Paleo and low-carb diets: what the science says

There are a number of popular diet plans which promise a variety of health benefits. These include ‘low-carb’, ‘keto’ and ‘Paleo’ and are diets which all limit the consumption of carbohydrates to various degrees, replacing it with protein or fat, usually from animal sources.

A ketogenic diet is the most extreme form of ‘low-carb’ and is an eating plan that is very low in carbohydrate and very high in fat. It was originally designed to treat diabetes and epilepsy and still plays a role in the treatment of severe, treatment-resistant epilepsy with high rates of seizure control. The basic premise is that if the body does not have access to glucose, glycogen stores will be used up and the body will turn to an alternative fuel called ketones, which are produced to meet the body’s energy needs. It is a popular approach currently for weight loss. Stored fat is utilised for energy as there is limited availability of glucose/glycogen and hence weight loss occurs. True ketogenic diets have <5% of calories (around 20–25g per day of carbohydrates) from carbohydrates. More recent versions for weight loss have 5–20% of calories from carbohydrates. These diets have a potential for nutritional deficiencies due to the lack of whole plant foods — folate, vitamin C and K, magnesium, fibre. There are certainly some short term benefits for weight loss and diabetes control and with this some improvement in metabolic markers in the blood. However, the long-term data does not support this diet pattern as a healthy option, with a long list of complications, including increased risk of heart disease, cancer and increased mortality.

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It is unfortunate, in my view, that the UK continues to promote low-carbohydrate diets for the treatment of type 2 diabetes. I agree that the short-term data are compelling, partly because patients are encouraged to give up refined sugar and grains, resulting in reduced calorie intake and weight loss, which improves blood sugar control. In fact, any diet that results in weight loss will have benefits for diabetes control with the potential for remission. However, to exclude carbohydrates from whole plant foods, including whole grains, fruits and beans is storing up longer term problems.

Large prospective cohort studies confirm that low carbohydrate diets increase the risk of cardiovascular disease and death. A study published in the European Heart Journal in April 2019 examined the relationship between low carbohydrate diets, all-cause death, and deaths from coronary heart disease, cerebrovascular disease (including stroke), and cancer in a nationally representative sample of 24,825 participants of the US National Health and Nutrition Examination Survey (NHANES) during 1999 to 2010. Compared to participants with the highest carbohydrate consumption, those with the lowest intake had a 32% higher risk of all-cause death over an average 6.4-year follow-up. In addition, risks of death from coronary heart disease, cerebrovascular disease, and cancer were increased by 51%, 50%, and 35%, respectively. The results were confirmed in a meta-analysis of 7 prospective cohort studies with 447,506 participants and an average follow-up 15.6 years, which found 15%, 13%, and 8% increased risk in total, cardiovascular, and cancer mortality with low (compared to high) carbohydrate diets. One of the lead authors Professor Banach has been quoted saying: ‘Low carbohydrate diets might be useful in the short term to lose weight, lower blood pressure, and improve blood glucose control, but our study suggests that in the long-term they are linked with an increased risk of death from any cause, and deaths due to cardiovascular disease, cerebrovascular disease, and cancer.

A further paper from the Dr Kevin Hall’s team at the National Institute of Health has shed some light on why low carbohydrate diets, high in fat, might have a detrimental effect on health. He conducted a study in 17 participants who were admitted to a metabolic ward where all the food was provided, and detailed metabolic tests were performed. The study found that a ketogenic diet was associated with an increase in markers of inflammation and in LDL-cholesterol and did not improve insulin sensitivity or glucose regulation.

The National Lipid Association in the US has reviewed the evidence for low-carbohydrate and very low-carbodydrate diets (including ketogenic diets) for weight loss and cardiometabolic risk factors and written a statement based on their findings. These are the take home messages.

  • Low- and very-low-carbohydrate diets, including ketogenic diets, are not superior to other weight loss diets and may severely restrict nutrient-dense foods that offer cardiovascular benefits.
  • While they may have advantages on appetite and reducing triglyceride levels and diabetes medication use, current evidence showed mixed effects on low-density lipoprotein cholesterol levels.
  • There is no clear evidence for advantages related to other cardiometabolic risk markers.
  • While some patients prefer a low-carbohydrate eating pattern, which may be reasonable for short periods of time (<6 months), long-term compliance is challenging, and long-term benefits and risks are not fully understood.
  • Regardless of the weight loss strategy chosen, a patient-clinician discussion about the risks and benefits and patient preference is of vital importance, since many patients follow the diets without medical supervision, which can increase the risk of adverse effects.

Some further points of clarification include:-

  • There may be some short term advantages in preventing weight gain but the initial weight loss is due to water loss and there is a greater loss of lean muscle mass. There is no advantage over other diet patterns in the longer term.
  • Effects on blood cholesterol and LDL is variable with some having a rise in LDL due to high intakes of saturated fat. Triglycerides levels do fall and HDL levels rise, although the latter is not maintained in the longer term.
  • No advantage for type 2 diabetes when compared to low fat diets but there is a short term advantage in helping people get off diabetes medication. Short term gains do not translate to a greater benefit at 1 year.
  • Effects on blood pressure are inconsistent and most studies do not show an advantage for those with hypertension.
  • No advantage for lowering CRP, a marker of inflammation, and negative changes in the gut microbiome, including greater generation of TMAO, a compound associated with increased risk of chronic disease.
  • Low carb diets are associated with an increased risk of death, adverse side effects and people find it difficult to adhere to in the longer term.
  • AUTHORS CONCLUSIONS:patients should be encouraged to eat a diet composed of fruits, vegetables, whole grains, legumes, nuts and seeds due to consistent long term benefits for health. This diet pattern can also be lower carb if desired with the help of a dietitian’.

The reasoning behind the ‘low-carb’ approach for diabetes is that if you limit the availability of sugar in the diet, then blood sugar will remain stable and there will be a reduced requirement for insulin. Something that is often forgotten is that protein can stimulate insulin secretion and some protein-rich foods can stimulate insulin secretion to the same extent or more than some carbohydrate-rich foods. For example some protein and fat-rich foods (eggs, beef, fish, lentils, cheese) induce as much insulin secretion as some carbohydrate-rich foods (e.g. beef equal to brown rice and fish was equal to grain bread). Substituting carbohydrates for protein and fat, particularly if from animal sources, does not reduce insulin resistance beyond that achieved through weight loss. Therefore, if people with diabetes on a low-carb diet decide to eat a carbohydrate rich food, such as a banana, their blood sugar will still become abnormally elevated. In contrast, a high carbohydrate, high fibre diet in people with diabetes has been shown to reduce insulin requirements and, in some, reverse diabetes even without weight loss, thus demonstrating the power of whole plant foods to reverse insulin resistance.

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The Paleo diet is one that attempts to mimic the diet of our Paleolithic ancestors. The premise of this diet is that what humans ate in pre-agricultural times is the diet best suited to our genetic make-up. Paleo advocates believe that the shift from a hunting and gathering diet (rich in wild meat and wild plants) to an agricultural diet (rich in grains and legumes) gave rise to the obesity and chronic disease epidemics. The diet includes eating meat, poultry, fish, eggs, vegetables, fruits, nuts, and seeds and avoiding processed foods, grains, legumes, and dairy products. It contains more protein and carbohydrates (around 100g) than low-carb diets, but is still a high fat diet. It also includes more plant foods than the ketogenic/low-carb diet and is therefore is of better quality, including more fibre, prebiotics, phytochemicals, plant sterols, antioxidants. It is often healthier than the typical Western diet pattern as it avoids highly processed foods, refined carbohydrates, fried foods, and fast foods, and encourages some fresh fruits and vegetables, nuts, and seeds. However, the diet is high in animal fat and protein, thus increasing the risk of developing chronic disease.

The nutrient content of today’s popularised Paleo diet does not really resemble the actual Paleolithic diet, in which people ate wild plants, animals, fish and insects and in fact did have the tools and skills to process plant foods. The balance of these foods varied with time-frame, location, season, hunting and gathering skills, tools and culture. This was a long historical period. During the first 80% of the Paleolithic period, human’s were mostly vegetarian and also did not consume the milk of other species. In the last 20% of the period they ate a more hunter/gatherer diet, with more gathering than hunting. Nutritional anthropologists have estimated the nutrient intakes of prehistoric humans and it was much closer to a vegan diet in terms of the macro- and micronutrient intakes than the modern Paleo diet. In the current Paleo diet, protein, vitamin A, and zinc intakes are similar to the historic diet, but fat and saturated fat levels are about double, cholesterol almost triple, and sodium five times higher. In addition, current paleo diets contain about a third of the carbohydrates, and half the vitamin C, calcium, and fibre of the original. Fibre consumption in the true Paleolithic diet was around 100g/day. The pre-agricultural diet would certainly not have included any processed meats.

The reason for some of these differences is because the meat and vegetables consumed today are very different from that eaten in the Paleolithic era. The wild animals eaten then provided around 6% to 16% of calories from fat compared to about 40% to 60% in today’s farmed animals and the meat from these wild animals had a higher omega-3 fatty acid content. They were also free of hormones, antibiotics, and environmental contaminants. All animal organs were consumed, and insects provided significant amounts of protein. In addition, virtually all fruits and vegetables available in supermarkets are more palatable, more digestible, and easier to store and transport than their wild cousins, at the expense of certain nutrients. Wild plants provide about four times the fibre of commercial plants.

Grains and legumes are avoided in the current Paleo diet, even though study after study have shown these foods promote health and longevity and prevent chronic disease. People who live in regions of the world known as the Blue Zones — the longest lived, healthiest populations in the world — all consume legumes and whole grains as part of their traditional diet. Paleo advocates claim that these foods weren’t part of Paleolithic-era diets, but new research suggests otherwise. They also argue that lectins naturally present in these starchy foods are harmful to human health. Consuming too many lectins can cause significant gastrointestinal distress. However, because legumes and grains are almost always consumed in a cooked form — and lectins are destroyed during cooking — eating beans and grains doesn’t result in lectin overload.

Listen to this talks by Christina Warinner on Debunking the Paleo Diet

In summary, the keto/low-carb and Paleo diets are far removed from the international consensus on an optimal diet. There may be some short-term benefits, but the longer term data do not support their role in promoting health or treating disease and there are genuine concerns about long term safety.

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