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Which is best – moderate or very-low-carbohydrate eating?

6th September 2023

When it comes to reducing carbohydrates in the diet, should people with diabetes opt for moderate intake or very low? Two viewpoints were presented at the recent American Diabetes Association’s 83rd Scientific Sessions. Lisa Buckingham reports.

Arguing that moderate carbohydrate eating is best, Dr Carol Kirkpatrick, Clinical Scientist at Midwest Biomedical Research Adjunct Faculty at Idaho State University, began by discussing the variety in definitions of carbohydrate-restricted dietary patterns within randomised controlled trials, which make it hard to evaluate evidence. Her team created a clearer definition and this is what she referred to throughout: Based on 2,000 calories per day: Moderate = 26-44% of calories from carbohydrate per day (130-220 g); Low = 10-25% (50-125 g); Very low = <10% (20-50 g). 

Starting with weight management, she highlighted a study comparing carbohydrate-restricted diets with higher carbohydrate diets, which found that while short-term weight loss was greater with low or very-low-carbohydrate dietary patterns, the effect was lost beyond 12 months. This can be partly due to difficulties with adherence to severe carbohydrate restriction (although we can acknowledge that it’s difficult for patients to adhere to any dietary pattern, she said).

A 2022 meta-analysis looked at the dose-dependent effect of carbohydrate restriction on different cardiometabolic risk factors. There was a significant decrease in bodyweight at six months as carbohydrate intake reduced; however, by 12 months, the significant effect did not remain. 

My stance, said Dr Kirkpatrick, is that moderate carbohydrate is best – this was demonstrated by a meta-analysis, which showed that 35% carbohydrate at 12 months showed the greatest difference in bodyweight compared with the lower and higher intake of carbohydrate. Perhaps, she said, people were better able to adhere to the moderate intake as carbohydrate restriction is difficult to adhere to, especially in the long-term. 

With regard to effect on lipoprotein lipids, a review of the evidence and several meta-analyses since showed that when it comes to LDL cholesterol levels, there is a very low response. Some patients do achieve lower LDL, especially if they’re able to lose adiposity. However, she said, some experience an increase, especially if they have a genetic susceptibility, and a high intake of foods rich in saturated fatty acids can also contribute. It’s therefore important that if a patient chooses to restrict carbohydrates, they replace those foods with unsaturated fats. In general, triglycerides decrease and HDL cholesterol goes up. 

Returning to the dose-response study, the greatest reduction in LDL cholesterol at six months was seen at 40% carbohydrate, which is line with Dr Kirkpatrick’s ‘moderate carbohydrate is best’ stance. 

She drew attention to another 2020 meta-analysis, which showed that a moderate carbohydrate intake resulted in the lowest increase in LDL cholesterol, compared with low- and very-low-carbohydrate intake. For triglycerides, with each reduction in carbohydrate intake, triglyceride levels decreased, so the most significant decrease was seen in the very-low-carbohydrate bracket. 

However, said Dr Kirkpatrick, when it comes to cardiovascular health, we’re not entirely sure what that means – we know that elevated triglyceride levels are associated with cardiovascular risk but what we’re looking for is the overall atherogenic particles in the bloodstream and LDL and non-HDL cholesterol gives us that information. Therefore, we should look at the results in the presence of both triglyceride lowering as well as what’s happening with LDL and non-HDL cholesterol. When we do this, we see that a moderate-carbohydrate intake allows a decrease in triglycerides without an increase in LDL cholesterol. 

Next, she looked at the effect of carbohydrate intake on HbA1c. Research consistently shows beneficial effects at all levels of carbohydrate restriction. The lower they go, the greater the decrease in HbA1c although this does wane at 12 months at all levels of intake.

Dr Kirkpatrick highlighted the PREDIMED and CORDIOPREV studies on cardiovascular health – both of these are dietary intervention studies showing a decrease in cardiovascular events with a Mediterranean dietary pattern – in both, she pointed out, the dietary pattern was moderate carbohydrate. 

In terms of observational studies, which she acknowledged have their limitations, we see that extreme intakes of carbohydrate is associated an increased risk of all-cause mortality – here, she showed a 2018 cohort analysis showing that lower carbohydrate intake of less than 40% was associated with an increased risk of all-cause mortality, compared with a moderate carbohydrate intake of 50%, and at the other end of the spectrum, too high an intake (>70%) was also associated with an increased risk of all-cause mortality.

Her take-home messages were:

  • Benefits of carbohydrate-restricted dietary patterns were achieved at moderate carbohydrate intakes – triglycerides and HbA1c values improve more with severe carbohydrate restriction but the effect wanes with longer duration. 
  • Very low carbohydrate intakes are difficult to maintain and lower carbohydrate intake is associated with increased mortality in cohort studies.  
  • Recommended healthy dietary patterns can be moderate in carbohydrate, with replacement being healthy protein foods and unsaturated fatty acids and the Mediterranean diet pattern has the strongest evidence of benefit.

The argument for very-low-carbohydrate intake

Dr Dina Griauzde, Assistant Professor of Internal Medicine at the University of Michigan, stepped in to present on behalf of Professor William S Yancy from the Duke Lifestyle & Weight Management Centre, to make the case for very-low-carb eating. 

The case for very-low-carb eating centres around improvement of glycaemic control and not increasing cardiovascular risk (likely lowering it). 

Low-carb diets are often referred to as fad diets, she said, but it’s important to note that they have been around for over a century, showing a quote from Dr Elliott Proctor Joslin in 1923 discussing the restriction of carbs. It was the only treatment for type 1 diabetes before the advent of exogenous insulin and the Joslin Diabetic Diet recommended just 15g of carbohydrate per day (2% of daily calories).

The first study she drew attention to was a small inpatient feeding study of adults with type 2 diabetes who initially ate a high-carb diet for seven days followed by a transition to a two-week period of a very-low-carb eating pattern (20g of carbohydrate per day). During the low-carb phase, glucose and insulin levels were substantially lower and HbA1c decreased from 7.3% to 6.8% over 14 days. We don’t see that effect with medication, she said – a very-low-carbohydrate diet is the most potent tool we have to help patients achieve glycaemic control.

She then outlined the Carbohydrate-Insulin Model, which explains why carbohydrate is considered more obesogenic:

  • Dietary carbohydrate (sugar or starch) raises serum glucose and insulin.
  • A carbohydrate-restricted diet reduces the diet contribution to serum glucose, which then lowers insulin levels.
  • Insulin is a potent stimulator of lipogenesis (fat storage) and a potent inhibitor of lipolysis (fat burning).
  • Lowering insulin level (or dose) leads to burning of stored body fat, raising serum ketones and lowering body weight. 

The idea that dietary carbohydrate can drive overeating was represented in data from a paediatric feeding trial in which a small cohort of boys with overweight or obesity had three evaluations. During each evaluation period, they consumed a diet with different carbohydrate content. They found that those who had consumed a higher carbohydrate meal ate more and had higher glucose and insulin responses, as compared with those who ate a lower carbohydrate meal. 

Next, she highlighted a systematic review of 13 trials in which dietary carbohydrate was restricted to less than 45% of total daily energy per day and these data were consistent with Dr Kirkpatrick’s data – the degree of improvement in glycaemic status was associated with greater carbohydrate restriction. 

A meta-analysis of 56 trials was next, comparing diet effects on glycaemia in type 2 diabetes with a total of 4937 participants and comparing nine diets. It concluded that: ‘For reducing HbA1c, the low-carbohydrate diet was ranked as the best dietary approach followed by the Mediterranean diet and Palaeolithic diet, compared with a control diet. 

She then moved on to discuss the role of a very-low-carbohydrate diet in relation to cardiovascular disease (CVD). One of the criticisms of this diet pattern is that it’s high in saturated fat and this potentiates CVD risk. However, in a meta-analysis of 21 studies, the intake of saturated fat was not associated with coronary heart disease (CHD), stroke or CVD.

We can take this one step further, said Dr Griauzde, by saying that saturated fat (when consumed as part of a very-low-carb diet) may actually improve key measures of cardiometabolic health. She showed a small study of 40 adults with metabolic dysfunction – it involved 12 weeks of eating 1500 calories per day. Participants on the very-low-carb diet had significantly greater improvements in key measures of cardiometabolic health, such as body mass index and triglycerides. She drew attention to the fact that small LDL (which is more atherogenic than larger LDL particles) increased to a greater extent in individuals following the low-fat diet. Total serum fatty acids (SFAs) decreased to a significantly greater extent in the low-carb group despite a three- to fivefold increase in saturated fat intake. 

The take-home points were: 

  • Low carbohydrate intake leads to rapid reduction in glucose and lower insulin levels.
  • The lower the carbohydrate, the lower the blood glucose trends. 
  • In patients with diabetes, very-low-carb eating also lowers medication requirements more than other eating plans.
  • Blood pressure decreases, serum HDL increases and triglycerides decrease.
  • LDL cholesterol does not typically increase AND small LDL decreases.

Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.