This information was written by Harriet Smith, Registered Dietitian on 22nd October 2019. It was updated and reviewed by Cordelia Woodward, Registered Dietitian, of MyNutriWeb in August 2025.
Oral nutrition supplements (ONS) are commercially produced products, prescribed for patients with disease-related malnutrition who are struggling to meet their nutritional requirements through an oral diet alone.1 Registered dietitians determine a patient’s effective dose of ONS requirements based on clinical judgement; as an evidence-based profession the dietitian needs to justify the type, volume and duration of ONS used. Currently the optimal dosage of ONS is likely to vary considerably according to patient need.
London audit data indicates 57-75% of ONS prescriptions are inappropriate.2 Of those initiated appropriately, few patients are followed-up to ensure ONS remain indicated, resulting in significant costs to the NHS.2 Malnutrition continues to be a significant economic burden in the UK, estimating to cost the UK health and social care system more than £23 billion each year.3 This equates to 15% of total expenditure on health and social care.3
The National Institute for Health and Care Excellence (NICE) suggests that improving identification and treatment of malnutrition has the third highest potential to deliver cost savings to the NHS,4 therefore, it is important that the amount of ONS being prescribed is effective, both clinically and financially. This article will provide an overview of the current evidence relating to optimal dosage of ONS in the context of current prescribing guidelines.
Current ONS prescribing guidelines
Most ONS prescriptions range from one to three supplements per day, with benefits typically seen in patients receiving between 300 - 900kcal from ONS per day.3 Prescribing Guidelines for ONS differ according to local trusts and Integrated care systems (ICSs); although many trusts consider a clinically effective dosage of ONS to be two sachets/bottles per day (depending on the product), which provides approximately 600 calories per day. Many of these recommendations are based upon national prescribing guidance from PrescQIPP,5 the Malnutrition Universal Screening Tool (‘MUST’) Explanatory Booklet6 and the Malnutrition Pathway3 and/or clinical experience.
ONS compliance
ONS is only effective if it is consumed and there are many factors (contextual, personal and product related) that impact patient compliance and perceived palatability of ONS.7 A systematic review paper from 2012 by Hubbard et al. looked at ONS compliance across different care settings and the influence of patient and ONS-related factors. They identified 46 studies (n=4328) which reported compliance or allowed compliance to be calculated and found mean compliance was 78% (67% in hospitals and 81% in community settings) and mean intake of ONS was 433 kcal/day.8
Patient compliance across a heterogenous group of unmatched studies was positively associated with higher-energy ONS and greater consumption of ONS and total energy intakes. However, compliance was unrelated to the amount or duration of ONS prescribed; this suggests the number of calories provided by the ONS is an important factor when it comes to improving patient compliance.8
What is an optimal dosage of ONS?
Based on the Hubbard et al. paper,8 the 46 studies included in the review reported a wide range of calorie prescriptions from ONS. A simple analysis of the ONS prescriptions reported in the paper shows that calorie prescriptions ranged from less than 300 calories/day to more than 900 calories/ day. The most frequently reported calorie range from ONS prescriptions was 400-600 calories per day.
The Hubbard et al. paper would suggest that it is likely to be a range of calorie intakes that have been associated with improved clinical outcomes, depending on the patient group and setting. This aligns with ranges recommended in national guidance.3,6 Standard ONS typically provides around 300 kcal per serving,3 so obtaining 600 kcal daily - usually by prescribing two servings - is widely regarded as a clinically effective dose. This is reflected in the PrescQIPP guidelines,5 which state: “To be clinically effective, ONS should be prescribed twice daily or at 600 kcal per day.” Given this evidence, it is easy to see why prescribing standard ONS twice daily has become recommended practice.
High-energy oral nutritional supplements (ONS) - which provide more calories per ml - are now available in once-daily formats, providing a clinically effective amount of energy in a single bottle. This one-a-day approach offers greater flexibility in how and when supplements are taken. Evidence shows that when ONS are consumed in smaller doses, this can aid compliance.8,9 This may be particularly useful for individuals with reduced appetite or those unable to tolerate larger volumes, such as patients with COPD, who may struggle to consume two standard ONS bottles (400 ml) per day.
Complementing this, the 2024 NHS Advisory Committee on Borderline Substances (ACBS) consultation response highlights that the now widespread smaller 125ml units ONS emerged as an innovation specifically aimed at improving patient compliance. This smaller format was introduced in response to clinical and industry recognition that energy-dense, low-volume presentations can enhance adherence.10
Previous concerns around the one-a-day concept
It is recommended that two ONS (or 600kcal) per day be prescribed as clinical effects are often seen with one to three servings per day. However, one ONS per day (300kcal) prescribing provides amounts that can be met with food fortification alone which is the preferred treatment.5 Whilst there is some evidence for managing malnutrition with dietary advice alone (dietitians are encouraged to adopt a food-first approach), data on clinical outcomes or cost is limited.11
Additionally, care should be taken when using food fortification to ensure requirements for all nutrients including macro and micronutrients are met. Finally, acute and chronic disease may adversely affect appetite and a patient’s ability to purchase, prepare and consume home-made fortified meals and drinks.11 Thus, food fortification techniques are only effective if it is realistic and sustainable for the patient and carer alike.
Interestingly, a 2023 dietitian survey conducted by AYMES found that whilst a food first approach is always recommended, only 36% of dietitians felt that food first principles were followed by at least half of their patients.12
Benefits of ONS
Existing research into the benefits of ONS has shown that:
- ONS can improve energy and protein intakes, reduce complications and readmissions to hospital, improve hand grip strength and body weight, with little reduction in normal food intake13
- ONS help to improve functional status (handgrip strength) and quality of life and reduces hospital readmissions14
- ONS are a clinically and cost-effective way to manage malnutrition, especially in patients with a low BMI, living in the community setting15
- Clinical benefits of ONS include reductions in complications (e.g. pressure ulcers, poor wound healing, infections) mortality (in acutely ill older people) hospital admissions and readmissions.13,15,16,17,18
Future studies into the effectiveness of ONS
A major new study, the £3.5 million REFRESH trial (2025–2028), will test prescribed ONS against fortified foods and routine care for older adults at risk of malnutrition. Led by the University of Plymouth with the BDA, it will involve around 1,530 participants across 90 sites, measuring outcomes such as quality of life, nutrition, muscle strength, weight, daily activities, and healthcare costs to identify the most effective, cost-efficient use of ONS in practice.19
Conclusion
In summary, determining an effective dose of ONS depends on both clinical evidence and individual patient needs, with 600 kcal per day (typically two standard servings) widely regarded as a clinically effective prescription. While a food-first approach remains the preferred starting point, practical barriers often mean ONS plays a vital role in supporting nutritional intake, improving clinical outcomes, and reducing healthcare costs. Flexible prescribing strategies, including the use of high-energy, low-volume formats, may further enhance compliance, particularly for those with reduced appetite or tolerance to larger volumes. Ongoing research, such as the REFRESH trial, will help strengthen the evidence base and guide future best practice in the management of malnutrition.
Download a Dietitian's Checklist for Improving ONS Compliance below:
