages of the study, plasma β-hydroxybutyrate concentrations increased significantly more in the LC diet group than in the LF diet group, and, although concentrations decreased over time in the LC diet group, they remained higher than those in the LF diet group throughout the intervention (Figure 2), which indicated adherence to a low-carbohydrate intake in the LC group.” - Brinkworth (B) 2009 “Evidence of the adaptation and compliance to VLCHF diet is usually evaluated by the βHB concentration in plasma.” - Cipryan 2018 “Dietary analysis and compliance Adherence to the dietary interventions was established by concentrations of ketone bodies, the ratio of urinary urea to creatinine, and the dietary analysis data.” - Keogh 2008

Studies in elite athletes, metabolic wards, and with close follow-up

There are also several studies that had tight control over what the participants ate. We didn’t include these in the primary analysis because they do not tell us anything about real-life adherence (external validity).

In other words, we can’t tell if people can adhere to a diet if they are being fed pre-planned meals in a hospital while being monitored. Yet, these tightly controlled studies are very useful if we want to figure out mechanisms related to ketosis (internal validity). For example, if we wanted to test how the ketogenic diet affects energy expenditure, we would prefer to do it in a metabolic ward.

In these types of studies, people have much higher BHB levels:

Here are ketone graphs from two of these studies:



Figure: Hall et al., 2016. Ketones remain elevated in a metabolic ward

Figure: Wilson et al., 2017. Ketones remain elevated for two months before refeeding carbohydrates. Note, this is not a metabolic ward study.

The metabolic ward studies show us that ketones can stay elevated when participants are “forced” to adhere to the diet. This is contrasted with the free living studies where participants’ BHB levels typically drop over time. The difference seems to be adherence.

If there is an adaptation that decreases BHB levels over time, it has not shown itself in these studies. At the very least, we must reflect on why BHB levels are consistently higher in metabolic wards, feeding studies, and studies with motivated athletes.