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Are Low-Carb Diets Deficient in Micronutrients?

A common criticism of low-carb and keto diets is that they do not supply adequate amounts of essential nutrients, but is this justified?

Assessing the nutrient intake of a low-carbohydrate, high-fat (LCHF) diet: a hypothetical case study design

Caryn Zinn, Amy Rush, Rebecca Johnson

BMJ Open 2018;8:e018846

http://dx.doi.org/10.1136/bmjopen-2017-018846

Abstract

Objective 

The low-carbohydrate, high-fat (LCHF) diet is becoming increasingly employed in clinical dietetic practice as a means to manage many health-related conditions. Yet, it continues to remain contentious in nutrition circles due to a belief that the diet is devoid of nutrients and concern around its saturated fat content. This work aimed to assess the micronutrient intake of the LCHF diet under two conditions of saturated fat thresholds.

Design 

In this descriptive study, two LCHF meal plans were designed for two hypothetical cases representing the average Australian male and female weight-stable adult. National documented heights, a body mass index of 22.5 to establish weight and a 1.6 activity factor were used to estimate total energy intake using the Schofield equation. Carbohydrate was limited to <130 g, protein was set at 15%–25% of total energy and fat supplied the remaining calories. One version of the diet aligned with the national saturated fat guideline threshold of <10% of total energy and the other included saturated fat ad libitum.

Primary outcomes 

The primary outcomes included all micronutrients, which were assessed using FoodWorks dietary analysis software against national Australian/New Zealand nutrient reference value (NRV) thresholds.

Results 

All of the meal plans exceeded the minimum NRV thresholds, apart from iron in the female meal plans, which achieved 86%–98% of the threshold. Saturated fat intake was logistically unable to be reduced below the 10% threshold for the male plan but exceeded the threshold by 2 g (0.6%).

Conclusion 

Despite macronutrient proportions not aligning with current national dietary guidelines, a well-planned LCHF meal plan can be considered micronutrient replete. This is an important finding for health professionals, consumers, and critics of LCHF nutrition, as it dispels the myth that these diets are suboptimal in their micronutrient supply. As with any diet, for optimal nutrient achievement, meals need to be well-formulated.1

Comment

A common criticism of ketogenic and low-carbohydrate diets is that they lack essential nutrients. This criticism lacks evidence.

In this study, Caryn Zinn and colleagues at AUT evaluated a hypothetical meal low-carbohydrate (<130 g carbohydrate per day) plan designed for an average man and woman.

View the meal plans here

Omega 3 intake

The omega 3 fatty acid intake in the low-carb diet plans exceeded recommended minimum intakes by 400-1200% from alpha-linolenic acid alone, with an almost ‘perfect’ omega 6:3 ratio of between 2:1 and 3.5:1.

Fibre

It is also commonly suggested that low-carbohydrate diets are restrictive for fibre intake. As this study demonstrated, a low-carbohydrate diet that is focussed on nutrient-dense foods like vegetables, berries, and other whole foods (in other words, a ‘good’ diet!) exceeds the recommended intakes for fibre by approximately 50%.

Essential vitamins and minerals

All vitamins and minerals were supplied in amounts exceeding the nutrient reference values recommended intakes with the exception of iron for the women’s diet plan. In this plan, 16 mg of iron was provided per day, in contrast to the daily recommended intake of 18 mg.

All vitamins and minerals were supplied in amounts exceeding recommended intakes with the exception of iron

Protein intake in this study was sufficient, and protein sources used were mixed (for example, the protein for major meals being primarily derived from either fish or beef) but perhaps a greater focus on beef and lamb for women is desirable to improve iron status of the diet overall?

perhaps a greater focus on beef and lamb for women is desirable to improve iron status of the diet overall?

How does this compare to ‘usual’ diets?

In New Zealand, government data shows2:

  • Around 20% of people fail to get enough vitamin A, B1 and B6
  • 8% of people fail to get enough B12
  • Nearly 10% of women don’t get enough iron
  • Around 25% of people don’t consume enough zinc, and 40% of males may not get adequate zinc from their diet
  • 45% of people don’t get enough Selenium, a mineral lacking in New Zealand soils

People are not on balance getting sufficient amounts of the health and performance-supporting micronutrients from the ‘average’ diet.   

What does this all mean?

There is no good reason why a low-carbohydrate diet should not be replete in all the essential nutrients. Typically, when they are, it is because the diet is either unnecessarily restrictive, insufficient in total food volume and energy, or simply founded on poor-quality, processed and refined foods. The same though could be said about any diet. The standard ‘western-style’ diet consumed in Australia and New Zealand, similarly to the US, is deficient in one or more of the essential vitamins and minerals and lacks sufficient protein, omega 3 fats, and fibre and resistant starch to enable people to truly thrive.

Take home points

  • Any diet can be deficient in nutrients if highly energy-restricted
  • Any diet can lack sufficient nutrients if based on processed and refined foods
  • A low-carbohydrate diet can be replete in all essential nutrients
  • Any diet, whether low-carb or higher- should be based on a foundation of nutrient-dense, whole foods

References

1.            Zinn C, Rush A, Johnson R. Assessing the nutrient intake of a low-carbohydrate, high-fat (LCHF) diet: a hypothetical case study design. BMJ open. 2018;8(2):e018846.

2.            University of Otago and Ministry of Health. A Focus on Nutrition: Key findings of the 2008/09 New Zealand Adult Nutrition Survey. Wellington; 2011.

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