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Ketogenic (or keto diet) diets are one of the latest dietary trends to sweep the diet market. The idea is you reduce your carbohydrate intake to below 50g per day allowing your body to shift mostly to fat as it’s primary energy source.
During usual metabolism the muscles and organs use a combination of glucose and fat as it’s main energy sources. The central nervous system on the other hand uses only glucose as it’s fuel source.
During periods of very low carbohydrate intake glucose is unavailable meaning our body’s need to think fast. Luckily it has an answer. Our body’s begin metabolising more fatty acids for energy.
Usually fat metabolism follows a sequence of events which ultimately provides energy. During periods of low glucose availability this sequence is disrupted. Some of the fat is converted to glucose in the liver to help with glucose supply. With the usual process of fat breakdown disrupted the liver needs to find an alternative way of meeting it’s fat/energy quota. It does this by producing what we call ketone body’s.
Ketone body’s are an alternative energy source to glucose made from fat breakdown. These can fuel the central nervous system and take up the slack in times of low glucose. When ketone body’s are high you are said to be in ‘ketosis’.
It is this process the keto diet manipulates. Ketosis is a natural process usually seen during periods of starvation, low carbohydrate diets, in type 1 diabetes when insufficient insulin is present and in some cases during excessive alcohol intake or exercise.
Supporters of such diets argue because we can manage without glucose intake it is not an essential nutrient. Others argue glucose is so important we have to be able to make it from fat and protein sources. However, such arguments are based on theory and so I like to delve into the evidence and see if there is anything to this.
Do they work?
The research into keto diets is unfortunately limited. Most studies I found directly looking into keto diets consisted of very small numbers. These studies were also for relatively short periods.
Most of what I found from these trials was keto diets actually appear superior for weight loss in the short term. This is particularly true when compared against low fat diets. However, the Mediterranean diet wasn’t far behind the keto diet for weight loss outcomes.
This is consistent with previous research I have done on low carbohydrate diets. Therefore, I can say with modest certainty low carbohydrate diets appear to be more beneficial for losing weight over the short term.
Some of these studies also looked at cholesterol and triglycerides to see if keto diets had any effect on these markers of health. The worry is when rapidly increasing your fat intake in place of carbohydrates there may be scope to increase your bad cholesterol and blood lipids.
Contrary to popular belief the diets looked as though they actually reduced bad cholesterol, improved good cholesterol and reduced triglycerides. This is another argument supporters of keto diets use as evidence of their safety. Often they will say no direct evidence exists between high fat intakes and cardiovascular disease.
However, keep in mind the people studied here also lost a significant amount of weight. Therefore, it’s more likely these improvements are down to the weight loss rather than the keto diet itself. Longer term data is needed before we can draw conclusions, which fortunately I found.
The long term effect of keto diets
I found 2 much larger studies looking at more longer term effects of diets with varying amounts of carbohydrates in the diet.
The first study I looked at was called the ‘PURE study’. This didn’t actually look at keto diets but instead focused on the effect of high carbohydrates diets. This study looked at 135,000 people over 18 countries over an median time span of 7.4 years.
Although still not massively long term the authors found a direct relationship between higher carbohydrate intakes and increased mortality and cardiovascular events (bad things). They also reported no adverse consequences of replacing carbohydrates with fat and protein foods from animal sources and/or saturated fat.
The temptation here is to then infer that low carbohydrate diets must be better for you. It is this kind of thinking where a lot of pseudoscience pops into existence.
When you actually delve a little deeper into this study a few cracks emerge. First there was no breakdown of the type of carbohydrates reported by the participants. You may be thinking with 135,000 people there’s probably a broad spectrum. However, many of the countries and people studied were from lower socioeconomical areas or countries. This means the majority of their dietary intake was from inexpensive refined carbohydrates. Something we already know is not good for our health.
The authors also only looked at carbohydrate intakes of 30% and above of total dietary intake. So although they looked at slightly lower intakes of carbohydrates compared to current recommendations, nothing resembled a keto diet.
Why is this relevant? Well, first of all this study doesn’t answer our question about the safety of very low carbohydrate diets. Second, they found no ill effects of replacing carbohydrates with saturated fat. The problem is though as carbohydrates is further reduced more reliance is placed upon saturated fat and animal products. This is particularly relevant with keto diets where potentially most of the diet will consist of animal based protein and fat and therefore be high in saturated fat.
For me what this study shows is eating high intakes of refined carbohydrates is not good for your health or lifespan. Replacing refined carbohydrates with saturated fat sources at modest carbohydrate intakes has no ill effects over a 7-8 year period.
But what about our question regarding keto diets?
I found one more study published after the PURE study. This study looked at 15,000 people over 25 years. So certainly more long term. The nice thing about this study was it looked at carbohydrate intakes across the whole spectrum.
Here’s what this study found.
If your not a graph person let me explain this to you. Harzard ratio on the left looks at risk of health hazards. If the hazard ratio is above 1 it means greater risk and if it is below 1 it means lower risk. A hazard ratio of exactly 1 means no greater risk.
As you can see the greater risk for all cause mortality (death by all causes) is actually greatest in carbohydrate intakes of under 20%. This is more representative of the keto diet.
As the percentage of total carbohydrate intake increases there is a steady drop in risk until around 50% of total energy intake. Here the participants were at no greater risk of dying. As carbohydrates start to make up more of the total energy intake above 55-60% the risk begins to increase again (but it may also reduce risk as you can see from the graph).
The researchers also found one crucial point. The risk of low carbohydrate diets was only if replacing carbohydrates with animal sources of fat and protein (meat, cheese, butter etc). When replacing with plant protein and fat (beans, pulses, lentils, nuts, oils etc) the risk was removed.
What does this mean?
There’s quite a few things to take from this research. The first thing I took from it was low carbohydrate diets may be superior in the short term for weight loss. Studies tend to be small but there’s probably enough small studies now to form a general consensus on this. However, more sustainable diets like the mediterranean diet are not far behind in terms of weight loss.
The second thing I took from this is eating at extreme ends of macronutrient intakes is likely bad for health. Having too few or too many carbohydrates appears risky for long term health outcomes. Therefore, a more balanced approach is likely optimal.
Interestingly, very low carbohydrate diets under 20% of total energy intake had the worst all cause mortality outcomes. This was if replacing the carbohydrates with animal sources which is the case with keto diets. When reducing carbohydrates to ketosis levels you will also need to sacrifice nutritious foods such as fruit and vegetables. Remember, these foods have many benefits including vitamins and minerals and are a great source of fibre.
This risk was eliminated if replacing carbohydrates with plant based protein and animal sources. Such diets would unlikely be ketotic because many plant sources of protein and fat are accompanied by carbohydrate. They also provide vitamins, minerals and fibre.
A personal note:
For me keto diets are short term and lock people into one dimensional thinking usually about weight loss or blood glucose levels. They forget the bigger picture that cutting out entire food groups may not be overly healthy even if it induces weight loss.
If I told you to stop eating for 2 weeks or to only eat eggs for a month you’d also lose weight and improve your blood glucose levels if you have type 2 diabetes. Does this mean it is some revolutionary approach - no.
From the research I have done the mediterranean diet appears to be one of the better dietary approaches. This is quite a balanced approach and is actually quite similar to the UK dietary advice we provide.
Some critics of UK dietary advice suggest it is the dietary guidance itself which is responsible for the obesity epidemic. They say low fat advice and reliance on carbohydrates is responsible for the problems we see today.
Let me address this quickly before signing off.
First of all, the UK dietary advice recommends 50% of total energy intake comes from carbohydrates. As we can see from the above, this might just be the sweet spot. Furthermore, it does not recommend this comes from refined carbohydrates. Instead we recommend wholegrain, fibrous sources. 15% should come from protein foods ideally choosing oily fish and red meat/alternatives once weekly and aim for 5 a day minimum of fruit and vegetables.
For the low fat critics out there. The UK dietary advice also recommends 35% of total energy intake comes from fat. So I would argue hardly ‘low’ fat. It is more about choosing the correct sources of fat (good fats) and choosing ‘lower’ fat options when the source is from bad fats. This keeps one simple message.
The advent of inactivity, fast food, high sugar food, refined carbohydrates etc is another conversation but UK dietary guidelines do not endorse these either. So UK dietary guidelines are actually quite consistent with the mediterranean diet. They also ensure you get all your essential vitamins and minerals.
Therefore, a balanced approach seems best. However, if you want to kick start some weight loss an interesting method may be to cycle between periods of lower carbohydrate intake and more sustainable approaches such as the med diet. You can then repeat this as often as you like.
I will do another blog specific to keto diets diabetes as I think this is a topic in itself. I hope this has been useful.
A Ketogenic Diet Favorably Affects Serum Biomarkers for Cardiovascular Disease in Normal Weight Men
Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study
Beyond weight loss: a review of the therapeutic uses of very-low-carbohydrate (ketogenic) diets.
Body Composition Changes After Very-Low-Calorie Ketogenic Diet in Obesity Evaluated by 3 Standardized Methods.
Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis.
Dietary carbohydrate restriction as the first approach in diabetes management: Critical review and evidence base
Effects of a high-protein ketogenic diet on hunger, appetite, and weight loss in obese men feeding ad libitum.
Ketosis and appetite-mediating nutrients and hormones after weight loss
Long term effects of ketogenic diet in obese subjects with high cholesterol level
Long Term Successful Weight Loss with a Combination Biphasic Ketogenic Mediterranean Diet and Mediterranean Diet Maintenance Protocol
Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials
Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet