Q&A: Low-Carb Diets: Effective for Weight Loss but at What Cost?

​Take-home messages:

Introduction 🍽️

We derive energy from foods containing carbohydrates, proteins and fats, collectively known as macronutrients. The primary principle for achieving weight loss is to consume less energy than is expended, creating an energy deficit that prompts the body to utilise its fat stores. This is why weight loss can occur on any diet that restricts portion sizes, specific foods or overall energy intake, whether it be low-carbohydrate, low-fat or balanced (1), as well as through exercise programmes that burn energy, with or without dietary restrictions. Diets that exclude particular foods or entire food groups, or emphasise single foods, can be challenging to maintain long-term due to their lack of flexibility. Most diets reduce access to energy-dense, highly palatable foods, which aids in weight loss. While any diet that restricts energy intake can lead to weight loss, it is essential to consider the associated disease risks, whether the diet meets nutritional needs and its sustainability for long-term success. Adherence to restrictive diets is often low outside of controlled research settings, affecting their real-world effectiveness.

In the nutrition community, low-carbohydrate diets—which limit carbohydrate intake in favour of increased protein, fat, or both (such as Atkins or Keto diets)—along with low-fat and balanced diets, are widely debated due to their potential impact on health and disease (2). Normally, the body uses glucose (derived from carbohydrates) as its primary energy source. However, when carbohydrates are restricted, the body shifts to using fatty acids and ketones for fuel (3). This metabolic state, known as ketosis, occurs because the lack of glucose forces the body to break down stored fat into fatty acids, which are then converted into ketones for energy. This shift is one reason why low-carbohydrate diets are heavily debated in terms of their long-term health effects.

Here, I will present evidence supporting the effectiveness of low-carbohydrate diets for weight loss, while also discussing the potential health consequences that dieters should consider.

Effectiveness of Low-Carb Diets on Weight Loss: Evidence from Studies Summarising Weight Loss Trials 🏋️‍♀️

When interpreting results from low-carbohydrate diets trials one must consider the significant water weight loss due to glycogen depletion, which can affect body weight results and that must be properly accounted for. In their meta-analysis Nordmann and other researchers (4) compared the effects of low-carbohydrate diets followed for 6 months without energy restriction versus energy-restricted low-fat diets on weight loss. They concluded that low-carbohydrate, non–energy-restricted diets are at least as effective as low-fat, energy-restricted diets in inducing weight loss.

Similarly, Bueno et al. (5) summarised 13 studies that compared the long-term effects of very-low-carbohydrate ketogenic diets (with no more than 50 g of carbohydrates per day) and low-fat diets. Their findings showed that individuals on very-low-carbohydrate ketogenic diets experienced greater weight loss than those on low-fat diets.

In another meta-analysis (6) low-carbohydrate diets, were shown to be effective for weight loss, with significant reductions in body weight at both 6-month (8.73 kg) and 12-month (7.25 kg) follow-ups. However, the differences in weight loss between low-carb and low-fat diets were minimal, reinforcing the idea that adherence to the chosen diet plays a more crucial role in achieving weight loss than the specific macronutrient composition. Similarly, Naude and colleagues (7) summarised 19 good quality randomised controlled trails and concluded that weight loss could be achieved irrespective of whether the diet is low in carbohydrates or balanced. 

Overall, the evidence suggests that low-carbohydrate, low-fat and balanced diets are all effective in reducing weight, with adherence being the key factor in long-term success.

Tailoring Diets Based on Insulin Sensitivity and Genetics: Personalised Weight Loss Strategies 🧬

In the table below I summarise findings from various studies on the relationship between diet, insulin sensitivity and genetics. The key takeaway is that insulin sensitivity and genetic factors like the IRS1 genotype can influence how effective certain diets are for weight loss and insulin resistance improvement. Tailoring diets based on these factors may lead to better outcomes, particularly with low-carbohydrate diets being more beneficial for insulin-resistant individuals. However, in some cases(8), genetics and insulin secretion were found to have no significant effect on weight loss between different diets.

Table: Summary of Studies on Diet, Insulin Sensitivity, and Genetic Factors

Health Implications of Low-Carb Diets ❤️

When selecting a weight loss strategy, it’s crucial to consider not only the immediate effects on body weight but also the broader health impacts. Different diets, particularly those with low-carbohydrate approaches, can influence various aspects of health, including cardiovascular risk factors, metabolic health and even mortality. Therefore, dieters must balance weight loss goals with the potential long-term health consequences of their chosen dietary approach.

Cholesterol Changes and Cardiovascular Health

Nordmann and colleagues (4) warned against the unfavourable changes in low-density lipoprotein cholesterol (the bad cholesterol) values when consuming a low-carbohydrate diets to induce weight loss. Bueno et al. (5) found that the very low carbohydrate diet decreased triglycerides (triacylglycerol) and diastolic blood pressure, but increased both the good (HDL) and the bad (LDL) cholesterol. 

Animal vs. Plant Protein in Low-Carb Diets

Song et al. (13) showed that high animal protein intake was positively associated with cardiovascular mortality and high plant protein intake was inversely with all-cause and cardiovascular mortality. Therefore, dieters considering following a high protein diet could substitute animal protein especially that from processed meat and red meat with plant protein because they are linked to increased cancer, cardiovascular and type 2 diabetes risk (14-16).

Low-Carb Diets and Type 2 Diabetes Risk

A recent study investigated the association between a low carbohydrate diet score and the incidence of type 2 diabetes in a large Australian cohort (17). The findings indicated that a higher low carbohydrate diet score was associated with an increased risk of type 2 diabetes, largely mediated by obesity, suggesting that low carbohydrate diets may elevate type 2 diabetes risk, especially in individuals predisposed to obesity (17).

Mortality Risk and Macronutrient Balance

In a summary of observational studies low-carbohydrate diets were associated with a significantly higher risk of all-cause mortality but not with cardiovascular mortality (18). Likewise, Seidelmann et al. (19) found that both high and low percentages of carbohydrate diets were associated with increased mortality, with minimal risk observed at 50–55% of total energy provided by carbohydrates. Similar to Noto and colleagues, Seidelmann et al. (19) observed that low carbohydrate dietary patterns favouring animal-derived protein and fat sources (lamb, beef, pork and chicken), were associated with higher mortality, whereas those that favoured plant-derived protein and fat intake (from sources such as vegetables, nuts, peanut butter and whole-grain breads) were associated with lower mortality.

Cognitive Effects of Very Low-Carb Diets

Furthermore, a study by Wing et al. (20) showed that a very low carbohydrate ketogenic diet reduced weight, but worsened higher order mental processing and flexibility impairing problem solving. However, more research on the cognitive effects of diets are needed to understand the relationship better.

Importance of Macronutrient Quality

It is not just the quantity (% contribution to total energy intake) but also the quality (type and nature) of macronutrients like carbohydrates that are critical in weight loss diets (7) and these profoundly affects health outcomes of dieters. Substituting refined carbohydrates, such as sugary snacks and white bread, with high-quality alternatives—such as whole grains, legumes, fruits, and vegetables, which provide more fibre, nutrients, and slow-releasing energy—is always a smart strategy. Similarly, replacing unhealthy fats, such as trans fats and excessive saturated fats found in processed foods, with healthy fats from sources like nuts, seeds, avocados and olive oil, can s enhance cardiovascular health and overall well-being.

The Role of Balanced Diets and Macronutrient Quality in Sustainable Weight Management⚖️

The set point theory suggests that the body has a natural weight range, or "set point," that it tends to maintain, making it difficult to sustain weight loss or gain outside of this range. This is why maintaining weight loss can be challenging, and the more drastically a diet is altered to achieve weight loss, the easier it may be to regain weight once the diet is stopped. A balanced diet that meets the body’s micronutrient needs—providing 50–55% of total energy from high-quality carbohydrates (complex carbs with low glycaemic indexes), protein primarily from plant sources, and healthy fats such as mono-unsaturated fatty acids (MUFAs) and poly-unsaturated fatty acids (PUFAs)—appears to be the most effective for long-term health. Adopting balanced dietary changes and increasing physical activity levels are recommended both for achieving weight loss and for keeping the weight off long-term (21).

Conclusion 🥗

While low-carbohydrate diets have demonstrated effectiveness in promoting weight loss, particularly in the short term, their long-term health implications and sustainability remain debated. Evidence supports that weight loss can be achieved with various dietary approaches—whether low-carbohydrate, low-fat, or balanced—provided there is adherence to the chosen regimen. However, individual factors such as insulin sensitivity and genetics can significantly influence the effectiveness of these diets, particularly for insulin-resistant individuals who may benefit more from low-carbohydrate approaches.

Importantly, beyond weight loss, the quality of macronutrients plays a critical role in determining long-term health outcomes. Substituting refined carbohydrates with nutrient-dense, high-fibre alternatives and replacing unhealthy fats with healthier options can contribute to better cardiovascular health and overall well-being. Ultimately, a balanced diet that includes good-quality carbohydrates, proteins, and fats, combined with physical activity, offers the most sustainable approach to weight management and long-term health.

Reflection Exercises 🤔

·         Reflect on your current diet. How much of it is composed of carbohydrates, proteins, and fats?

·         Consider your own or someone else's weight loss journey. Were specific diets like low-carb or low-fat used? Reflect on how successful these diets were in terms of both weight loss and long-term maintenance.

·         Reflect on how much you know about the importance of macronutrients. How has your understanding of carbohydrates, proteins, and fats changed after reading about their role in both weight loss and overall health?

·         Reflect on the association between high animal protein intake and cancer and cardiovascular risks. How could substituting plant-based proteins in your diet impact your long-term health?

·         Reflect on how a very low-carb diet could affect cognitive functions, such as problem-solving and flexibility. Have you ever noticed a change in mental performance when following a specific diet?

·         Reflect on the idea of balance in a diet. How do you currently balance your carbohydrate, protein and fat intake? What changes could you make to ensure you are meeting your micronutrient requirements?

·         Consider the challenges of maintaining weight loss after dieting. Have you or someone you know experienced weight regain after a restrictive diet? Reflect on why balanced diets may be more sustainable.

References📝

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2.         Ornish D. Was Dr Atkins right? Journal of the American Dietetic Association. 2004;4(104):537-42.

3.         Westman EC, Feinman RD, Mavropoulos JC, Vernon MC, Volek JS, Wortman JA, et al. Low-carbohydrate nutrition and metabolism. The American journal of clinical nutrition. 2007;86(2):276-84.

4.         Nordmann AJ, Nordmann A, Briel M, Keller U, Yancy WS, Brehm BJ, et al. Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials. Archives of internal medicine. 2006;166(3):285-93.

5.         Bueno NB, de Melo ISV, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British journal of nutrition. 2013;110(7):1178-87.

6.         Johnston BC, Kanters S, Bandayrel K, Wu P, Naji F, Siemieniuk RA, et al. Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. Jama. 2014;312(9):923-33.

7.         Naude CE, Schoonees A, Senekal M, Young T, Garner P, Volmink J. Low carbohydrate versus isoenergetic balanced diets for reducing weight and cardiovascular risk: a systematic review and meta-analysis. PloS one. 2014;9(7):e100652.

8.         Gardner CD, Trepanowski JF, Del Gobbo LC, Hauser ME, Rigdon J, Ioannidis JP, et al. Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial. Jama. 2018;319(7):667-79.

9.         Cornier MA, Donahoo WT, Pereira R, Gurevich I, Westergren R, Enerback S, et al. Insulin sensitivity determines the effectiveness of dietary macronutrient composition on weight loss in obese women. Obesity research. 2005;13(4):703-9.

10.       Pittas AG, Das SK, Hajduk CL, Golden J, Saltzman E, Stark PC, et al. A low-glycemic load diet facilitates greater weight loss in overweight adults with high insulin secretion but not in overweight adults with low insulin secretion in the CALERIE Trial. Diabetes care. 2005;28(12):2939-41.

11.       Qi Q, Bray GA, Smith SR, Hu FB, Sacks FM, Qi L. Insulin receptor substrate 1 gene variation modifies insulin resistance response to weight-loss diets in a 2-year randomized trial: the Preventing Overweight Using Novel Dietary Strategies (POUNDS LOST) trial. Circulation. 2011;124(5):563-71.

12.       McClain AD, Otten JJ, Hekler EB, Gardner CD. Adherence to a low‐fat vs. low‐carbohydrate diet differs by insulin resistance status. Diabetes, Obesity and Metabolism. 2013;15(1):87-90.

13.       Song M, Fung TT, Hu FB, Willett WC, Longo VD, Chan AT, et al. Association of animal and plant protein intake with all-cause and cause-specific mortality. JAMA internal medicine. 2016;176(10):1453-63.

14.       Farvid MS, Sidahmed E, Spence ND, Mante Angua K, Rosner BA, Barnett JB. Consumption of red meat and processed meat and cancer incidence: a systematic review and meta-analysis of prospective studies. European journal of epidemiology. 2021;36:937-51.

15.       Kennedy J, Alexander P, Taillie LS, Jaacks LM. Estimated effects of reductions in processed meat consumption and unprocessed red meat consumption on occurrences of type 2 diabetes, cardiovascular disease, colorectal cancer, and mortality in the USA: a microsimulation study. The Lancet Planetary Health. 2024;8(7):e441-e51.

16.       Wang DD, Li Y, Nguyen X-M, Ho Y-L, Hu FB, Willett WC, et al. Red Meat Intake and the Risk of Cardiovascular Diseases: A Prospective Cohort Study in the Million Veteran Program. The Journal of Nutrition. 2024;154(3):886-95.

17.       Kabthymer RH, Karim MN, Itsiopoulos C, Hodge AM, De Courten B. Association of low carbohydrate diet score with the risk of type 2 diabetes in an Australian population: A longitudinal study. Diabetes & Metabolic Syndrome: Clinical Research & Reviews. 2024:103049.

18.       Noto H, Goto A, Tsujimoto T, Noda M. Low-carbohydrate diets and all-cause mortality: a systematic review and meta-analysis of observational studies. PloS one. 2013;8(1):e55030.

19.       Seidelmann SB, Claggett B, Cheng S, Henglin M, Shah A, Steffen LM, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. Lancet Public Health. 2018;3(9):e419-e28.

20.       Wing R, Vazquez J, Ryan C. Cognitive effects of ketogenic weight-reducing diets. International journal of obesity and related metabolic disorders: journal of the International Association for the Study of Obesity. 1995;19(11):811-6.

21.       Macedo RC, Santos HO, Tinsley GM, Reischak-Oliveira A. Low-carbohydrate diets: Effects on metabolism and exercise–A comprehensive literature review. Clinical nutrition ESPEN. 2020;40:17-26.

In developing this work, the author utilised ChatGPT-4 to assist with language editing, not for content generation.