WO2018029199A1

WO2018029199A1 - A body hydration monitoring system and method

Info

Publication number: WO2018029199A1
Application number: PCT/EP2017/070086
Authority: WO
Inventors: Qiao HUA; Jun Shi; Ming Sun; Donghai Yu
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2016-08-08
Filing date: 2017-08-08
Publication date: 2018-02-15
Anticipated expiration: 2019-02-08

Abstract

A body hydration monitoring system comprises an infrared light source adapted to be applied against a lip of a user of the system and an optical sensor for sensing reflected infrared light. A hydration level is determined from the relative intensity of the reflected infrared light. This system is based on detecting the reflection of infrared light from the tissue beneath the lips. The amount of reflection is correlated with the hemoglobin density, which itself depends on the level of hydration of the body. The system provides a non-invasive measurement which enables a user to track their hydration level over time in a simple and unobtrusive way.

Description

A body hydration monitoring system and method

FIELD OF THE INVENTION

This invention relates to a body hydration monitoring system and method.

BACKGROUND OF THE INVENTION

Dehydration refers to a deficit of total body water, with an accompanying disruption of metabolic processes. Dehydration occurs when water intake is insufficient to replace free water lost due to normal physiological processes (e.g. breathing or urination) and other causes (e.g. diarrhea or vomiting).

Hydration is even more essential for a number of special groups, namely women during pregnancy and lactation, children, athletes and the elderly.

Chronic dehydration may for example cause high blood pressure, high cholesterol, constipation, kidney problems, weight gain and premature aging.

There are various physiological signs of dehydration. The most outwardly clear sign is drying of the lips. The lips do not contain oil glands like the skin, so they can dry and become chapped very easily. Thus, a condition of dry lips is one of the signs of body dehydration.

Many consumers reach for lip balm or lipstick when they feel their lips are dry. However, this is not addressing the underlying problem of dehydration, and some products contain allergic or even toxic compounds that may be harmful to human health.

There are products which provide hydration coaching or hydration tracking through smart apps and smart bottles. These solutions aim to record the drink volume of a subject, and assess this volume against a target such as 8 glasses of water per day.

Clinical methods to determine dehydration include monitoring body mass changes, measuring sweat, blood markers and urine analysis. However, they are invasive methods and few of them can perform real-time hydration monitoring.

US 2008/0076983 discloses a system and method for determining tissue hydration. In the system disclosed, near infrared light is used to measure tissue hydration by determining a hydration index (a ratio of water to the sum of water and protein). Tissue with hemoglobin is preferred to be avoided due to its contribution to the absorption spectrum.

In US 2008/0076983, it is proposed to correlate the hydration with the bandwidth of a spectral absorption feature (the absorption being caused by water and protein) for example the bandwidth of an absorption peak around 1450nm. This requires complex analysis of a frequency spectrum and therefore provides a complex approach.

There remains a need for a non-invasive hydration monitoring system able to provide real time monitoring which is easy to perform by a consumer without the need for medical assistance.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a body hydration monitoring system comprising:

an infrared light source adapted to emit infrared light against a lip of a user of the system;

an optical sensor for sensing the infrared light reflected; and

a controller for determining a hydration level from the difference between the intensity of the infrared light emitted and the intensity of the infrared light reflected.

This system is based on detecting the reflection of infrared light from the tissue beneath the lips. The amount of reflection, determined from the emitted intensity and reflected intensity, is correlated with the hemoglobin density, which itself depends on the level of hydration of the body. The system thus takes advantage of the properties of hemoglobin, in particular the intensity change of infrared light. The intensity change correlates with hemoglobin levels, and hemoglobin further correlates with water. This provides an alternative approach to the known system outlined above which correlates a bandwidth variation with protein and water. Hemoglobin concentration level is correlated by hydration level, since water is base in blood and hemoglobin locates in blood. Thus when dehydration occurs, water supplied to tissue decreased, so the concentration of hemoglobin will increase; when the tissue is hydrated, the concentration of hemoglobin will decrease.

The system provides a non-invasive measurement which enables a user to track their hydration level over time in a simple and unobtrusive way. The lip has the thinnest skin of the face and provides the best site for hemoglobin density monitoring. The system is able to operate at a single selected frequency, or set of frequencies (as explained further below). This simplifies the hardware and the signal processing required.

For example, the infrared light source has an output wavelength in the range 840nm to 950nm, at which hemoglobin has good optical absorption. Thus, the output wavelength may be 910nm, which can accurately detect concentration of hemoglobin. This provides a simple apparatus.

The system may further comprise a second light source for providing reference light for which the reflection does not depend on the hydration level. The reference light for example is red light with a wavelength in the range 600nm to 720nm, for example in the range 650nm to 670nm, for example 660nm. This red light provides an offset signal so that even small reflections in the infrared range may be measured as a small signal superposed over a larger reference signal in the red wavelength range.

The infrared reflectance can thus be distinguished from the skin reflectance at red wavelengths.

The benefits of providing hydration monitoring are that it becomes possible to remind people to drink properly whenever they check their smart phone / smart watch. An optimized liquid intake can be advised, for example during pregnancy and lactation an intake of 3 L and 3.8 L of total fluid is advised, respectively. A child for example needs increasing fluid intake during their growth, from 1 L to over 3 L. The elderly often experience a diminution of their thirst sensation, and office workers who are busy at work often forget to drink.

The system may further comprise an image sensor for taking an image of the lips, wherein the controller is further adapted to take into account the image in determining the hydration level.

Dry lips are a sign of dehydration as mentioned above, and this condition can be detected based on image analysis, at least once it has become severe. This provides an additional source of information for determining a level of dehydration, or it may be used as a way to calibrate the optical sensing measurement to a particular individual.

The controller may be adapted to provide compensation for ambient light. This may be based on further sensing or based on information about the lighting arrangement used in the space where the system is being used. This provides a simple compensation approach which does not require additional effort from the user.

The system may further comprise an output interface adapted to provide output information which advises a user when to drink in order to prevent dehydration. In addition to determining a hydration level, the system can be used as a reminder or warning system, to prompt the user to take on enough fluids to remain hydrated. The advice may be based on the expected behavior (e.g. exercise routine) of the user.

The system may further comprise a temperature sensor and/or a humidity sensor. Temperature and/or humidity information may assist the system in predicting how quickly a hydration level will change over time, so that the system can give indications in advance of the need to take on fluid.

The system may further comprise an input interface adapted to receive input information from the user which identifies volumes and timings of liquid intake by the user.

This enables the system to calibrate a recommended amount of fluid intake for a particular user based on historical information about the user, in particular how their fluid intake alters the measured hydration level.

An input interface may also be adapted to receive input information from the user which identifies a thirst perception of the user.

This also enables the system to calibrate a recommended amount of fluid intake for a particular user based on historical information about the user, in particular what levels of dehydration give rise to thirst for that particular person.

The input interface may also be used to provide information about the plans of the user, for example with regard to eating, sleeping, exercising etc. The input information from the user generally enables a tailored database to be created, so that a continuous real time monitoring function can be provided, with advice provided to the user on a continuous basis.

The system may be integrated into or connected to a portable wireless device such as a smart phone or tablet. Some portable wireless devices may already include an infrared light source, in which case the system may only need to add a sensor to the existing hardware of the portable wireless device. Alternatively, an external camera and sensor module may be provided for connection to the portable wireless device, which then performs the required signal processing.

Examples in accordance with another aspect of the invention provide a method of determining a body hydration level comprising:

emitting infrared light into the lip of a subject;

sensing infrared light reflected from the lip using an optical sensor; and

determining a hydration level from the difference between the intensity of the infrared light emitted and the intensity of the infrared light reflected. The skin on the lips is the thinnest skin on the face, and thus provides the strongest penetration of infrared light into the tissue beneath.

The method may further comprise emitting reference light to the lip of the subject, for which the reflection does not depend on the hydration level.

The method may further comprise analyzing an image of the lip of the subject in determining the hydration level. It may also comprise providing compensation for ambient light.

Output information may be provided which advises the subject when to drink in order to prevent dehydration. The method may further comprise performing temperature and/or humidity sensing. Input information may also be received from the user which identifies volumes and timings of liquid intake by the subject, and/or a thirst perception of the subject.

The invention may be implemented at least in part in software.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows the cellular structure of normal lips;

Figure 2 shows the cellular structure of chapped lips;

Figure 3 shows an example of a body hydration monitoring system.

Figure 4 show the system of Figure 3 implemented as an accessory to a mobile telephone;

Figure 5 shows the optical paths in more detail;

Figure 6 shows the reflection spectrum of general indoor lighting and of the light provided by the two light sources of the body hydration monitoring system of Figure 3;

Figure 7 shows a reflection spectrum of the lips;

Figure 8 shows a hydration level and a weight level over time;

Figure 9 shows a body hydration monitoring method; and

Figure 10 shows a general computer architecture for implementing the processing performed in the system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a body hydration monitoring system which comprises an infrared light source adapted to be applied against a lip of a user of the system and an optical sensor for sensing reflected infrared light. A hydration level is determined from the intensity change of the reflected infrared light. This system is based on detecting the reflection of infrared light from the tissue beneath the lips. The amount of reflection is correlated with the hemoglobin density, which itself depends on the level of hydration of the body. The system provides a non-invasive measurement which enables a user to track their hydration level over time in a simple and unobtrusive way.

Figure 1 shows the cellular structure of normal lips and Figure 2 shows the cellular structure of chapped lips. The skin surface becomes damaged and there is a reduction of inter-cellular fluid.

Figure 3 shows a body hydration monitoring system 10.

An infrared light source 12 is for application against a lip 14 of a user of the system, and an optical sensor 16 is used for sensing reflected infrared light.

As explained below, the light source 12 may further include light of a second wavelength (in particular a red light source) so that light of two different wavelengths is provided to the user.

A controller 18 then determines a hydration level from the reflected infrared light, and in particular from the absorption characteristics, as determined from the intensity difference between the emitted infrared light and the reflected infrared light.

This system is based on detecting the reflection of infrared light from the tissue beneath the lips. The amount of reflection is correlated with the hemoglobin density, which itself depends on the level of hydration of the body. The system enables a non-invasive measurement which permits a user to track their hydration level over time in a simple and unobtrusive way. The lip has the thinnest skin of the face and provides the best site for hemoglobin density monitoring.

Figure 3 also shows an optional image sensor 20 (i.e. a camera) for taking an image of the lips. The controller 18 takes into account the image in determining the hydration level. Dry lips are a sign of dehydration, and this condition can be detected based on image analysis. This provides an additional source of information for determining a level of dehydration, or it may be used as a way to calibrate the optical sensing measurement to a particular individual. Tissue under the lip is taken as a sample of body tissue for hydration monitoring.

In an example, the controller 18 also provides compensation for ambient light by processing a measure or else previously determined indication of reflected light outside the infrared wavelength band. An input/output interface 22 provides output information which advises a user when to drink in order to prevent dehydration. The output information may give advice about fluid intake amounts and/or timing information, for example providing a reminder or warning system. The timing advice may for example be based on the expected behavior (e.g. exercise routine) of the user.

For this purpose, the input/output interface may also receive input 24 from the user, such as their exercise calendar, information about how much they have drunk and when, and their current thirst level. All of this information enables the system to be calibrated to a particular user based on historical information about the user as well as future activity plans of the user, such as eating times, drinking times, exercise amounts and times.

Figure 3 also shows a temperature sensor and/or a humidity sensor 26. Temperature and/or humidity information may assist the system in predicting how quickly a hydration level will change over time, so that the system can give indications in advance of the need to take on fluid.

The system provides a non-invasive solution for monitoring body hydration level through a sensor that can be integrated with smart phone, smart watch and any other electronic devices.

Figure 4 shows the system of Figure 3 implemented as an add-on to a mobile phone. The mobile phone functions as the controller 18 (by means of an application loaded onto the mobile phone), the input/output interface 22 and the (visible light) image sensor 20. The addon 30 comprises the infrared light source 12, the optional red light source, and sensor 16. It may communicate wirelessly with the mobile phone, for example using Bluetooth. The mobile phone may also include the temperature and/or humidity sensors or these may be part of the add-on module.

In another example, the mobile phone may already include the infrared light source and/or the red light source and/or the infrared sensor. In this case, only those hardware components not already present in the mobile phone need to be provided as part of the external add-on.

In order to be able to accurately monitor personal hydration changes, a record of initial personal data may be stored as part of a calibration process. For this purpose the drinking volume, drinking time and the perception of thirst are provided as inputs when the lip sensing is carried out. By this calibration process, a tailored hydration database is established. Subsequently, a continuous and real-time hydration monitoring function can be performed (without requiring further user input) to remind the user when to drink and how much to drink.

A lip hydration index based on the infrared analysis, appearance changes, and skin changes may all be recorded to contribute to the evaluation of the body hydration level. The advisory feedback takes account of the personal database established previously, but also the environmental conditions (present and/or forecast), consumer activities (present or planned) and any user-requested functions.

Whenever a user checks their phone, hydration data and drinking advice may be presented automatically.

The sensor may detect multiple wavelengths, for example red and infrared.

Figure 5 shows an infrared 910 nm light source 12 and a red 660 nm light source 42 mounted within a housing 40. Hemoglobin absorption is maximum in the infrared light range such as the wavelength of 910 nm which penetrates the lip skin 14-1 and reaches the body tissue 14-2 under. Therefore this is the best candidate infrared light to be used for body tissue hydration monitoring, since the infrared light intensity difference will be maximized and then easier to be observed. More generally, the infrared wavelength lies in the range 840nm to 950nm. When dehydration occurs, the volume of a blood vessel decreases and the concentration of hemoglobin increases. Thus, the absorption of infrared light increases and the reflected light intensity reduces.

The red light is used as a reference so that the overall signal does not drop below a detection threshold. More generally, the red wavelength is in the range 600nm to 720nm, for example in the range 650nm to 670nm.

Light influences from the environment may be compensated by subtracting the environmental light intensity in the range of 400nm to 750nm obtained by measurement or by previous calibration of the system. A measurement may for example be made without the 660nm and 910nm light sources turned on (e.g. 400nm to 750nm ), but equally an ambient light measurement below the 660nm wavelength (e.g. 400nm to 630nm) may be made with light sources turned on.

Figure 6 shows a plot of light reflection versus wavelength (in nm) of the hydration status of the tissue beneath the lips. The spectrum of environmental light reflection is shown as plot 60. It has various peaks generally in the wavelength range 420nm to 630nm. This environmental light reflection is for example from artificial light sources in the space in which the system is to be used. The reflection spectrum from the two LED sources at 660nm and 910nm (or slightly higher for Figure 6) in combination with the environmental light is shown as plot 62. The environmental light intensity (as detected or instead as previously determined as part of a calibration process) can be subtracted since there is no overlap with the reflections at 660nm and 910nm. This leaves a clean signal representing the sum of the intensity peaks at 660nm and 910nm. The peak at 660nm may be measured by a wavelength selective sensor, for example which measures the range 630nm to 700nm.

Thus, there is measurement of the peak at 660nm and measurement of the combined peaks at 660nm and 910nm. These measurements may be made with two different sensors having different bandwidth responses. The broader bandwidth sensor will also pick up signals resulting from ambient light reflections as well. There is thus compensation for any environmental (ambient) light reflections based on a further measurement or else use of a database which provides the required information. In this way, the magnitude of the reflection peak at 910nm can be determined without needing a sensor with a small detection threshold.

In particular, the system may include a database of environmental light intensity values for the space in which the system is to be used, effectively providing the information shown by plot 60. The database of environmental light intensity may include several scenarios including sunlight and artificial lighting. The environmental light contributions to the received reflected signal may thus be subtracted. The processing will take account of the reception bandwidth of the sensor.

The reflection at 660 nm for one person remains constant for different hydration levels. Thus, the 660nm light functions as a reference for which the reflection does not depend on the hydration level. This reflected peak thus acts as an offset value on top of which the 910 nm reflection is provided.

In particular, the light reflection peak at 910nm could be much smaller than at 660nm due to absorption by hemoglobin in the lips. The purpose of the 660nm light is thus to prevent the system dropping below the limit of detection when there is a small reflection signal of interest at 910nm for example if the majority of light at 910 nm is absorbed by hemoglobin in the lips. This small signal of interest is in effect superposed over a larger signal at 660nm.

Of course, ambient light compensation is only needed if the system is to be used in a way which means ambient light reflections will be able to reach the optical sensor. It is instead possible to provide a system which avoids ambient light reaching the sensor, but this will generally result in a more cumbersome operation for the user.

In order to tailor the system to a particular user, firstly, an original personal database for the consumer needs to be established. For this purpose, the consumer measures their hydration level just after drinking a certain volume of water (e.g. in 5, 10 or 20 minutes). Afterwards, based on the original personal database, the healthy hydration level for the individual is determined so that predictions and suggestions can be made for consumers when they need to drink and how much volume is suggested. The suggestion will also take into account environmental conditions, for instance, if the relative humidity level in the environment is quite low (e.g. <30%), the user should be recommended to drink with greater frequency.

Figure 7 shows the absorption spectrum for the object with low density of

hemoglobin, where the peak the of infrared light at 910nm is almost the same as the peak of the red light at 660 nm, indicating that the infrared light at 910nm is almost fully reflected back due to the low absorption of the hemoglobin.

Figure 8 shows experimental data. Plot 70 shows the measure of hydration (with the left y-axis scale) using the lip hydration measurement. Figure 8 is based on measurement of the red and infrared reflections only.

The left y-axis shows a hydration index with no units, which takes into account the light intensity at both around 660nm and 910nm. As an example, the hydration index may be the melanin values generated by Mexameter MX18, where the melanin values indicate the light intensity absorbed by melanin. In this example infrared light at 870nm and red light at 660nm are used as light sources for the measurement. An absolute value of hydration index will change if different infrared wavelengths are used. However, the trend for different infrared wavelength in the range of 840nm-950nm should be similar.

Plot 72 shows the weight over time, with the right y-axis scale in units of Kg. There is overall a 1% weight loss from the peak to the minimum. The body mass changes provided as a dehydration reference. Drinking times are shown as D.

The weight increase resulting from drinking can be seen. The weight loss is due to skin evaporation, breathing and urination.

The signal for the lip hydration levels can be seen to be closely related to the body weight changes. For example, after drinking 200 ml of plain water, the weight increased from 48.7 kg to 48.9 kg, and the lips hydration index increased from 40 to around 55 in 30 minutes. Afterwards, the lips hydration index gradually decreased with weight loss due to no consumption of any food or drink. After 150 minutes, after drinking 300 ml of plain water, the lip hydration index increased again. Thus, the lips hydration index can be used to continuously monitor the body hydration changes.

Figure 9 shows a method of determining a body hydration level. The method comprises providing infrared light into the lip of a subject in step 80. In step 82, reflected infrared light is sensed using an optical sensor. In step 84 a hydration level is determined from the reflected infrared light. In step 86, output advisory information is provided.

As explained above, the method preferably (but optionally) further comprises emitting reference light to the lip of the subject, for which the reflection does not depend on the hydration level.

The method optionally also includes the step of analyzing an image of the lip of the subject for use in determining the hydration level in step 84.

The system described above makes use of a controller for processing the collected data.

Figure 10 illustrates an example of a computer 90 for implementing the controller or processor described above.

The computer 90 includes, but is not limited to, PCs, workstations, laptops, PDAs, palm devices, servers, storages, and the like. Generally, in terms of hardware architecture, the computer 90 may include one or more processors 91, memory 92, and one or more I/O devices 93 that are communicatively coupled via a local interface (not shown). The local interface can be, for example but not limited to, one or more buses or other wired or wireless connections, as is known in the art. The local interface may have additional elements, such as controllers, buffers (caches), drivers, repeaters, and receivers, to enable communications. Further, the local interface may include address, control, and/or data connections to enable appropriate communications among the aforementioned components.

The processor 91 is a hardware device for executing software that can be stored in the memory 92. The processor 91 can be virtually any custom made or commercially available processor, a central processing unit (CPU), a digital signal processor (DSP), or an auxiliary processor among several processors associated with the computer 90, and the processor 91 may be a semiconductor based microprocessor (in the form of a microchip) or a

microprocessor.

The memory 92 can include any one or combination of volatile memory elements (e.g., random access memory (RAM), such as dynamic random access memory (DRAM), static random access memory (SRAM), etc.) and non-volatile memory elements (e.g., ROM, erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), programmable read only memory (PROM), tape, compact disc read only memory (CD-ROM), disk, diskette, cartridge, cassette or the like, etc.).

Moreover, the memory 92 may incorporate electronic, magnetic, optical, and/or other types of storage media. Note that the memory 92 can have a distributed architecture, where various components are situated remote from one another, but can be accessed by the processor 91.

The software in the memory 92 may include one or more separate programs, each of which comprises an ordered listing of executable instructions for implementing logical functions. The software in the memory 92 includes a suitable operating system (O/S) 94, compiler 95, source code 96, and one or more applications 97 in accordance with exemplary embodiments.

The application 97 comprises numerous functional components such as computational units, logic, functional units, processes, operations, virtual entities, and/or modules.

The operating system 94 controls the execution of computer programs, and provides scheduling, input-output control, file and data management, memory management, and communication control and related services.

Application 97 may be a source program, executable program (object code), script, or any other entity comprising a set of instructions to be performed. When a source program, then the program is usually translated via a compiler (such as the compiler 95), assembler, interpreter, or the like, which may or may not be included within the memory 92, so as to operate properly in connection with the operating system 94. Furthermore, the application 97 can be written as an object oriented programming language, which has classes of data and methods, or a procedure programming language, which has routines, subroutines, and/or functions, for example but not limited to, C, C++, C#, Pascal, BASIC, API calls, HTML, XHTML, XML, ASP scripts, JavaScript, FORTRAN, COBOL, Perl, Java, ADA, .NET, the iOS development tool, and the like.

The 170 devices 93 may include input devices such as, for example but not limited to, a mouse, keyboard, scanner, microphone, camera, etc. Furthermore, the I/O devices 93 may also include output devices, for example but not limited to a printer, display, etc. Finally, the I/O devices 93 may further include devices that communicate with both inputs and outputs, for instance but not limited to, a network interface controller (NIC) or

modulator/demodulator (for accessing remote devices, other files, devices, systems, or a network), a radio frequency (RF) or other transceiver, a telephonic interface, a bridge, a router, etc. The I/O devices 93 also include components for communicating over various networks, such as the Internet or intranet.

When the computer 90 is in operation, the processor 91 is configured to execute software stored within the memory 92, to communicate data to and from the memory 92, and to generally control operations of the computer 90 pursuant to the software. The application 97 and the operating system 94 are read, in whole or in part, by the processor 91, perhaps buffered within the processor 91, and then executed.

When the application 97 is implemented in software it should be noted that the application 97 can be stored on virtually any computer readable medium for use by or in connection with any computer related system or method. In the context of this document, a computer readable medium may be an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer related system or method.

This invention is of particular interest for home use or for use by non-experienced users.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A body hydration monitoring system comprising:

an infrared light source (12) adapted to emit infrared light against a lip (14) of a user of the system; and

an optical sensor (16) for sensing the infrared light reflected; and

a controller (18) for determining a hydration level of the body tissue under the lip(14) from the difference between the intensity of the infrared light emitted and the intensity of the infrared light reflected.

2. A system as claimed in claim 1, wherein the infrared light source (12) has an output wavelength in the range 840nm to 950nm, for example in the range 890nm to 930nm, for example 910nm.

3. A system as claimed in claim 1, further comprising a second light source for providing reference light for which the reflection does not depend on the hydration level.

4. A system as claimed in claim 3, wherein the reference light has a wavelength in the range 600nm to 720nm, for example in the range 650nm to 670nm, for example 660nm.

5. A system as claimed in claim 1, further comprising an image sensor (20) for taking an image of the lips, wherein the controller (18) is further adapted to take into account the image in determining the hydration level.

6. A system as claimed in claim 1, wherein the controller (18) is adapted to provide compensation for ambient light by subtracting the environmental light intensity.

7. A system as claimed in claim 1, further comprising a temperature sensor and/or a humidity sensor (26).

8. A system as claimed in claim 1, further comprising an output interface (22) adapted to provide output information which advises a user when to drink in order to prevent dehydration.

9. A system as claimed in claim 8, further comprising an input interface (22) adapted to receive input information (24) from the user which identifies:

volumes and timings of liquid intake by the user; and/or

a thirst perception of the user.

10. A system as claimed in any preceding claim, integrated into or connected to a portable wireless device such as a smart phone or tablet.

11. A method of determining a body hydration level comprising:

emitting infrared light into the lip (14) of a subject;

sensing infrared light reflected from the lip using an optical sensor (16); and determining a hydration level of the body tissue under the lip(14) from the difference between the intensity of the infrared light emitted and the intensity of the infrared light reflected.

12. A method as claimed in claim 11, further comprising emitting reference light to the lip of the subject, for which the reflection does not depend on the hydration level.

13. A method as claimed in claim 11, further comprising analyzing an image of the lip (14) of the subject in determining the hydration level.

14. A method as claimed in claim 10, comprising:

providing output information which advises the subject when to drink in order to prevent dehydration; and/or

receiving input information (24) from the user which identifies volumes and timings of liquid intake by the subject, and/or a thirst perception of the subject.

15. A computer program comprising code means which is adapted, when said program is run on a computer, to perform the method of any one of claims 11 to 14,