Tuesday, January 02, 2018

What Are People Eating (And Not Eating) While Following A Low-Carb Diet?

Photo by Laurel F
Today's guest post comes from Dr. Nicola Guess who I invited to write about her recent study which explored what people on low-carb diets were and weren't eating. The tl;dr version? At least according to their dietary recall, they ate more whole foods, and far less in the way of liquid sugars and combination high-fat, high sugars foods (the ultra-processed stuff) - a potentially more sating pattern, and definitely not one that raises any alarm bells. And just an FYI, Dr. Guess is also very active on Twitter and is definitely worth a follow.
Carbohydrate restriction can be very helpful in the management of type 2 diabetes, particularly with controlling blood sugar levels. However, there have been several concerns cited about low-carbohydrate diets which I think hinders the widespread recommendation of this dietary approach for management of type 2 diabetes. For example, people might be limiting nutrient-rich foods such as fruits, vegetables and pulses. Others might be consuming a very narrow range of foods such as diets characterised by sausages, burgers and butter, with the occasional fried tomato thrown in. Despite the apparent popularity of low-carbohydrate diets, I realised that outside of clinical trials where the diet is prescribed by researchers, very little is actually known about what foods people eat when restricting dietary carbohydrates.

I therefore sought to examine this question by using national dietary surveys in the UK. Earlier this year I examined the National Diet and Nutrition Survey and found that people on a reduced carbohydrate diet (fewer than 40% of kilocalories from carbohydrate) consumed more red and processed meat, butter, oily fish and vegetables, as well as a lower consumption of soft drinks and pulses than people on a higher carbohydrate intake. I could not examine the intake on a low-carbohydrate diet (of which a suggested definition is fewer than 26% of calories from carbohydrates) because only 15 people out of 2263 followed this pattern.

Fortunately, the UK BIOBANK offered the opportunity for me to examine this question in a larger dataset. The UK BIOBANK is a huge population study in the UK which has collected comprehensive demographic, biochemical and lifestyle data in over 500,000 people. Dietary data was collected using a short food frequency and dietary habits questionnaire on the first visit. This basically asked how often people have different types of meat, oily fish, fruits, vegetables and alcohol, and also what type of spread, cereal and bread people consumed. In addition, a 24-hour dietary recall was completed on up to five further occasions. A 24-hour dietary recall asks the participant what food or drinks they consumed over the previous 24-hours, including the portion size of each food or drink consumed.

Using this dataset I was able to compare the dietary intake of people consuming fewer than 26% of kilocalories from carbohydrates to those consuming 45% or more. Confirming the findings from my earlier study, I found that a low-carbohydrate dietary pattern was associated with a significantly higher intake of red meat, oily fish, nuts and seeds but fewer fruits, vegetables and pulses compared to the normal carbohydrate group. In general the consumption of nutrient-rich foods such as nuts, seeds, oily fish and pulses were low in both groups. However, most striking was the difference in the intake of high-sugar, high-fat foods and snacks such as doughnuts, biscuits, cakes, pastries and ice cream. People in the normal carbohydrate group reported a much higher consumption of these foods (a median of six across the five dietary recalls) compared to the low carbohydrate groups (a median of zero!). This mirrored a result from my earlier study - that total carbohydrate restriction is associated with a preferential restriction of foods high in added sugars - a dietary change almost certainly likely to improve health outcomes.

The other striking finding was how few people actually reported a dietary intake consistent with a low-carbohydrate diet. Out of over 210,000 people who had completed dietary data, only 444 had a low-carbohydrate diet as defined by fewer than 26% kcal from carbohydrate. 1953 people reported consuming less than 130g of carbohydrate per day, which is another definition used for a low-carbohydrate diet. In an earlier study in the US, out of over 10,000 people in the Continuing Survey of Food Intake by Individuals, only 412 reported consuming a diet with fewer than 30% kilocalories from carbohydrates. There could be many reasons for this. Potentially, these types of dietary surveys might attract people who are engaged with conventional health advice, and therefore more likely to follow the type of diet recommended by national guidelines. Another reason may be that a low-carbohydrate diet is difficult to follow for most people. It’s worth mentioning that the low-carbohydrate groups had a higher BMI in each of the three dietary surveys I mentioned above. It’s possible that people with a higher BMI were following this diet to try to help them lose weight, but the low overall number reflects the difficulty in sustaining this diet long-term.

A limitation of our study is that we cannot tell whether people were intentionally following a low-carbohydrate pattern. Therefore I next plan to follow people prospectively as they adopt a low-carbohydrate diet. I hope this research helps us understand what foods people consume on a low-carbohydrate diet (especially people on a low-income) and can aid us in offering dietary advice to people wanting to follow this dietary approach.

Shafique M, Russell S, Murdoch S, Bell JD, Guess N. Dietary intake in people consuming a low-carbohydrate diet in the UK Biobank. J Hum Nutr Diet. 2017 Dec 13.

Guess N. Dietary intake in people consuming a reduced-carbohydrate diet in the National Diet and Nutrition Survey. J Hum Nutr Diet. 2017 Jun;30(3):360-368.

Kennedy ET, Bowman SA, Spence JT, Freedman M, King J. Popular diets: correlation to health, nutrition, and obesity. J Am Diet Assoc. 2001 Apr;101(4):411-20. Review.

Dr Nicola Guess is a lecturer in the Department of Nutritional Sciences at King’s College London in the UK and a Registered Dietitian. Her research interests are on the effect of diet on the pathophysiology of type 2 diabetes.