Updated: Sep 28
There is a lot of interest in low-carbohydrates, for anything from weight loss to improved cognitive function. However, it’s important to recognise that there is no agreed definition of a low-carbohydrate diet. This is important because the degree of carbohydrate restriction determines any physiological effects; and so does the replacing food or macronutrient (i.e. when you take out carbs, what are you replacing it with, if anything?).
Currently, some definitions are used in the scientific and lay literature and they are: A low carbohydrate diet is <26% calories per day (typically less than 130g) while a low carbohydrate diet is <10% of calories (typically less than 50g of carbohydrate a day). There is no official consensus behind these definitions and none of these cut-offs are based on physiology (i.e. we don’t know that reducing carbohydrate intake below these cut-offs will definitely improve insulin sensitivity or lower blood glucose).
Furthermore, a carbohydrate is an umbrella term for lots of different dietary components: starch, sugar (including lactose and fructose naturally found in milk and fruit) and fibre. 50g of fibre is going to have very different effects on blood glucose compared to 50g of starch.
Finally, the replacing foods or macronutrient is important too: the studies available suggest that reducing carbohydrate and increasing protein probably lowers blood glucose more than reducing carbohydrates and increasing fat intake. Here’s a link to my study which is seeking to understand this more conclusively.
We also need to consider what we mean by “best” diet…best for what? For weight loss, gut health, mental well-being, cholesterol, blood pressure…? A diet which could be fantastic at lowering blood glucose (ie, if you ate nothing but bacon fried in butter…) could be terrible for your gut health, mental health, lipids, vascular health and so on.
So how can we integrate all these ifs and buts into a diet which you might choose to eat? Below attempts to form a summary of the data so far:
Undoubtedly there are multiple ways to lose weight and keep it off, but the most important factor is that someone likes and can follow a diet long-term. Nevertheless, there is some intriguing data which suggests that the ketones which are produced when a person restricts carbohydrate may be able to suppress the hormone ghrelin. This is the hormone which tells us we’re hungry, and its concentration in the blood usually increases when we lose weight (i.e. it makes us hungrier!) Two studies have show that when people are in nutritional ketosis (i.e. very few carbohydrates) that the usual weight loss-induced increase in ghrelin is not observed. This might explain why some people on a low-carbohydrate diet say they do not feel hungry anymore. However, other data suggest this suppression of gherlin does not last long term. People on low-carbohydrate diets probably also increase their protein intake and this might help people feel fuller as well.
Blood glucose management
The higher protein intake on a low-carbohydrate diet may also explain why this type of diet can be effective at lowering blood glucose. The amino acids in protein can help the pancreas to produce insulin. Studies in which carbohydrate in the diet is reduced and protein is increased show a reduction on glucose concentrations, even in the absence of weight loss.
Diets in which carbohydrate is reduced and protein is increased also show a reduction in liver fat compared to a higher carbohydrate and lower protein diet. The amino acids in protein protect against liver fat deposition under conditions which normally lead to excessive liver fat, such as high-calorie, high-sugar diets. The good news is that it does not seem to make a difference whether the added protein comes from animal or plant sources.
Polycystic ovarian syndrome
In polycystic ovarian syndrome it is thought that insulin makes the ovaries produce too much of the male sex hormones. Therefore a low-carbohydrate diet has been proposed as an effective dietary approach because reducing the carbohydrate content of a meal can reduce the amount of insulin produced. Results from studies are not completely consistent but in general a diet which reduces the glycemic load (like a low carb diet does) seems to improve menstrual regularity and lower male sex hormones compared to other diets.
Is there anything I need to be concerned about if I follow a low-carbohydrate diet?
Many people including doctors are concerned about low-carbohydrate diets because they are inevitably high in fat. It’s important to remember that it’s the type of fat which matters for long-term health not the amount. A diet high in polyunsaturated and monounsaturated fat from plant courses including olive oil, nuts, seeds, avocado, sunflower oil plus oily fish can promote good health. In contrast a diet high in saturated fats from foods such as red meat, butter, lard and cream can probably increase LDL cholesterol markedly in some people. So if you choose to follow a low-carbohydrate diet, it’s sensible to prioritise foods high in unsaturated fats and limit saturated fats: this way you get all the benefits of a low-carb diet and reduce any risks as much as possible.
Other concerns are that low-carbohydrate diets limit plant foods too much and are inevitably low in fibre. This does not have to be true at all, and my advice to patients in clinic is to increase their intake of non-starch vegetables on a low-carbohydrate diet to help make a meal look more appetising and to help fill them up. The fibre and plant-based content of the diet is probably very important for keeping a healthy gut microbiome, and a meat-only diet is probably not optimal for promoting long-term gut health. (Note the probably here is because 1) this is an emerging field 2) there is large interpersonal variability in gut microbiome and 3) many studies have important confounding factors).
An abundance of low-starch, nutrient-rich foods can be consumed on a low-carbohydrate including nuts, seeds, nearly all vegetables, olives, berries, and other low-glycemic fruits. The so-called phytonutrients (basically naturally-occurring chemicals) in these foods may help to explain why just supplementing with vitamins and minerals alone does not achieve the same health benefits as a varied, plant-rich diet.
If a low-carbohydrate diet is a dietary pattern you can enjoy eating, it can be a great choice to help you manage your weight or type 2 diabetes. As is true of all diets, whatever macro-manipulations you choose to make, make sure you get the majority of fat from unsaturated sources, and enjoy a variety of plant-based foods to optimise all aspects of your health.