We also acknowledged that the two most studied methods for turning back the metabolic clock on type 2 diabetes were bariatric surgery, and very low calorie diets. These are similar in that both involve a period of eating very little, but of course surgery involves an invasive procedure and in many cases irreversible or hard to reverse physical changes.
As I’m a nutritionist and clinical exercise physiologist and not a surgeon or medical doctor by training, I’ll leave the discussion of bariatric surgery behind and focus on what can be done without an operating theatre.
So this week we’ll start to look at very low calorie diets, what they are, what they do in terms of helping to improve and sometimes reverse type 2 diabetes, and how we think they work. As it’s a big topic, this will become a little miniseries of posts over the next few weeks.
What is a very low calorie diet?
There is no absolute definition for a very low calorie diet, so for practical purposes I’m going to say it’s between 300-1000 kcal/day. Most of the studies discussed below used liquid meal replacements to ensure adequate amounts of essential nutrients.
The truth is that very low calorie diets in type 2 diabetes are nothing new. So called ‘starvation diets’ were being used to treat people with diabetes at least as far back as 1915 before the discovery of insulin (1).
In this article we’ll look at most (I couldn’t get every paper) of the more recent work (2-21). For those of you interested in specific details about individual studies such as how many people were included, their age and body mass index (BMI), diabetes duration and treatment, and specific diet, click here for Table 1 in pdf format.
All the studies I discuss below enrolled people diagnosed with type 2 diabetes, and in general they confirmed this diagnosis as part of the study.
If you look at the table 1, you’ll notice that most of the studies had fewer than 20 people. This counts as a pretty small number, but that’s because this kind of study is expensive and time consuming to run. The more closely people are monitored and the more involved the methods used, the more expensive a study is.
Looking at the table, you might also notice that the volunteer groups in these studies varied quite a bit from study to study. Some studies looked at people with a recent diagnosis of type 2 diabetes on minimal or no medication, while other studies included people on insulin who had had diabetes for over a decade.
The BMIs also vary from mildly overweight to clearly obese even if you quibble about BMI as an indicator of body fat.
The studies also differ in duration from a few days to a few months. That’s why they also differ in weight change.
What effects did very low calorie diets have on blood glucose?
To see the results of individual studies, click here for Table 2 in pdf format.
All the studies showed a clear reduction in fasting blood glucose, and/or HbA1c. What’s worth knowing is that these improvements often came despite volunteers being taken off insulin and/or diabetes medication.
Few studies showed a complete normalisation of fasting blood glucose. To put this in context though, most studies that involved a very low calorie diet for more than a week produced average fasting blood glucose results that were below the 7.9 mmol/L that many organisations consider diagnostic for type 2 diabetes.
HbA1c also went down accordingly. As severe calorie restriction also reduces carbohydrates relative to normal recommended diets, this comes as no surprise.
From most doctors' perspectives, if they follow standard guidelines, many of the volunteers in these studies went back to just having impaired fasting glucose, or what some call prediabetes. Some also reversed their diabetes completely.
That’s a heartening result when you consider that some of these studies involved people who had relatively poorly controlled type 2 diabetes for many years. There was more to be undone than could be achieved in a few weeks or even months, but huge progress was made, and it's a shame the studies were limited by time or weight reduction targets.
Three studies reported having some or all of their participants doing exercise (6,8,19), but only one study included a diet only and diet + exercise group for direct comparison (8).
Interestingly, people who also exercised lost more body fat in total and also a higher proportion of body fat than those just doing the very low calorie diet (8). That means they kept more of their lean mass, probably muscle, which we’ll see in future posts is a good thing. What isn’t clear from this study is how much exercise helped with glucose control.
What happens with very low calorie diets?
An obvious result of very low calorie diets is weight reduction (see table 2), and more importantly fat reduction. The interesting thing is that the benefits often kick in before there’s been a big weight change.
Take a look at the graphs below to see weight change and then change in fasting blood glucose over time in the few studies that measured these at multiple time points. Have a close look at the x-axis (along the bottom), you’ll see it is evenly spaced.
The study with the highest blood glucose at the start only lasted seven days, but we still see a rapid and drastic improvement in blood glucose despite volunteers being taken off metformin and/or having their insulin doses adjusted down (18).
The middle study in the orange and red lines compared lower and higher carbohydrate diets (19). I wouldn’t read too much into the small differences you see, because: 1) at 1000 kcal/day the study was higher in calories than most of these kinds of studies; 2) the carbohydrate content was above that of very low carbohydrate diets in both groups; 3) this was in a group of Japanese volunteers only in the overweight category, whereas most of other relevant studies were done in those well into the obese category.
All that said, we still see a nice rapid reduction in the first two weeks, followed by a slower reduction in the third and fourth week (19).
In the next blog post we’ll look more under the hood of these studies to see what might explain the findings, and what that might mean for you.
This is all inspiring stuff, but these kind of diets need to be done with the help of an informed medical team. There’s a reason why many of the studies involved taking people off their medication and/or insulin. With too much medication, and how much is too much is an individual thing, there is a very real risk of blood glucose going dangerously low.
These diets and the associated weight reduction can also lower blood pressure by a lot. We’ll discuss that in future posts. For now just be aware that some people also have to adjust their blood pressure medication to avoid dangerously low blood pressure.
I have a simple rule I wish everyone understood: natural or synthetic, diet, exercise, or medication – if it has an effect it can have side-effects.
Click here to read part 2.
References: click here for a full list of references.
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