Research suggests we may not actually absorb up to 30 per cent of the kilojoules in nuts.

A series of studies, from the US Food and Drug Administration (USDA), looked at almonds, cashews, pistachios and walnuts, over a period of six years. The results suggest the average tree nut offers significantly less energy (or kilojoules) than what nutrition labels suggest.

In the laboratory, the Atwater system has been used (for more than 100 years!) to calculate the energy content of different foods. But this new research suggests the ‘Atwater factors’ assigned to certain foods, including nuts, may not be accurate. 

A new way of looking at energy in nuts

Over the past decade, studies have collected and analysed samples of urine and faeces from study participants on either a ‘control diet’ or a ‘nut-containing diet’ (1). They’ve then used this to work out how much of the fat from nuts is actually metabolised and converted into energy in the body, and how much is excreted.

These carefully-controlled ‘feeding trials’ have revealed that Atwater factors overestimate the actual energy content of nuts.

It’s been suggested that this is because the naturally-occurring fat in nuts is held (or trapped) within the cell walls of the nut, making it hard for the body to digest and absorb. Instead, nut eaters excrete some of this dietary fat in their stools – without it ever being used by the body as a source of energy.

Table: Energy absorption of certain tree nuts

Atwater factor calculation for energy content (kJ) / 30g serveAverage available energy content from feeding trials (kJ) / 30g servePossible overestimation of the energy from nuts
Almonds76058032%
Cashews73061416%
Pistachios7116785%
Walnuts82865421%

Note: References for the values above can be found below: Almonds (ref 1), cashews (ref 2), pistachios (ref 3) and walnuts (ref 4).

The energy available from nuts is thought to be around 5-30% lower than that estimated by the laboratory-based Atwater factors.

What about different forms of nuts?

Studies suggest that the more ‘intact’ nuts are, the less kilojoules we absorb from them. In an almond study (5), for example, less energy was available to the body after eating whole almonds, compared with almond butter. So, the form of nuts makes a difference.

Nuts and weight

Despite their high content of healthy fats, nuts are not associated with weight gain.

In fact, research shows the opposite – those who eat nuts are less likely to be overweight (7). This is likely due to much of the fat in nuts being ‘trapped’ in the fibrous cell walls of the nut, meaning it’s not absorbed by the body.

The bottom line

If you enjoy eating nuts, but worry about their energy content, these studies – which suggest that up to around a third of the kilojoules in nuts is not absorbed – is good news.

We now need more research into what this means in areas such as food labelling and dietary guidance for nuts. In the meantime, continue to enjoy the recommended healthy handful (that’s 30g) of nuts daily!

References

  1. Novotny JA., et al., Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets. Am J Clin Nutr, 2012. 96(2):296-301.
  2. Baer D., et al., Measured energy value of pistachios in the human diet. British Journal of Nutrition, 2012. 107(1): 120-25.
  3. Baer DJ., et al., Walnuts Consumed by Healthy Adults Provide Less Available Energy than Predicted by the Atwater Factors. The Journal of Nutrition, 2016. 146(1): 9–13.
  4. Baer DJ., Novotny JA., Metabolizable Energy from Cashew Nuts is Less than that Predicted by Atwater Factors. Nutrients, 2018. 11(1): 33.
  5. Mandalari G., et al., Understanding the Effect of Particle Size and Processing on Almond Lipid Bioaccessibility through Microstructural Analysis: From Mastication to Faecal Collection. Nutrients, 2018. 10: 213.
  6. Cassady BA et al., Mastication of almonds: effects of lipid bioaccessibility, appetite, and hormone response. Am J Clin Nutr, 2009. 89(3): 794–800.
  7. Flores-Mateo G, Rojas-Rueda D, Basora J, et al. Nut intake and adiposity: meta-analysis of clinical trials. Am J Clin Nutr, 2013. 97: 1346-55.
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