The Pros and Cons of Low-Carb Diets

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This guest post was written by Dr Harriet Holme (MA Hons Cantab, MBBS MRCPCH, PhD, RNutr). Harriet studied medicine at the University of Cambridge and has over a decade of experience as a paediatric doctor. Additionally, she is a Registered Nutritionist and has a PhD in genetics from University College London. Harriet now uses these uniquely developed skills for the benefit of her clients and students, consulting as a Registered Nutritionist and lecturing in culinary science and nutrition. Follow @healthyeatingdr on Instagram for more nutrition facts.

Dr Harriet Holme

This article explores the hot topic that is low-carbohydrate diets (A.K.A. low-carb diets). Before deciding whether a low carb diet plan is right for you, it is important to consider the pros and cons of this approach. 

What Are Low-Carb Diets?

Low-carb diets involve consuming less than 26% of daily energy from carbohydrates, compared to government healthy eating guidelines which advise about 50% of daily energy from carbohydrates (1).

There are lots of slightly different low-carb diets in the media such as the ‘keto’ and ‘Atkins’ diets, which have gained popularity due to short-term weight loss. 

The central aim of these diets is to exclude or reduce carbohydrates (sugars and starches) while focusing on eating more fat and protein to compensate.

Foods and drinks that are high in carbohydrates are excluded from low-carbohydrate diets, these include:

  • Sugary foods and drinks
  • Grains e.g. cereals, bread, pasta, rice, etc. 
  • Legumes  e.g. beans, peas and lentils
  • Root vegetables e.g. potatoes, sweet potatoes and butternut squash
  • Many types of fruit e.g. bananas, mango and pineapple
  • Alcohol e.g. beer
  • Fruit juice and dried fruit 

What Is Ketosis and Why Does It Matter?

Ketosis is a metabolic state which occurs to those following a very low-carb diet plan (i.e. including only 5-10% of energy from carbohydrates).

Instead of breaking down or ‘burning’ carbohydrates (the body’s preferred energy source), cells are forced to break down fat instead.

When this occurs, specific chemicals, called ketone bodies or ketones are formed, and this is why it is called ketosis or ketogenesis. So this type of low carbohydrate diet mimics the fasting state.

The ketogenic diet (or the keto diet for short) was developed for children with treatment-resistant epilepsy, as a short term measure to get their seizures under control.

The diet was designed to be used for a maximum of 2 years and to be stopped early if seizure control was not achieved.

However, this diet has become popular in recent years with the aim of promoting weight loss. For more information about the keto diet specifically, check out this post

Pros of Low-Carb Diets:

There is evidence that short-term use of these diets may lead to weight loss, but no evidence of superior weight loss in the long term (2-3).

Better short term control of blood sugar in type 2 diabetes has been associated with a low-carb diet, but again there is insufficient evidence of long term benefit (3-4).

Low-carb diets which are high in plant-based protein and plant-based fats have been shown to be associated with decreased mortality and cardiovascular risk (9).

There has been some debate that a ketogenic diet might be beneficial for cancer. There have now been many preclinical research studies performed, on just cells, and also using animal models of cancer, but the results are mixed (5). This has now progressed to clinical studies of patients with cancer, most notably investigating the effect of ketogenesis alongside standard treatment for a type of brain tumour called glioblastoma (6-8).

At present, it is certainly not advised for patients with cancer to start a ketogenic diet without discussion with their oncologist.

Cons of Low-Carb Diets:

Some people get the ‘keto flu’ when they follow a very low-carbohydrate diet, with symptoms such as diarrhoea, tiredness, cramps and headaches. 

The brain preferentially uses sugars from healthy carbohydrates, lack of these can lead to confusion and irritability.

Eating a limited range of food (especially fruit, vegetables and wholegrains) can lead to micronutrient deficiencies. While fruit and vegetables contain natural sugars, they also contain fibre, vitamins, minerals and antioxidants in the form of polyphenols. 

So a low-carb diet can lead to a lower fibre intake which can increase the risk of constipation, negatively impact the microorganisms which live in the gut and contribute to an increased risk of bowel cancer (11).

Low-carb diets can often become high in saturated fat, which can lead to an increase in Low-Density Lipoprotein (LDL), known commonly as the bad cholesterol. This can increase the risk of fatty liver and cardiovascular disease (9-10).

A restrictive diet is difficult to maintain, and therefore any weight loss is likely to be temporary. Furthermore, a lot of the initial weight loss due to reducing carbohydrate intake tends to be water weight.

Data from a large meta-analysis (grouped analysis of a number of studies) and long term prospective study which followed people for over 20 years, found that a low-carb diet, where less than 40% of daily energy was coming from carbohydrate, was associated with a higher risk of early death (9). However, within this data set, there were two very different groups. For those people who substituted carbohydrates with plant-based fat and protein, they had lower mortality and cardiovascular risk (9). While those people who substituted carbohydrate with animal-based fat and protein had a higher risk of mortality and cardiovascular risk (9).

This restriction also risks creating an unhealthy and obsessive relationship with food.

Should I Try a Low-Carbohydrate Diet?

While eating a diet rich in fat might sounds appealing if you wish to lose weight in the short term, I would advocate that a more balanced diet of wholegrain carbohydrates, fruit and vegetables is a less restrictive way of achieving long-term health.

However, as discussed above there are health benefits associated with a low-carb diet which is high plant-based fat and protein (9).

Different dietary approaches work for different people, so following a low carb diet plan can be helpful in some cases.

If you have any medical conditions you should consult with your doctor or dietitian before trying a restrictive approach like this.  

For more information on the different clinical uses for low-carb diets, check out this post


  1. Oh R & Uppaluri KR (2020) “Low Carbohydrate Diet” [Accessed May 2020 via:]
  2. Brouns, F. Overweight and diabetes prevention: is a low-carbohydrate-high-fat diet recommendable? Eur J Nutr 57, 1301–1312 (2018).
  3. Snorgaard, O., Poulsen, G. M., Andersen, H. K. & Astrup, A. Systematic review and meta-analysis of dietary carbohydrate restriction in patients with type 2 diabetes. BMJ Open Diabetes Res Care 5, e000354 (2017).
  4. Meng, Y., Bai, H., Wang, S., Li, Z., Wang, Q. & Chen, L. Efficacy of low carbohydrate diet for type 2 diabetes mellitus management: A systematic review and meta-analysis of randomized controlled trials. Diabetes Res. Clin. Pract. 131, 124–131 (2017).
  5. Khodadadi, S., Sobhani, N., Mirshekar, S., Ghiasvand, R., Pourmasoumi, M., Miraghajani, M. & Dehsoukhteh, S. S. Tumor Cells Growth and Survival Time with the Ketogenic Diet in Animal Models: A Systematic Review. Int J Prev Med 8, 35 (2017).
  6. van der Louw, E. J. T. M., Olieman, J. F., van den Bemt, P. M. L. A., Bromberg, J. E. C., Oomen-de Hoop, E., Neuteboom, R. F., Catsman-Berrevoets, C. E. & Vincent, A. J. P. E. Ketogenic diet treatment as adjuvant to standard treatment of glioblastoma multiforme: a feasibility and safety study. Ther Adv Med Oncol 11, 1758835919853958 (2019).
  7. Klement, R. J., Brehm, N. & Sweeney, R. A. Ketogenic diets in medical oncology: a systematic review with focus on clinical outcomes. Med Oncol 37, 14–12 (2020).
  8. Weber, D. D., Aminzadeh-Gohari, S., Tulipan, J., Catalano, L., Feichtinger, R. G. & Kofler, B. Ketogenic diet in the treatment of cancer – Where do we stand? Molecular Metabolism (2019).
  9. Seidelmann, S. B., Claggett, B., Cheng, S., Henglin, M., Shah, A., Steffen, L. M., Folsom, A. R., Rimm, E. B., Willett, W. C. & Solomon, S. D. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health 3, e419–e428 (2018).
  10. Mansoor, N., Vinknes, K. J., Veierød, M. B. & Retterstøl, K. Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials. Br. J. Nutr. 115, 466–479 (2016).
  11. Aune, D., Chan, D. S. M., Lau, R., Vieira, R., Greenwood, D. C., Kampman, E & Norat, T. Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies. BMJ 343, d6617–d6617 (2011).


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Founder of The Food Medic

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Maeve has written extensively for NHD magazine over the last few years, producing a wealth of dietetic and nutritional articles. Always evidence based and factual, Maeve creates material that is relevant and very readable. She provides high quality work with a professional and friendly approach. Maeve is a beacon of high quality knowledge and work within the nutrition writing community; and someone NHD magazine is proud to work with.

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