If you’ve been on social media recently, you’ve probably seen competing support for low-carbohydrate, ketogenic diets versus plant-based diets. Supporters argue that the benefits of “their” diet  include helping to maintain your weight while the other leads to increased hunger, overconsumption of calories, and ultimately, excess body weight. But who’s right? Until now, no one actually knew. A landmark study is one of the first to actually compare the two diets under very strict conditions.

The researchers found that participants ate more on an animal-based, ketogenic, low-carbohydrate diet compared to a plant-based, low-fat diet – even though the two diets were matched on calories, protein and non-starchy vegetables. The components of the diets obviously differ from the higher proportion of carbohydrates that largely characterise plant-based diets to the higher proportion of fat that makes up the majority of ketogenic diets.

What was even more interesting was that participants on the keto diet ate more calories even though this diet led to a greater increase in blood ketone levels and a greater reduction in glucose and insulin levels compared to the plant-based diet.

Ah, now we’ve got your attention! So what did researchers of this study do to shed more light on these contrasting diets?
They admitted 20 adults into a clinic for four weeks, where the researchers could meticulously control participants’ environment and food intake. Metabolic outcomes were also measured down to the finest detail, including blood ketones, glucose, insulin and lipids (e.g. cholesterol), blood pressure, body weight and composition and physical activity.

Participants were advised that this was not a weight loss study, so they should not try to lose weight. They also wore loose clothing and were not privy to their daily weight, ketone and glucose measurements. These strategies reduced the chance of participants altering their behaviour (i.e. how much they ate) to be more in-line with expected outcomes.

It is important to emphasise here that this type of study is considered the Rolls-Royce of clinical research, especially in the nutrition space where it is often difficult to fully capture what participants are eating versus what they say they are eating.

After being randomly assigned to one of the two diets, participants were instructed to eat as much or as little as they desired of either the keto diet or the plant-based diet for a period of two weeks. After this initial two-week period, participants swapped groups to have the alternative diet for another two weeks.

What did the participants eat?
The percentage of protein was similar between the diets at ~14%. However, the ketogenic diet contained ~10% carbs versus ~75% fat, which was the opposite to the plant-based diet with ~75% carbs and ~10% fat.

Perhaps unsurprisingly, over 80% of calories came from animal products in the keto diet, as well as containing higher levels of salt, whereas the plant-based diet was primarily made up of plant-based products and contained greater levels of fibre.

Example images of an animal-based, ketogenic, low-carbohydrate meal (zucchini pasta with meat sauce) versus a plant-based, low-fat meal (spaghetti with vegan pasta sauce)
Image Source: Hall KD, et al. (2020) osf.io/preprints/nutrixiv/rdjfb

What did the researchers more specifically find?
The keto diet led to greater blood ketone levels (which is thought to suppress appetite), whereas the plant-based diet led to elevated after-meal glucose and insulin levels (which is thought to induce food cravings).

Surprisingly, however, the keto diet prompted participants to eat ~700 more calories (~2800 calories) per day than the plant-based diet. More calories consumed in the keto diet occurred despite a greater mass of food being eaten in the plant-based diet and despite there being no reported difference in participants’ ratings of appetite or enjoyment of meals.

While the keto diet resulted in increased calorie consumption, the macronutrient composition of the consumed foods and beverages was as expected.

Although carb and fat intake obviously differed between the diets, protein intake was higher during the keto diet, both in absolute terms (+136 calories per day) as well as when expressed as a fraction of the energy consumed (15.5% with keto versus 14.0% with plant-based).

In terms of body weight, even though participants on the keto diet had more rapid weight loss during the first week compared to the plant-based diet, and increased calorie consumption overall, total weight loss was similar in both groups at the end of the diets.

Results of body composition assessments help explain how total weight loss between the keto and plant-based diets were similar despite differences in calorie consumption. Body composition assesses all body components, such as fat and ‘fat-free mass’ (i.e. water, bone, organ and muscle content).

Only the plant-based diet resulted in significant loss of body fat, whereas only the keto diet led to loss of fat-free mass. The researchers suggested that weight loss from cutting carbs in the keto diet may be largely due to the excretion of body fluids, such as water, gastrointestinal contents and glycogen (i.e. a form of glucose that serves as an energy store in the body).

During both diets, participants also experienced beneficial reductions in fasting blood glucose, insulin and markers of inflammation. However, participants on the plant-based diet experienced a reduction in blood cholesterol levels, blood pressure and pulse rate, whereas those on the keto diet did not.

Interestingly, from the start to the end of both diets, it was only the keto diet that led to lower levels of fasting triglycerides, whereas the plant-based diet gave rise to increased concentrations. A greater level of triglycerides (both in the fasting and after-meal state) is thought to increase the risk of cardiovascular disease.

What does all this mean?
These results go some way to challenging assumptions about these diets, including the premise that keto diets contain high-glycaemic carbs that increase after-meal insulin levels, which in-turn causes increases in body fat, hunger and overeating.

Although the plant-based diet in the current study was made up of foods with higher glycaemic load that increased after-meal glucose and insulin levels compared to the keto diet, participants on the plant-based diet consumed significantly fewer calories.

What can’t this study tell us?
Because the two diets were short-term (two weeks each), it is still unclear whether long-term intake of either diet would lead to different results.

It is hard to know whether the differences in body fat and blood levels were due to the different make-up of macronutrients between the diets (i.e. high-fat, low-carb versus low-fat, high-carb) or due to the difference in calories consumed per day (~700 calories more during the keto ketogenic diet). Also, it remains unclear why higher levels of fasting blood triglycerides resulted after the plant-based diet.

Participants were also instructed to eat as much or as little of each diet as they wanted, so it remains unknown whether results would hold up if individuals were pursuing active weight loss. This is particularly unknown in a context where individuals may undertake such diets in a home setting rather than a clinical environment.

What should we do?
This impressively robust study suggests that plant-based diets may benefit appetite control, whereas keto diets may benefit after-meal glucose and insulin levels.

But, before you hedge your bets, if you see something on social media saying one diet is better than another, it may be worth keeping in mind that it is often much more complex than simply pitting macronutrient profiles, such as high-fat, low-carb diets vs. high-carb, low-fat diets, against one another. More evidence from rigorous studies such as this is what is needed to inform our ideas about which diet might be best for ourselves or others.